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rsr911
08-10-2006, 11:26 PM
Hello all,

I've got a pair of no longer available Zenith 3 barrel carbs for my Porsche 911, I have completely rebuilt them and they don't run right. After some inspection I found a lot of play in one of the throttle shafts on each carb, there are three shafts per carb and they are steel riding in the diecast aluminum housings. My play is to set up a fixture on my lathe carriage and bore or ream them so I can bush the holes, my question is what material should I bush the holes with? Bronze would be easy but I don't know if it will wear more than the original aluminum, Teflon is another thought. I could possibly weld up the holes and redrill, any suggestions? Remember I need just enough clearance for the shafts to move so I don't get vacuum leaks around the shaft like I'm getting now.

On a similar note I plan to eventually make 3 barrel valve bodies for use with EFI, these would be CNC machined from 6061 and probably use readily available throttle plates such as Holley carburator plates. Since I'll be starting from scratch on these what should I use for those shafts, would sealed bearings work or would the vacuum pull through the seals and suck grease out of the bearings.

HWooldridge
08-10-2006, 11:46 PM
I have rebuilt old Carter and Zenith carbs for other applications...but nothing so sexy as a Porsche...:-)

Bronze or brass would be about the same as aluminum but the Teflon is a good thought and would probably last a lifetime. At work, we use Teflon washers to seal polyester resin from leaking on mixer guns and it is pretty impervious to most chemicals.

A thought that occurred to me if making a fresh start would be to put a threaded insert with an O-ring in the body. Drill and counterbore a stepped hole so the O-ring fits in the bottom of the hole. Tap the larger hole in the body, then make a suitable plug to fit (with thru hole for the t-shaft) that slightly compresses the O-ring to make it seal against the shaft. This method would provide the capability to replace the O-ring if a vac leak develops.

A.K. Boomer
08-10-2006, 11:50 PM
Those were the days,,, very common problem on those old dawgs,, i remember my bro re-bushing with bronze, and i think you could even go sintered bronze and they would probably wear better, sintered will vent slightly but its consistant so this should not be a problem for you (you just adjust idle mix to compensate and everything stays the same, unlike right now were your plate is dancing around changing everything) sintered also holds lube good.

Iv checked out some sites now on Delrin AF and gas will not bother it, this is amazing plastic with teflon and does not need lube and wears great...

Question; how do you plan on finding center? the bushes wear in the direction of the throttle return spring do they not? so if its ovalized you cant really get a reference off of that, just curious.

CCWKen
08-11-2006, 12:13 AM
Do they have to remain stock looking? If not, just countersink the shaft bore (from the outside) for about 1/8" and install shaft seals. I know someone makes them. I had them in a carb kit a few months back. Or just use bronze in the same place.

Unless the shafts are really sloppy, I suspect something else is wrong.

LarryinLV
08-11-2006, 12:18 AM
Not a job for the faint of heart Any differences between them will show up in the idle quality, but I suppose it can be tuned out.

Some new carbs actually use teflon sleeves around the shaft but that won'y fix a worn hole.

Lot's of options today, but I think I would use brass or bronze. The contrast with the aluminum looks cool when you point out what you did.

A.K. Boomer
08-11-2006, 12:27 AM
It creates two major problems, one being the vaccum leak past the shafts and the other being the massive T-plate shifting its position in the carb bore, sometimes shutting down flow almost completly and somtimes getting to much, also because the plate is so close to the idle circuit bores it is a fuqe,,, this is why the shaft holes must be bored very precisely.

torker
08-11-2006, 12:28 AM
Christion, we had quite a discussion about this a couple years ago.
Was mostly about Holley carbs but the idea is the same.
I can't remember the title of the post but a search may turn it up.
Russ

A.K. Boomer
08-11-2006, 12:37 AM
You know i dont remember much about the zeniths, i remember a little of the solex, but the webers are what stuck out to me, god did they go through the pains to cover the ranges of rpm and demand, very complex carbs to dial in but once you did they purred like a little ***** cat, im thinkin i remember Webers coming stock with bronze bushes.

rsr911
08-11-2006, 12:59 AM
All very good ideas!

The carbs do not have top appear stock as very little of the car will be stock once I'm finished. I don't think shaft seals will work since the shafts are pretty sloppy, I can wiggle the shafts and change the idle. The worn ones are only the ones that have the linkage attached to them. The shafts are in line with each other fastened together with spring collars in case of back fire.

