View Full Version : Small engine wont run!

04-18-2006, 06:30 PM
Alright, so i'm the president of a high mileage vehicle club at my school and have been put in charge of the engine, drivetrain etc because i have alot of expierence (compared to other students my age that is) with small engines. Now we've got about a week until the competition and we can't get the four cycle engine to run off of choke. It's a 2.4 hp B&S overhead valve engine. It has a mikuni carb. It ran early this year but has been sitting in a classroom where other kids could concieveably mess with it. One student in the club, before the teacher passed the "only tom is allowed to touch the engine" rule, removed both rocker arms for some unknown reason. (he's alittle add sometimes :) ) I reassembled it and called the factory for specs - .006 gap. I carefully gapped them at TDC. Tried to start it and the engine wont run off of choke unless its at high rpm. All three automotive teachers made me repeat the gapping proccess for them so they could make sure i was doing it right. No problems there. Based on the fact it would run off choke at high rpm (3300-4000) i decided to check for a vacuum leak. Once again, no probs there. Completely cleaned the carb, checked all the jets etc - all fine. Riched the mixture screw as much as possible (only about a half turn). Still no luck. When you give it full throttle via the throttle control cable, the govenor goes crazy trying to maintain the rpm. Generally i see this when an engine has compression problems so we checked compression. Once again first pull produced 35psi (well with in factory specs) and develops a max psi of about 65. It holds even at 65 psi. Changed spark plugs. I'm out of ideas short of a complete rebuild...i must be missing something but our small engines teacher couldnt think of anything either...help!! :)

04-18-2006, 06:43 PM
Just like in the other thread, an engine that won't run with the choke off is either starving for fuel or it's getting too much air. Check the fuel supply, carb and it's connection to the engine.

04-18-2006, 06:44 PM
Dribble some fuel in the intake when it is running. If that improves it then you have a fuel delivery problem. You can also just squirt a bit of raw gas in the intake and try starting it with no choke. If it runs a couple of seconds and then dies again, you have a fuel problem.

04-18-2006, 06:50 PM
Check for vaccume leaks with the engine runnin with a can of ether. Fire it where the carb-manifold-manifold-head connections. If engine behaivor changes...you're sucking air the carb doesn't know about. The others are correct for testing for lack of fuel.

04-18-2006, 07:03 PM
Thanks for all of the quick replies!!

No fuel delivery problem - on choke it starts to foul the spark plug and it will actually puddle slighty around the choke valve (normal for most small engines running on choke)

Checked for vacuum leaks exactly the way you described, a can of ether sprayed all around the intake and phenolic spacer. No problems there - held the same rpm with in a range of 8. No sound changes either.

BTW i realize its either starving for fuel or has too much air, or more concisely the mixture is no good, but i can't figure out for the life of me why it is. Its almost like the carb is atomizing correctly - that's why i checked for vacuum/compression problems. A bad compression causes a drop in vacuum and prevents the carb from atomizing the fuel correctly - but the vaccum and compression seem normal to excellent.

04-18-2006, 07:16 PM
A B&S will run with practically no compression if the mixture is right. I would guess you still have a plugged passage or jet in the carb. That is by far the most common problem if the engine has sat for a time with *any* fuel in it. The fact it will choke to death means nothing. Also, (I hate to say it) but did you drain the tank and put in fresh fuel?

04-18-2006, 07:22 PM
A B&S will run with practically no compression if the mixture is right. I would guess you still have a plugged passage or jet in the carb. That is by far the most common problem if the engine has sat for a time with *any* fuel in it. The fact it will choke to death means nothing. Also, (I hate to say it) but did you drain the tank and put in fresh fuel?

Yup! New fuel and cleaned the carb with carb cleaner - removed all jets and soaked them then sprayed them. I have found four B&S engines so far that didnt work because of compression - although these were drastically damaged. Two (a flat and an overhead) would run only at high rpm and the govenor would go crazy. The flat head had been run dry by some idiot <whince> i get these as people throw them away generally, and the other i was paid to fix. Turns out the overhead just need the intake valve readjusted to run properly. (it also would only run at high rpm or on full choke) The other two plumb didnt work - one had a piece of wire that was lifting the exhaust valve during compression and combustion stroke and the other had a warped valve stem that prevented it from closing all the way. Like i said these were all pretty drastic but i feel justified in checking compression since the behavior of the govenor suggested poor compression and fit the symptoms of the earlier overhead i worked on.

