300 lawn runs 20 mins then backfires and dies

[coloradotrout]

It will start right up, runs at least 10 minutes or longer, then starts sputtering, maybe backfires, and eventually dies. It won't start for awhile. I let it sit overnight.

Replaced plug, no change.

Has spark. Does not seem to have gas. At least the plug was dry when I took it out.

Any ideas?

 

[Yogi05]

Fuel cap not venting? Creating a vacuum in the tank starving it for fuel?

 

[Thomas]

Had somewhat the same problem,I added fresh gas and some carb cleaner after couple of mowing work fine also change fuel filter.

 

[coloradotrout]

This is the old 300 hydrostat "lawn" tractor. From the 70s I think. Kohler one cylinder engine. I see & "feel" spark at both ends of the plug so pretty sure it's fuel related.

Not sure if it has a fuel filter or not. Would the carb bowl have 15-20 minutes of fuel?

 

[tomplum]

No, the bowl will supply a minute or so of fuel. This should have the mechanical fuel pump which can cause this condition. Seeing as though there could also be another delivery problem, use my donor can trick and run a few feet of 1/4 " fuel line off the pump into a donor can and see if the problem persists.

 

[Midniteoyl]

It will start right up, runs at least 10 minutes or longer, then starts sputtering, maybe backfires, and eventually dies. It won't start for awhile. I let it sit overnight.

Replaced plug, no change.

Has spark. Does not seem to have gas. At least the plug was dry when I took it out.

Any ideas?

This is the old 300 hydrostat "lawn" tractor. From the 70s I think. Kohler one cylinder engine. I see & "feel" spark at both ends of the plug so pretty sure it's fuel related.

Not sure if it has a fuel filter or not. Would the carb bowl have 15-20 minutes of fuel?

Had a 70's 16hp B&S doing this, pretty much exact same thing. Turned out the exhaust valve seat in the head was loose (they were press fit then *maybe* staked). Once hot, it would leak causing sputtering and eventually dying, just as yours is. Letting it sit overnight lets it cool enough to work until hot again.

 

[Carl_NH]

Agree with midniteoyl, valve clearance can cause this too - easy to check and adjust.

Also if you suspect its fuel related, its easy to remove the air filter and with a small spray bottle of gas, spritz into the carb throat if it starts/runs like this then it could be the carb float or the fuel pump.

 

[coloradotrout]

On a side note, any suggestions on how to deal with backfires? We slow it to idle for a few seconds, but it still likes to backfire.

Also at idle this beast shakes pretty good. Thoughts on speeding up idle?

Im still chasing the stutter and die after 20 minutes, but figure now is the time to look at the other things too.

 

[tomplum]

Kohler used to say (back in the day when you actually went to update) to try a different throttle position to help lessen the backfire. There is no reason to idle an air cooled engine that low. Even if all you did was bring up the base idle, it would be better. Plus this carb is adjustable. You can likely still get the service manual from Kohler online as free download should you want it.

 

[Carl_NH]

On a side note, any suggestions on how to deal with backfires? We slow it to idle for a few seconds, but it still likes to backfire.

Also at idle this beast shakes pretty good. Thoughts on speeding up idle?

Im still chasing the stutter and die after 20 minutes, but figure now is the time to look at the other things too.

Unburned fuel causes backfire due to incorrect fuel mixture settings and or valve adjustment. But as Tomplum states air cooled machines should not be idled low as it affects cooling.