Finding center shouldn't be two hard since two of the shafts on each carb are still in great shape since they had no side loading. The plan is a long thin boring bar or reamer between centers on the lathe so I can line bore them I figure I'll need to make two since whatever I use for bushings will have to be pressed in place and I'll want to recut the inside to size. I'm kind leaning toward teflon glued and pressed in place with 3m's plastics glue. I plan to make an extension on the linkage end of the shaft to ride in another bearing which will take a lot of stress off the end bearing. I'm thinking teflon can be machined "tighter" providing a better seal but question it's thermal expansion and softening when hot compared to the cab bodies. I'm thinking heat soak between track sessions might give me trouble there but I really don't know.

Still wondering about sealed ball bearings for when I design and make the throttle bodies though, any thoughts on that idea?

What ever I do it needs to look professional and be highly functional. Since I'll be making a fixture to hold the carbs I plan to offer the service to other enthusiasts in the same boat I'm in but without machine tools or experience. These carbs are 34 years old at this point so I'm sure a lot of guys would jump at the chance to have there carbs bushed to restore that "like new" tunability and drivability. I can tune a pretty good idle on these and full throttle is awesome but light part throttle in any gear at any RPM causes light surging. I packed heavy grease around the shafts and the problem went away temporarily so I'm pretty certain it's the shaft bores that are causing the problem.

I love these little cars, they're very reliable, handle and stop fantastic etc. My 3.0 liter recently made 174HP and 180 ft/lbs on a chassis dyno, that's about 205 HP and 212 ft/lbs at the flywheel which is pretty fun in a 2000 lb car. :)

I was going to do the work on my mill but after thinking about it my large lathe has a T-slot table and it just makes more sense to support the cutting tool on both ends since the carbs are about 11" long and the throttle shaft bores run the end to end.

rsr911
08-11-2006, 01:04 AM
You know i dont remember much about the zeniths, i remember a little of the solex, but the webers are what stuck out to me, god did they go through the pains to cover the ranges of rpm and demand, very complex carbs to dial in but once you did they purred like a little ***** cat, im thinkin i remember Webers coming stock with bronze bushes.
These Zeniths are very similar to Webers. I've always been a fan of individual runner setups, the throttle response and power are unmatched by plenum type systems. The guys at the dyno said the throttle response reminded them of a sport bike.

winchman
08-11-2006, 03:32 AM
If some of the bores are still in good shape, you could make a tool to fit them so you can use them as a guide. The tool would have something similar to a back spot facing cutter on it. You'd turn it and pull it from the other end, where it's supported by the good bearing surfaces. That would enlarge the worn hole to recieve the bronze bushing you're going to make, and it would keep all the holes lined up.

Roger

John Stevenson
08-11-2006, 04:06 AM
Used to do these years ago on large petrol generating sets.
Because of the vibration and the fact they were usually in a fixed position they wore like crazy.
Problem was it was pushed by the linkage at one side ans pulled bu a big spring at the other so you got a forward facing slot one side and a rearward facing slot on the other.

First couple worked but weren't right, they stuck at low revs with not being accurately centralised, didn't matter too much on these gennies as they were flat out all day.

Next batch I did I turned a slug of steel up and very accurately drilled a cross hole same size as the shaft.
This had two 'O' rings fitted into grooves, one either side of the cross hole so it would stay where it was fitted in the bore.

This was pushed into the card and by using a piece of rod as a dummy spindle I managed to get the hole location in exactly the right position.
I found bolting to the cross slide of the lathe, [ mine had tee slots ], on packings was the quickest way to set these up as one axis was controlled by the plane of the flange. All you had to do was set the height by packings and the centralisation by the dummy bush.

Used to use brass for bushes but in light of modern materials I'd go for Delrin now as it wears far better and doesn't wear the shafts as much as metal to metal.

.

wierdscience
08-11-2006, 10:04 AM
I fixed a few old carbs,by reaming the old hole out just enough to include a sintered bronze sleeve,but oversized to allow some epoxy.All I did then was coat the bores with JB weld,slip the bushings in and used the throttle flap to set the position.A dab of grease on the shaft kept things from sticking while it cured.Never had an problems with them and the 1/16" egg shape being gone improved things radically.

On the new throttle body I would use graphite impregnated fiber bushings that have several advantages.They don't cut shafts,they tolerate vibration and they need no lube since it's built in.

Oh,and the're cheap:D

LarryinLV
08-11-2006, 11:06 AM
Since you mention Webers. I used DCOE's on both a Jaguar and an MG (3 on the Jag, only 1 on the MG).