Oh - it doesnt normally choke to death - while it is running it still puddles. If its running full throttle it is possible to kill it on full choke but on idle it runs on full choke. (I don't mean to be offensive or anything - i sound like i'm just shooting down everybody's suggestions and i dont mean to. Its just that i have already checked it and it still doesnt work :( exasperating !!)

04-18-2006, 07:39 PM
It sounds like the idle and/or low speed circuit in the carb is plugged. These are only .006 to .010 and can get clogged very easily. Drop the bowl on the carb and see if there's any dirt in it. You said you "cleaned" the carb with carb cleaner. Was this a dip or just spray?

When you clean the carb, you should back flush the venturi circuits. Blowing through the jet drillings will just lodge the dirt deeper. Don't poke wires through the openings either. That will enlarge them.

04-18-2006, 07:40 PM
Back to 'did you try squirting in a bit of fuel' while it is running?

04-18-2006, 09:20 PM
The float bowl was clean when i took it off and i removed all of the jets and soaked them in carb cleaner and then carefully blew them out - they all seemed clean and clear

I removed the carb and soaked it in some carb cleaner that was actually intended to be used from a spray can. After a brief time soaking, and after removing the float bowl gasket, i sprayed it with a can of carb cleaner and then reinstalled the jets. It was not, however, a real carb dip. When i sprayed the carb, I shot cleaner in from the opposite side (the side that bolts to the engine); would this be considered back flushing it?

Evan - firstly i really appreciate all of your help! second we did, in fact, try squirting fuel into the carb. It would fire once but not really run unless the choke was on. Thats what has be baffled - when you run it on choke, you can take the carb off directly afterward and there is liquid fuel puddled in the phenolic spacer/intake manifold before the intake port. Even though the manifold is getting plenty of fuel it doesnt seem to be atomizing correctly. It seems like it needs more vacuum/air velocity to properly atomize the fuel as its drawin into the carb. Does that seem right?

04-18-2006, 09:46 PM
Is the throttle flap position set right and when you looked at the mixture screws did they come out to a needle point or were they blunted?

Reason I ask is sometimes someone will crank down on the mixture screws with a screwdriver and the tips will blunt or even break off.The flap positon will sometimes make the carb draw fuel only when the choke flap is closed(max vaccum)

Duct Taper
04-18-2006, 09:54 PM
Maybe bent, sticking or burned valve, sticking lifter, bad head gasket that allows compression but lets vacuum leak in, bad condenser, oil on the points... ?

04-18-2006, 10:37 PM
I think you need to set that rock arm lash with the lifter on the base circle of each lobe, not just TDC. You also should do that warm if possible.

04-18-2006, 10:38 PM
Wierdscience - what do you mean by throttle flap position? On this particular carb i dont think you can toy with that any - but i'm all ears! The mixture screws can't actually come all the way out - it has a stop molded on it for max leaness and richness. Jets are in good condition however and the passage that the needle of the mix screw was cleaned thoroughly. Thanks though - that sounds like something that would happen around here (somebody cranking down on the screw way to hard).

The engine has solid state magneeto ignition so no probs w/ condenser or contact points although there could possibly be a burned valve but i would think that would show on compression test. I'll have to see about the head gasket...the lifters all seem to be working fine when turned by hand - but at a higher rpm i have no idea - i guess they might flex and then bind or stick?

JKilroy- I had it durring the compression stroke at TDC so, in my mind anyway, it would be at the lowest point and it would seem to me that that point would be essentially the base circle for the cam since it doesnt vary until the lobe begins...i'll check it again though

04-18-2006, 11:37 PM
Miserable things...never tell ya whats wrong with themselves. When ya only got 1 hole, it's harder to figure the problem.

Quick and expensive would be a new carb. I hate to suggest that but it might save ya some troubleshooting time.

04-19-2006, 12:25 AM
The mixture screws can't actually come all the way out - it has a stop molded on it for max leaness and richness.

Jets are in good condition however and the passage that the needle of the mix screw was cleaned thoroughly.

How did you clean it?

