View Full Version : Help with homemade manometer, please

04-28-2005, 08:58 PM
I'm trying to get the wife's '75 CB400F Honda back on the road, after 17 years of (improper) storage. Rebuilt the carbs last winter, and the engine fired right up. The Clymer manual gives plans for a homemade manometer, using 1/4" poly tubing, some small jars of colored water, and a board to mount it all on. To get a reading, you make an orifice plate about 75% of the carb's throat diameter, with brass tubing to hook to the poly tube up. It is within the throat, and draws a bit of vacuum, sucking water up tube.

For the first attempt, I built it exactly as shown. But the water column only moves about 1/4" total. For the second attempt, I went to 3/16" I.D. tubing, and got maybe 1/2" differential, idle to a handful of throttle. Still not enough sensitivity to balance those four carbs.

Do any of you ladies or gents understand airflows, pressure differentials, and the like? I have exhausted the readily available sizes of tubing from the local hardware emporiums. I can buy smaller stuff from the specialty houses, but it's still a guessing game for me. Some help would definitely help the learning curve along.

Is there a better way to get more 'suck' at the orifice plate? Don't want to starve it for air and screw up the air/gas mixture. Should my sample tube be smaller or larger?

I'm hoping for about 6" travel in the water column so I can balance the four carbs at idle. The carbs are small, about 26mm across the air throat, and I'm at 6,000 feet if that means anything. Commercial four place manometers are available, but spendy. I would still like to get this Rube Goldberg one to work. Any and all enlightenment most appreciated. Thanks, Keith.

J. R. Williams
04-28-2005, 09:51 PM
Where is the pressure sensing point located? Nothing wrong with the 1/4" tube as changing the tube would not change the pressure. How is the orifice plate attached? The hole may be too large and not providing enough pressure drop.


3 Phase Lightbulb
04-28-2005, 10:04 PM
If you go down to a Honda service center, you can buy a 4x carb sync guage for ~$10... It's not worth fussing around with a homemade one (unless you want to of course)..


04-28-2005, 10:44 PM
'to get a reading you build an orifice plate'
I'm not understanding that I guess. My cb400t has a port for each cylinder where the manometer tubes get hooked up to. I think your bike does too, not sure, though. Mine's an '80, so maybe that's something that was added afterwards to these models. I haven't done that, but I watched a bike mechanic (my dad) do that once to my own bike. You do have to be sure there's not vacuum leaks at any of the fittings.
The thing sure ran well after they did this tuneup. I still have the bike almost 25 yrs later, but have had carb problems since I parked it a few times for too long, and didn't use fuel conditioner.

04-28-2005, 11:50 PM
How about this?


04-29-2005, 07:51 AM
When I worked at Honda Don's years ago, we used 4 vacuum gages mounted on a rack. They were all hooked up simultaneously and the carb slides were adjusted to yield the same reading on all four gages. There were small screws in the runners between the carbs and engine that were removed and small tubes threaded into the holes. The gages were then attached to the tubes and adjustments were made.


04-29-2005, 10:54 AM
Having never seen the orifice plate in the manual you are describing it seems that it would be a pain.

If you intakes between the carb and the engine are made of aluminium why not drill and tap for these? http://www.brasparts.com/hose_barbs/hose_barbs/hose_barbs.htm

I don't know if they'd work if the manifolds are made of that hard dense rubber though. I'm sure space is pretty limited between the carb and cylinders so this may not work at all but if it does with some thread sealant and caps you'll have a permanent fix.

[This message has been edited by KENZ (edited 04-29-2005).]

04-29-2005, 11:53 AM
My hint. Adjust the slides physically with the carbs off the bike. Those Keihn slides are a real pain. The pressure needed to loosen the locknut, turn the screw, and retighten the nut causes all kinds of havoc. If you have been trying it you'll know what I mean.

Also set all the mixture screws the same and only use those to balance them.

Also you have to make sure all the valves are set right first.


04-29-2005, 12:22 PM
The one I use has an adjustable venturi.
You just adjust it downward decreasing the amount of air that can flow, this causes the vacumm to rise and then it will work properly. I have also used a stethoscope to
hear the difference in airflow which works well. The idea Chris has should work perfect.
Goal is to have uniform vacumm throughout the manifold causing equal cylinder charging so that each cylinder is not fighting the other.

04-29-2005, 11:40 PM
Daryl, if I can find a $10 4x carb sync I'll be happy as a pig in slops. So far, no luck. It doesn't help that our local Honda dealer is also the Harley dealer; if they're not selling you a $20,000 motorbike or a $20 t-shirt they won't give you the time of day. I'll try some of the other shops in town. If I strike out, I'll try the yardstick balancer; only two carbs at a time will require a few extra steps but I like the price. Thanks to all for your
responses, I know a bit more now and am ready to go back at it!

04-30-2005, 01:27 AM
slant the tubes (so one leg is above the other). This makes any differential more easily seen/measured. Called a slant-tube manometer http://bbs.homeshopmachinist.net//smile.gif
Rgds, Lin

04-30-2005, 06:01 AM
Try this.

04-30-2005, 06:05 AM
or this.

04-30-2005, 11:39 AM
In instrumentation, I just used a loop of 1/4" tubing about 24" tall. Ruler schedule marks between them. Colored koolaid in them. Outside loop open to air.

You do that with 4 of them and you'd have a 4 port guage. When you hook the vacuum to them you'll see the differential from one loop to the other side. That is inches vacuum.

I used a wally-box (Wallace & Tierman) on my last multi-carb setup. The old Honda 750's had a adjustment on top of the carb slide rod. Do the new ones?

Anything you rig up is not going to be as nice as one you can purchase.

I kinda remember Indian Larry setting up dual carbs on a Harley using a tube to "listen" to the whistle inside the venturi of the carbs. I think he was one of the "real" people on the biker shows on tele. Not to put up a false front for apperance.

Have you ever used a screwdriver or plastic tube to listen to a engine? Harleys with thier ball bearing bottom ends sound like a siren.

I have some real good books that explain instrumentation in a manner a layman can understand. No way to scan them thou. I'd be really happy for someone to scan them into PDF.

04-30-2005, 11:54 AM
Ohh and whoever said that a "angled" loop gives more "resolution" is correct, but no longer a proper way to calibrate in inches/h20 until you calibrate the angle meter with something of known value.
It will move further up a incline than straight up vertical inches.

I have a couple of old H2O guages around here somewheres, I plan on using them on my Smelter thou.

I always wondered why bubbler guages are not used with Oil level on autos. You hook a known air pressure up to a bubbler tube and tee back to your manometer. This is how I used to do Sewage levels too. This allows remote mounting of the level guage/switch. A rotameter (like the one on a mig gas) would be used in the air-feed, then you'd just adjust up the pressure till you had forward flow any excess flow would escape out the weighted tube in the bottom of the channel.

(then something passed through bending a 3/4" guide pipe over) Must've been a alligator or?