DCOE's are side draft. They come with sealed bearings on each side of the throttle shaft - they're bullet proof, the rest of the carb will fall apart before there is any play in the throttle shaft.

Instead of making bushings and then extending the shaft to place bearings, is there enough meat on a Zenith to cut a pocket for bearings in a similar fashion to the DCOE? Once you have sized the shaft (either turned or sleeved) to fit the inner race of the bearing (assuming you might not find a bearing the exact size needed) it should never have to be done again.

A.K. Boomer
08-11-2006, 11:38 AM
"I love these little cars, they're very reliable, handle and stop fantastic etc. My 3.0 liter recently made 174HP and 180 ft/lbs on a chassis dyno, that's about 205 HP and 212 ft/lbs at the flywheel which is pretty fun in a 2000 lb car."


I think it is amazing what porsche acomplished back then and dont get me wrong i have respect for that --- but i have always looked at them as a dog chasing their tail, yes they do handle but god help you if you have to lift off the throttle in a turn, as you know a porsche will hunker down and bite into the pavement with power on but if you have to "lift" all that rear weight wants to swap ends,, and if you have to brake things really get scary, oversteer is an understatment --- just the same if you know what your doing and have the balls yes those cars handle,
The engine is where i end up really scratching my head, all the pains that porsche went through to "buck the system" and in many ways im in awe of the crouts just because they found a way,,, Air cooled and high performance dont mix, porsche on the other hand ignored this fact and found ways around it but in the long run things got very impractical and they had to adapt ----- exhaust valves ran at such high temps that they had to be sodium filled, I think a typical 911 S ex. valve was $125.00 and that was 20 years ago, the engines of that era held 14 quarts of oil! they had an oil cooler the size of a mini radiator, and to pump air to all the cylinders and throught the cooler they had to use a massive forced air fan that gobbled up an incredible 14 horsepower at 6,000 RPM's, for such a horsepower conscious company to use this kind of aproach it made me wonder even as a kid -- when are these guys going to adapt? I mean - porsche knew how to make HP's but they also knew how to throw it away in the form of to high of cylinder head temps and all previously mentioned,
Then come's the big anouncement in the year of 2,000 -------- ladies and gentelman ------- weve water cooled the 911, DUH

As far as being reliable i personally think them as one of the worst, constant valve adjustments because of all the aluminum and magnesium squirming around from all the heat, warped magnesium valve covers constantly leaking oil --- a major design flaw in the hyraulic cam chain tensioners that cause them to relax and not pump up on a cold winters day resulting in the chains wrapping around the crank drive and snapping all camshafts, unless you install aftermarket limiters your days were numbered and were not talking cheap repair,

on the positive side --- the attention to detail in certain area's was impresive, nobody had a lower Drag C.E. on a sports car at that time, the 911 was very slippery,, you could hold all five lug nuts in your hand and not even know they were there, the style was cool, (i hated that bump on the floor at the left side of the clutch pedal though and really didnt like the way the car shifted from first to second,)

I remember everyone creaming thier pants over them but if you had to work on them it sucked ---i would take girls out in them and hide my inner feeling about the car just to get laid...

Im sorry i beat on your car ---- and back in the day it was amazing what porsche pulled off, there is no doubt that its an all time classic, forgive me but iv just got to many years in....

ammcoman2
08-11-2006, 06:39 PM
How is this for co-incidence: Friend brings around his '65 Corvair convertible today. Ride around the block. Going around a corner - feel rear end twitch. Brings back vivid memories of an uncontrolled 360 in a 1961 model (in 1961).

The only difference between this car and a Porsche is that everything happens faster in a Porsche. Heh Heh.

I offered to rebuild a spare generator he picked up and had almost forgotten what they looked like, what with Gits oil cups, and a rear bushing. One also realizes how much space they took up compared to alternators.

Geoff

Evan
08-11-2006, 06:49 PM
Friend brings around his '65 Corvair convertible today. Ride around the block. Going around a corner - feel rear end twitch.

The '65 Corvair Monza was a great car. By then the suspension was basically the same as the Corvette since they abandoned the standard swing axle in '64. It had a 150 hp turbo engine and pretty decent performance. I drove a lot of miles in various Corvair models and we had a Corvair van when I was young.

CCWKen
08-11-2006, 07:43 PM
It wasn't the Monza that had the turbo--That was the Spider. I had three Monzas; a 62, 64 and 65 but no turbo. I still have a scar on my right index finger from adjusting the timing. Those spinning pulleys were like flails!