04-19-2006, 12:36 AM
You know, I just thought of something else. It could be that the float valve is not sealing and actually flooding the engine. I know this sounds strange but I've seen in old cars and tractors. That could be the reason it runs at high rpm and not low. I did a carb for a neighbor a while back that had that problem. His tractor would start then go right to high speed then die. Fuel would then dribble out of the intake. A carb rebuild with a new float valve fixed it.

Check the float--Make sure it does indeed float (Use the warm water test on brass and hollow plastic floats). And check the float valve for a ring or hardened seal.

04-19-2006, 01:43 PM
I was going to suggest the float and float valve also. Seen it happen many many times. Ken beat me to it tho.

J Tiers
04-20-2006, 12:42 AM
I do not know the mikuni carb, so this may be inappropriate...

When an engine won't run except on choke, there is a problem with fuel delivery, no question about it. MIGHT be too much fuel, I suppose, but usually it is too little on my stuff (B&S, Tecumseh, Kohler, power products, etc, newer and older).

Many of these have carbs with extra passages and drilled jets, that get clogged. Then there is nothing for it but to pull out the welch plugs and wash out. Your jet removing on that one might do the same thing.

So, the choke does two things.... it increases vacuum, and it increases fuel flow, as a result of vacuum directly, due to choke plate position relative to small jets, or due to a throttle interconnection that positions the throttle plate relative to jets. The jet position is for high air velocity.

If it won't run off choke, but does run with choke, then only with choke is the fuel flow and atomization correct.

Does the carb have a ball valve in teh main jet? Lauson carbs do, and it likes to get cemented by goo. Washing out fixes if immediately tried, but delay means remaining goo sticks the ball down, and it won't run without choke.

Could the carb have been disassembled and put back together wrong? Like throttle or choke wrongly assembled?

Float at wrong height so that normal operation does not suck in enough fuel, but choke does? There is a height adjustment, and they often really mean it.

04-20-2006, 02:44 AM
Tell me what kind of Mikuni carb this is. If this is a CV carb you are missing the boat here. So I need to know if it is a Mikuni flat slide or a CV...(the one with the big diaphragm on the top). You say choke...if you are saying choke...then it better have a butterfly closing the carb throat. If it doesnt then it is a mixture enrichment circuit that opens a passage that dumps bowl fuel into the throat.. Perhaps you could post a picture so I can see it. Depending on what kind it is there are tons of things you should be looking for. I am assuming this is some sort of after market application carb ...in that I don't ever recall seeing a Briggs with anything but a Briggs carb on it. As for the mixture screw ..you have to get it out. It doesn't matter if there is a stop involved...remove the stop if you have to. You cannot clean a carb with out removing that screw. 99.99 times out of 100..that passage just below the idle mix screw gets glued shut with varnish. That passage can be as small as .020-.030...and you can't clean it with the screw in it.


Norman Atkinson
04-20-2006, 04:16 AM
Dirt somewhere or


I bought an ancient old SV Reliant Rebel car- in a fit of madness.
Built up the new engine, you name it. Would start on a fresh prime but died immediately. Finally, after finally, I tried to ignite some of the fuel with a lighter. It would barely burn. Drained the tank- and voila- no problem.

You might have petrol oil mixture. Start again with fresh fuel.


J. Randall
04-21-2006, 02:04 AM
Fasttrack, I am not familar with the OHV Briggs or that particular Mikuni carb, but 2 things that have not been mentioned are restricted exhaust, carbon or loose baffle or timeing. Most small Briggs have aluminun flywheel key and I have seen them deformed enough to affect the way they run . James

04-21-2006, 02:19 AM
in a fit of madness That's the way I felt today when the Craftsman would not fire. No spark and it feels like the brake is on even when it is in neutral. My wife helped me push it back to it's parking spot. Went in the house, had a beer, guess what; it's still in the same condition as it was before the beer. Proves my point, beer does not solve all life's problems; sure makes them seem easier to fix. Tomorrows another day!

04-21-2006, 04:10 AM
Most of my problems with Briggs and Stratton engines have involved that little lightweight spring that is in the throttle linkage. It sounds like Fasttrack has done all the right things but he has missed something. To me this sounds like he has checked something but maybe he underestimated just how small a particle can clog a port. Maybe he didn't appreciate how small "small" is. He could be looking for a particle just a fraction of the size of a grain of salt. I can't wait to hear what he eventually finds.

