Mower engine problem

[JohnR66]

I have a 3.5 hp B&S "classic" engine. While mowing, it started sputtering and surging like it was about to run out of gas but it has plenty. It does this in any throttle position. I took off the carb and cleaned the strainer. It had some junk, but wasn't very dirty. After I put the carb back on, it ran fine for 3 minutes then started surging again. One thing I noticed is that it will run smooth seconds before it runs out of gas. Sounds like a carb problem, but I'm not sure.

Thanks for any help. John

[Jeff Wisnia]

You didn't mention checking the air cleaner. Did you? If not, maybe it's gunked up and "choking" the engine? Try running it for a bit with the air cleaner removed.

Jeff

[Rich]

Sounds to me like the gas cap isn't vented correctly either the holes are plugged or you replaced the cap with a non vented cap. What happens is everything is fine for a few minutes then the engine quits you wait five minutes (the vacuum in the tank has equalized by then) and then it will run fine for a few minutes then repeat. Try unscrewing the cap a little so that it doesn't pull a vacuum on the tank to eliminate this as a possible cause.

Good luck, Rich

[Ralph Mowery]

It could be the cap vent is stopped up. Last year a fellow at work had a mower that would run for a short time and quit and repeat. I told him to check the cap. He found he had swapped the cap from the can he filled the mower with the one on the mower tank.. The filling can lid was not vented so it gave the same effect as you describe.

[JohnR66]

Thanks for the replies so far. I failed to mention that I ran it with the air cleaner off and had the same result. After reading the posts about the gas cap, I checked and can see that the holes are not plugged. I should mention that the carburator is the type with the priming bulb. The carburator appears to be molded out of nylon plastic and bolts to the gas tank with 5 screws. John

[ds549]

the rubber gasket between the carb and the tank is called a diaphram. it pumps fuel up to the carb, try putting in a new diaphram and that should fix it. if your primer bulb is the least bit deteriorated replace it also. ..... in some cases, the tube that the carb slides onto gets loose or blows the gasket where it attaches to the engine block, that little gasket cost little but can cause problems if bad... also snug up the head bolts,the ones by the ex tend to loosen... good preventative maintenance is to take off the sheetmetal and clean the grass out of the cooling fins, this is a good time to check the intake tube and gasket... lucas

[ds549]

forgot to say, change the air filter. i dont think your carb is designed to work with the air filter bolt out. lucas

[Gary Dyrkacz]

Although there are other possibilities, you might also check to see if the head gasket is leaking. I had a mower with similar problems and when I happened t o pass my hand past the side of the engine I felt this hot blast coming from the gasket. Replaced it and the engine worked fine. Gary Dyrkacz [e-mail] Radio Control Aircraft/Paintball Physics/Paintball for 40+

[awiecek]

Hi all... Hoping that someone could point me in the right direction.

My parents' Toro gas lawnmower doesn't want to operate.... it will start running but dies within a second. I checked the float in the carb and it works just fine, the needle valve is not stuck and the reservoir has plenty of gas in it. The engine will only start if I prime it about 5 or so times, if I try to start the motor without priming it - it won't budge. I tried different plugs to no avail. However, I noticed that once the motor starts and dies, the plug is wet, which tells me that the mixure is too rich.

Couple of questions at this point:

- what does the prime button do? Does it only fill up the float chamber with gas? Or does it also pump some gas into the engine for quicker starts?

- do lawnmower carbs have an adjustment somewhere where I can reduce the amount of gas sucked into the combustion chamber?

Any help would be greatly appreciated. Alex

[Tony Hwang]

Hi, First thing to check then is air filter. How does the exhaust smoke look? Newer carbs don't have much in adjustment due to emission control standard. Try to run it without air filter for temporary.

[greg6755]

This will probably cost you a big whooping 50 cents but try it.

Replace the rubber fuel line.

It may have a crack in it that opens and draws air after the engine starts.

My mower did the same thing and that was the problem.

Hope this helps.

[awiecek]

Hi Tony... sorry about that, I should have also mentioned that I did run it without the air filter. I can see the throttle valve so I'm assuming air supply is OK.

[awiecek]

I'll give it a try, thanks. I have a spare fuel line handy. However, the float reservoir is full of gas (gas is fed from the top and drawn into the carb from the bottom). Would a cracked line have any effect on this? Also, I'm kinda baffled as to why the spark is wet after the engine dies. It's as if it was getting too much gas in there.

[Oren]

If you loosen the gas cap, will the mower stay running?

Check the vent hole on the gas cap, clear with a paper clip or other small wire.

-- Oren

"The voices in my head may not be real, but they have some good ideas!"

[jacko]

[awiecek]

message

Hey guys... thanks for the replies.

I tried two brand new spark plugs with same effect. Checked for spark

- she's there too. I thought about trying to do it without the gas cap but it was too late as I was already on the way home. I can understand the clogged cap but how would that explain the wet plug though? In any case, anything is worth trying and I'll get at it next weekend. I'll even swap out carbs with a spare one I have here to see if that is causing the problem.

I should have mentioned that the gas in there is over 6 months old (was not stabilized over the winter), but my parents are telling me that it ran ok two weeks ago on the same gas.

[Oren]

message

Did you gap the plug correctly? A minor change in the gap can make a difference.

Plugs can vary. One might burn hotter than another. The right plug and gap are important.

The power stroke is not burning the fuel.

(A bad valve/seat?)

-- Oren

"The voices in my head may not be real, but they have some good ideas!"

[jacko]

[awiecek]

I did not gap the plugs... probably should have. Also, will try the gas.... Quickie question though: what does the prime bulb do? Does it pump the gas into the carb from the float reservoir for quicker starts? Looks like the gas is gravity fed from the tank so that's the only purpose of the prime bulb that I can see. This may explain why the motor will start after about five presses of the bulb and then dies.... it might have something with gas starvation. Hmmmm..

[Stormin Mormon]

I think the mower gap is either 025 or 030. I'd try 025 first.

I've seen mowers wtih water in the gas. They behave just like this. Please try taking off the float bowl from the carb, and see if you've got water.