I pulled a trailer from Ohio to Texas in 1970 with the 65. Made it to Waco, Texas and had to stop and have the rear bumper welded back on. :eek: Still, the 65 was great car. I wish I had it back now.

Evan
08-11-2006, 07:49 PM
The only real difference between the Monza and the Spyder was the turbo and 40 extra horsepower. Otherwise they were the same.

wierdscience
08-11-2006, 08:40 PM
I just gave away two Corvair trans axles the otherday,I had them,why I don't know.

rsr911
08-11-2006, 10:51 PM
I think it is amazing what porsche acomplished back then and dont get me wrong i have respect for that --- but i have always looked at them as a dog chasing their tail, yes they do handle but god help you if you have to lift off the throttle in a turn, as you know a porsche will hunker down and bite into the pavement with power on but if you have to "lift" all that rear weight wants to swap ends,, and if you have to brake things really get scary, oversteer is an understatment --- just the same if you know what your doing and have the balls yes those cars handle,
The engine is where i end up really scratching my head, all the pains that porsche went through to "buck the system" and in many ways im in awe of the crouts just because they found a way,,, Air cooled and high performance dont mix, porsche on the other hand ignored this fact and found ways around it but in the long run things got very impractical and they had to adapt ----- exhaust valves ran at such high temps that they had to be sodium filled, I think a typical 911 S ex. valve was $125.00 and that was 20 years ago, the engines of that era held 14 quarts of oil! they had an oil cooler the size of a mini radiator, and to pump air to all the cylinders and throught the cooler they had to use a massive forced air fan that gobbled up an incredible 14 horsepower at 6,000 RPM's, for such a horsepower conscious company to use this kind of aproach it made me wonder even as a kid -- when are these guys going to adapt? I mean - porsche knew how to make HP's but they also knew how to throw it away in the form of to high of cylinder head temps and all previously mentioned,
Then come's the big anouncement in the year of 2,000 -------- ladies and gentelman ------- weve water cooled the 911, DUH

As far as being reliable i personally think them as one of the worst, constant valve adjustments because of all the aluminum and magnesium squirming around from all the heat, warped magnesium valve covers constantly leaking oil --- a major design flaw in the hyraulic cam chain tensioners that cause them to relax and not pump up on a cold winters day resulting in the chains wrapping around the crank drive and snapping all camshafts, unless you install aftermarket limiters your days were numbered and were not talking cheap repair,

on the positive side --- the attention to detail in certain area's was impresive, nobody had a lower Drag C.E. on a sports car at that time, the 911 was very slippery,, you could hold all five lug nuts in your hand and not even know they were there, the style was cool, (i hated that bump on the floor at the left side of the clutch pedal though and really didnt like the way the car shifted from first to second,)

I remember everyone creaming thier pants over them but if you had to work on them it sucked ---i would take girls out in them and hide my inner feeling about the car just to get laid...

Im sorry i beat on your car ---- and back in the day it was amazing what porsche pulled off, there is no doubt that its an all time classic, forgive me but iv just got to many years in....

No worries it's all those oddities that make the car fun. Take the over steer for example, once you learn how to use it to your advantage it can be a big advantage especially on a track. Also a lot of the troubles with the early cars was worked out by the late '70s with the 911SC, it's 3.0 is considered bulletproofm, often going 300K miles without even a top end rebuild. I run the 3.0 in my car but with carbs instead of the CIS for a performance increase. In '84 they went to pressure fed chain tensioners eliminating the chain problem and offer it as a kit for earlier cars, I've got one to install this winter.

What really makes these cars enjoyable is the "not so wealthy" guys who are buying them up and do all sorts of practical modifications for street and track and it's the track where they really shine. Two years ago a friend from another forum won the Open Track Challenge on the west coast with a modified 1972 911 using a late 90's engine. He didn't trailer the car to the track like the competition, no he drove it there complete with a roof rack loaded with track tires! ;)

In the end I can appreciate someone who has tried something and is honest enough to admit they didn't care for it. I bought this car from my brother when it still had a 2.0E engine in it, I was less than impressed at the time since my toy back then was a low 11, high 10 second Mustang drag car, the Mustang did it all "on the motor" with a stroker 351W (410 CI), big headers, race heads and cam, the works. I was used to having 600+ HP under my right foot and no brakes to speak of, handling? what was that. Well after several seasons of blowing up engines, transmissions and drivelines I got a bit tired of it, then one afternoon on the way back from the drag strip I ran into a guy coming from Mid-Ohio in a 911 at a truckstop. We talked for a bit and he gave me a ride. In the hands of a capable driver and obviously more power than my brother's car I was impressed, he took a corner that I could of sworn most cars would have slid right off of, and the brakes? my god could that car slow down in a hurry. About a year later the drag car was in pieces when my brother offered my the old 911, he said take it for a week to see if I like it and I did. Fast forward 5 years and I've got a car that still needs a new paint job but the power is much greater, the brakes have been upgraded along with the suspension and wheels. I rebuilt the tranny and worked out all the little bugs my brother never had time to and now the car is a blast. Sure it's only marginally faster than a stock V8 Mustang in a straight line but a nice fast drive down the twisty country roads near where I grew up and I forget all about drag racing.