I remember one time I had a breakdown in a van I was driving. I could barely keep it running enough to pull into a gas station (back in the day when you could find a mechanic in one!) The mechanic hardly said a word, but pulled the air cleaner off and put a blast of air in a certain spot in the venturi. He put the cover back and had me turn the key. It ran perfectly. I was impressed, and he wouldn't accept any payment. That mechanic probably never got rich, but he deserves a place in heaven if there is one.

One more thing. I started my mower last weekend on the same fuel that it had in it last summer. Mine has a rubber primer button. Two presses on that, and one pull of the cord was all it took to begin my summer rote work.

Duct Taper
04-21-2006, 11:58 AM
I think I might have your solution. A friend just had exactly the same problem with his motorcycle with a Mikuni carb, the carb with the pot on the top. That pot is a vacuum chamber that raises the tapered fuel mixture rod. With the throttle fully closed there is no vacuum directed to the pot and the mixture rod stays down. The idle circuit is in operation then. When the throttle butterfly is moved slightly the idle circuit no longer is in operation, vacuum is directed to the pot and the high speed circuit is operational.

After spending a lot of $$$ on new coils, points, etc., all that was wrong was that the throttle cable was too tight. It opened the butterfly just a little. Just enough to cut out the operation of the idle circuit. Without the engine at high speed yet the carb gave too much gas for it to use and it flooded out.

Check the play in your throttle.

04-21-2006, 08:02 PM
Alright so three people here really nailed it right on the head i think - or rather a combination of these things.

First off - thanks everyone for your help!
Secondly - The mikuni carb is actually stock on these new b&s ohv small (and i mean small - 2.4hp) engines. Needless to say i was surprised when we first opened the box a few years ago. Its constructed the same way as the normal b&s carbs are except they have removable jets so you could use them with methanol or other fuels by changining jets. Also the mixture screw actually has a molded plastic top that prevents it from being removed or turned too much - Even called the factory on that one but no luck getting it out.

So today durring a meeting i was still disturbed by this and had come to the conlcusion that it absolutely had to be in the carb (i had already checked timing; as was mentioned, these small engines have a dinky little woodruff key to hold the flywheel on). I sat everybody down and had them say exactly what they did to it. I found one kid who filled it with gas and ran it for a little while. The engine ran out gas and he put it away. Later i was working on the engine and asked for gas but no one could find a can of gas so we went and got fresh stuff - it ends up that kid who put gas in it the first time either put in E85 or two-cylce. We're not sure but it wasn't your normally variety of gas :). A teacher at school got a dip tank and fluid and we dissassembled the carb as completely as possible w/o removing welsch plugs (we figured if we had to do that we would, but try the simple stuff first). We let it soak overnight and then removed it, reassembled everything, added fresh gas and we were good to go!

Now my rant-
For the last three meetings i've messed with this engine. The first thing i asked about was gas but the kid didn't fess up because he was affraid he screwed something up. Because we assumed that it only had good gas, and it had run a few weeks prior, i didn't pay huge attention to the carb. Sure i cleaned the best i could w/o a dip tank but not really the way a carb should be cleaned for like a rebuild or such. In this time i adjusted valves and made essentially a straight pipe. Now what ticks me off is that i got nothing but crap from the teachers. Even the power mechanics teachers had no ideas - you guys proved more helpful than they did. They know what the books say but they dont have any real expierence with engines. One teacher made me adjust the valves literally six times and each time it was exactly with in specs. He made me check it, then he would check, then we'd try it. Then he made me do the whole thing over because he didnt believe i had it at top dead center. Then another teacher wanted me to put the muffler back on, which i did happily enough, but that has no effect on a small engine w/ low compression. I understand that makes a difference in high comp engines - like motorcycle engines - but i've been making straight pipes for small engines since i started building go-karts six years ago. Basically everything i did i had to redo several times and then they still didn't believe me or trust me. I understand they were just trying to help but it's irritating when they don't know jack about them - and dont get me wrong, i listened repsectfully to thier advice and opinions but then when they still wouldn't listen to anything i had to say.....grrr oh well at least it works now! Rant off

Thanks again everybody!