Next on my agenda is a 1973 shell I have in storage, it's going to be a street/track car with the emphasis on track. The ride will be rough and the exhaust noisy, the interior will be non existant and the only "radio" I'll have is the symphony behind me, there will be no creature comforts like air or heat or even a cup holder and you'll have to climb over a roll cage to get in, yet to me the car will be perfect, all business just like my drag car used to be only it will last longer and get driven more.

rsr911
08-11-2006, 10:54 PM
Since you mention Webers. I used DCOE's on both a Jaguar and an MG (3 on the Jag, only 1 on the MG).

DCOE's are side draft. They come with sealed bearings on each side of the throttle shaft - they're bullet proof, the rest of the carb will fall apart before there is any play in the throttle shaft.

Instead of making bushings and then extending the shaft to place bearings, is there enough meat on a Zenith to cut a pocket for bearings in a similar fashion to the DCOE? Once you have sized the shaft (either turned or sleeved) to fit the inner race of the bearing (assuming you might not find a bearing the exact size needed) it should never have to be done again.

Larry, you've answered my second question about the bearings for the throttle bodies. Unfortunately there isn't enough meat around the shaft bore on the carbs for bearings so it will have to be bushings.

DCOE's are great carbs, i had a pair on a 2.3L Ford about 15 years ago, lots of fun.

A.K. Boomer
08-11-2006, 11:30 PM
RSR 911, you are a good sport, Dont worry about your 73 not having any creature comforts inside -- remember the ferrari F40's didnt even come with a radio or door panels and they were $400,000.00 , They were actually meant for a certain type of racing that got cancelled, when this happend they asked Enzo what to do --- he said put bumpers and turn signals on them and sell them to the public, little while later two of the members of pink floyd have one each sitting in there garage (lucky bastards)

Anyways, getting back to your 911, i just thought of one of the bennies of that car, there are many engine configurations but its very hard to beat a horizontally opposed six cylinder for smoothness (of course a horizontal twelve will do it)

The H-6 is almost perfect in the two catagories that it takes to balance an engine, mechanical and power impulses,,, its near perfect, and it really shows when you take them up in the R's and it almost feels like you have an electric motor powering you, nuther cool thing i remember cuz i been into there trannies to, all helical gears side load the drive gear and the recepter gear (countershaft) and in almost every aplication you will see a bronze thrust washer with oil retention grooves cut out to handle these loads, The 911 was no exception --- till you get to fifth gear, then they are needle bearing thrust washers, Yup --- the krouts were thinking autobahn...

rsr911
08-11-2006, 11:50 PM
Boomer,

LOL, I remember a quote from Enzo about the F40 not having a radio, he asked why one would need a radio with a 12 cyl behind them.

I believe it's H-6, I-6 and V-12 that are all naturally balanced but I could be wrong. The H-6's main benefit is low center of gravity. Subaru is another manufacturer who uses this advantage although I believe their rally cars are H-4s. My dream engine would be the water cooled GT3 engine. The first watercooled 911's used a pseudo dry sump system and the block was inferior to the last of the air cooled engines so for the GT1, GT2 and GT3 they used the 993 turbo case (993 = last air cooled 911) with water cooled cylinders and 4valve heads. That engine will bolt up to an early transmission. Speaking of transmissions, they are good but the synchros where always lackluster, finally the Krauts gave in and used Borg Warner sychros in the G50 gearbox that came out in the late '80s. I'm using one of those in the '73.

Sounds like you know your cars, what is your favorite?

A.K. Boomer
08-12-2006, 12:24 AM
Believe it or not the F-40 was a V/8

Your right on the inline 6, it to has good balance in both area's, the reason I dont like that configuration is what you stated -- to tall, yes you can slant them but still, also dont like running a long crank...

If I had a few extra bucks id like to own an NSX, i know there not the most powerful cars out there but i like refinement, the crome-mo birdcage frame, the aluminum body, the titainium connecting rods -------- and superb predictable handeling thanks to the engine midship (little more to the rear actually)

i never had the balls to push my brothers 911s in the turns because every time he'd let me drive it he'd say --- if you crash it you better hope you die in the flames...

rsr911
08-12-2006, 01:18 AM
Believe it or not the F-40 was a V/8..


Can't believe I forgot that. I even saw one in person once at a restaurant.

I like th NSX also and the power is not a big concern. Last time I was at Mid Ohio I got the chance to ride in a BMW M3 race car, this was club racing and the guy who owned it told me the engine was stock 175HP at the wheels. Funny thing was he was one of the fastest cars on the track that day, handily beating vettes, Vipers and the rest. I noticed between rounds he'd check the tire pressure and that was about it while the other guys where all over thier cars fixing and changing things. Once I got a closer look at the M3 i could see why he was so fast, huge brakes, obvious suspension upgrades, a well thought out cage etc. He didn't need a lot of power because he could brake later and carry more speed through the turns. The vettes would roar down the back straight and get on the brakes early then heavy throttle after the turn. Also the M3 driver was a "finesse" driver who obviously had a lot of experience, he told me he kept the motor stock for dependability.

Anyway, back on topic. I took a close look at the carbs today and there isn't much meat around the shaft bores to fit bushings. I'm thinking I'll make some thin brass bushings so I don't have to bore the holes too large.

Evan
08-12-2006, 02:24 AM
About the idea of using teflon for the bushings: It should work but there is a gotcha to watch out for. PTFE has an unusual response to temperature change. The coefficient of linear expansion isn't linear at all. It undergoes an important change right at around room temperature. PTFE is a crystalline polymer and makes a phase change from the triclinic form to the hexagonal form starting at 19 degrees C. That's around 67 F. Starting at that temp through to about 72F it suddenly expands by about one to two percent as the crystal form switches. After that it shrinks slightly and then stays pretty stable over the next 200 degrees.

You have to take this into account for any close tolerance machining of PTFE. For your application I would machine it at or below 65 degrees F only and install it cold. Any tightness when warm will not be a problem because of the extremely low friction plus if too tight it will cold flow to fit exactly. If it seems tight just wait several days as cold flow doesn't happen suddenly.

The maximum operating temp of PTFE is around 500F so that is not an issue. It isn't a thermo plastic and will not melt.

rsr911
08-12-2006, 02:56 AM
Ok, so I need to air condition the shop. Actually not that big a problem as my shop is at work an our coater room is cooled by a huge air condtioner, If I were to close the garage door to the ware house and open the door to the coater room this would allow the shop to get cool.

Or,

I could just freeze everything and work quickly.

Or what about running coolant at say 60 degrees with pre-cooling of the parts. I have a cooler for my tig torch that I could drain and put coolant in, then run the return through one of my 5 micron filter housings and back into the cooler. Alternatively I could put the 25 gallon carboy that my CNC uses for a sump into a fridge over night and run coolant out of that.

Would you glue the teflon in using 3M's plastics glue or just machine it for a tight press fit? How well will Teflon handle gasoline?

Evan
08-12-2006, 10:10 AM
I wouldn't freeze the part. Below the phase change it still has a linear change with temperature. Cold coolant would work although cold air would be much better as no coolant is required with PTFE. A stream of compressed air blowing on it should keep it below the critical temp as the air will be cooled by expansion.

PTFE is basically impervious to all common solvents except benzene. Gasoline does contain some benzene but the percentage of benzene today in gasoline is limited to no more than 1% maximum and is due to be reduced even further. It shouldn't pose a problem.

I wouldn't bother with the glue if there is some mechanical method of retaining it in place, like a snap ring or something.

A.K. Boomer
08-12-2006, 11:57 AM
I know you dont have the room but somebody mentioned ball bearings before and now that were on the subject of bearings again Id like to point something out, Ball and roller bearings are superior in many ways but also take a back seat to the "plain" bearing in a few area's, one being the obvious extra room required --- the other being surface area and bearing footprint or pattern, this gets critical with bearings that are kept in a stationary position, throttle plate bearings usually have enough variation to avoid "brinnelling" of the bearing races but it still can occur and set up a pattern, once brinneling gets started the pattern wont stop but will seek its deviation (full throttle and idle) and settle in, My point is is just because you have ball or roller does not mean things will last forever, And they would be a No-no in a generator type system,
Ball - roller and needle bearings require movement to lubricate themselves, if any of these bearings are kept stationary and even under a slight load and subject to any vibration at all then they will "frett" there roller or ball into their races --- all grease gets pushed out of the contact area and its metal to metal and vibration, its why brand new cars traveling long distance on trains and such were showing up at the dealers with their wheel bearings trashed, they should be blocked in the air or at the very least moved back and forth after so many miles, This distruction is not a simple pit in a race but a pattern that all balls or rollers created so even slight brinnelling is a total fuqe because all balls or rollers dive into the same deviation at the same time...
Iv seen some carbs with this problem --- and when i peeled back the bearing seal i expected to see a dry bearing , not so - plenty of good clean grease.

I learned this lesson the hard way as a Kid, I had a BMW 1600-2 , it had independent rear suspension, I hopped the engine up a little and lowered the car down some, i put great emphasis on lowering the car so that it kept my rear axles totally at the same level as the rear wheels and the diff. --- Iv always known how to get the most power to the wheels and this was just one of the many little tricks that made my little bimmer perform well for the little changes I did on the power plant itself (which was basically just high compression Colbenshmitt (sp) pistons, some head porting and letting it breath both intake and exhaust)
Things were great for a little while and then i started trashing the needle bearings in the rear axle U-joints (the car had a triple yoke inner CV by the diff. and U-joint outers by the wheels)
At first i kept wondering why, Like -- God they dont even have a tough life, their not even moving except over the ocasional bump --- then "bing" thats when the lightbulb went off - yeah dummy, there not even moving --- I made some plain bearing cups that were greasable and they lasted till I put the bimmer on its roof (for the second time)
Believe it or not those little cars had oversteer, if you see one next time look at the front wheel - its right in back of the F. bumper, then look at the rear wheels, they are way foreward, plus I had my battery in the back right above the rear wheels --- this helped in Colo. winter driving but also helped the bimmer swap ends a couple times when i had to finally lift on a progressive mountain turn...

Evan
08-12-2006, 12:23 PM
helped the bimmer swap ends a couple times

I just hate it when that happens. :D

I used to spend a fair amount of time road racing with my friends in the Berkeley hills. There is/was a road over the hills into Orinda that was hardly ever used by anyone since the Caldecott tunnel was built and it was perfect for racing, no traffic at all. One day I was well in front driving an MGB. Came around a blind turn that I knew exactly how fast I could take and was met by the road covered in loose gravel that had rattled off the hill side. There was absolutely nothing I could do except go along for the ride. Amazingly all that happened was that I did about two complete 360s down the center of the road and didn't go over the cliff side. I must have had several horsehoes stuffed up my butt back then because I got away with a lot of stuff that should have killed me.

A.K. Boomer
08-12-2006, 01:09 PM
I too cringe when i think about the past, first time I rolled my bimmer my friend was with me and he flew out the side window, all he got was a scratch on his shoulder and he was pissed because he ripped his brand new shirt, I lost it at 70mph around a turn but clung to the road for a long long time going backwards, it was surreal watching all the tire smoke roll off the front of the car while traveling backwards --- I took all four continentals down to the steel belts, we scrubbed allot of speed before the car went off the road and rolled over, when it came to a stop i remember that Denny was no longer in the car, I sat there upside down afraid to call out his name, then after what seemed like an eternity one of us did and the other replied (cant remember who broke the silence) I remember undoing my seatbelt after that (had one of those quick release levers) and i almost broke my neck because it was holding me in the air upside down, Denny obviosly wasnt wearing his, Five minutes later we flipped the BMW over and drove home, years later he re-payed me by almost getting me killed in his trans-am... Pair of dorks...

rsr911
08-12-2006, 01:35 PM
Thanks guys, the more I think about this the more I like the Teflon idea.

I had that argument about the bearings in a rear driveshaft with a mechanic friend recently. I'm helping my brother in law doa ground up on a TVR 2500M and we are installing a Ford 2.3 turbo, T5 trans, and the center section out of a IRS Mustang SVT. I've got the driveshaft properly phased but my friend suggested I just run the driveline straight. This is going to be a track car so the engine, tranny, and center section are mounted solid to the frame so the drive shaft will not move, just rotate. I told him I angled it for several reasons but the big one was keeping the bearings moving so they wouldn't pound into the races. In the end he conceded that I had a point.

It didn't occur to me that this might be a problem on a carb. With Teflon I can run tight clearance and probably get the best seal. I still want to investigate putting a bearing out board of the linkage end to reduce deflection of the shaft. Both carbs are worn at the linkage end and not worn on the other cylinders. When I do make my throttle bodies I plan to put the linkage arm between two cylinders just for this reason.

Evan, the inherant dangers of street racing I've been there. I've got a very fun increasing radius curve on my way home from work everyday. It's a oneway ramp like a freeway off ramp and I used to hit it hard all the time. One day I spun my car around 180 after going over a 2' diameter oil slick, I wound up inches from the guard rail with the car that was behind me inches from running up over the front of my car.

A.K. Boomer
08-12-2006, 02:05 PM
"I still want to investigate putting a bearing out board of the linkage end to reduce deflection of the shaft. Both carbs are worn at the linkage end and not worn on the other cylinders. When I do make my throttle bodies I plan to put the linkage arm between two cylinders just for this reason."



Sounds like a winner, just as critical as linkage is your throttle return spring type and positioning...

also you can reduce wear by running a separate return spring to retrieve the throttle cable and pull the gas pedal back up --- this will enable you to go lighter tension springs on the carbs.

LarryinLV
08-13-2006, 10:45 AM
If you can't machine a pocket for your bearing, I'll bet you could press in a "button".

The button stem would provide you with a new bushing and the button head would provide a large enough surface to install the bearing.

Do not make the shoulder from the stem to the button head sharp and creat a stress point,

Happy throttling.

bluegrass
09-08-2006, 08:26 PM
RSR911,

Sorry to get in on the discussion so late. I actually had a set of bushings made of Delrin back in about 1984, for some 2 barrel Solexes on a 912. Unsatisfactory. I don't remember all of the details, but it seems like they were intermittently "sticky". They would feel fine sometimes, and tight other times. We never did figure out exactly why. At first we just used big return springs, but ultimately we had the body sleeved... the Solex had lots of metal.

I've never used the TIN's, but they sure have a following. I am partial to the Weber 40 IDTP 13C-1's I have now. Are you sure it wouldn't be easier to go to Webers? If you really want it to scream, you could always go to the RSR mechanical injection...

Bluegrass

rsr911
09-09-2006, 02:17 AM
Bluegrass,

Currently the best carb available for the 911 are the PMO carbs. PMO took the best features of Webers and Zeniths and combined them into a new carb, sadly they didn't go with the Zenith's accel pump set up with a pump for each throat. Zenith's also have tall booster venturis which were a rare and prized option for Webers and they have a much better float design that won't starve in hard corners. In days of old no performance parts like larger jets and venturis were available for the Zeniths but that is no longer the case, I'm running 36mm venturis in mine with larger jets on a 3.0SC engine. My baseline HP numbers were about 175 at the wheels with about 185 ft/lbs torque. That's without dialing in the carbs or the ignition, I expect to pick up at least another 5-10 HP at the wheels. After that I'm limited by the factory CIS pistons which don't work well with carbs and aren't clearanced for bigger cams. Cams, pistons and headers should put me over 220 at the wheels with a proper tuneup and still remain streetable. At this point the only thing I'd consider swapping the Zenith's for would be IR throttle bodies and EFI. RSR MFI would be wicked but I'm no millionaire LOL. A properly tuned Porsche MFI setup is damn hard to beat from a throttle response and power standpoint.

Thanks for the tip on the Delrin, I've got the whole winter to decide. ;)

John Stevenson
09-09-2006, 02:44 AM
Has anyone ever studied the cross section drawings of the Porsche cylinder head with valve gear and the 1949 onwards British AJS 7R motorcycle ?

Amazing how they are SO similar that even some parts interchange :rolleyes:

.

Peter S
09-09-2006, 09:05 AM
rsr911, I wouldn't get too carried away trying to make a really snug fit, a bit of an air leak doesn't matter, a stuck throttle does.
Teflon doesn't sound right, not usually used for bushing is it? I think I would go for bronze myself.
Line boring doesn't sound practical to me on such a small diameter, how about setting it up as John S suggests, but in a dividing head or similar so you can bore one side then index accurately 180*, bore the other, possibly finish to size going right through with reamer?
Never done it, just nit picking...

rsr911
09-09-2006, 11:36 AM
My current temptation is to just ream the carbs and cut new shafts from nitrided stock. All I'd need is 3 flats for the throttle blades and the surfaces on the ends for the linkage. I could do this on my CNC and offer it as a service to fellow enthusiasts. There's zillions of carbs out there without bushings or bearings and it's never a problem until the mileage is real high or the linkage is setup wrong putting funny side loads on the shafts. I haven't measured but I assume the shafts are metric so the simple solution would be the closest larger size in either metric or standard for a new shaft then just ream the bores to fit.