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Ok, I'm going to give as much information as I can, but unfortunately the details will be somewhat limited. The back story is that my grandfather's tractor died last year, so my dad put a motor on it from an identical model tractor that ran fine when parked (deck rusted away, and tractor generally rusty and in poor condition from sitting outside). I don't know specific model numbers of either the tractors or the engines, but they are the Home Depot variety John Deere Sabres with Briggs and Stratton engines (either 38" or 42" cut), probably 10 years old or so. Again, I apologize for limited information - I live about 2 hours away from my parents and grandfather.

Since switching the engines, the tractor runs fine at an idle - I mean perfectly smooth - and will drive around the yard fine, but after mowing for a few minutes (maybe 5-10 minutes typically), it will start to kind of surge/sputter and want to shut off. Sometimes it can be saved by putting it in neutral, disengaging the blades, and playing with the throttle to keep it running, but other times it shuts off completely. Usually, even if it is kept running it will continue to sputter/surge even in neutral with no load on it for a while. Sometimes it will straighten itself out after a few minutes, other times as you back the throttle off it will die. If you let it set for a minute it will fire right up and idle perfectly smoothly again.

They've put fresh gas in it, cleaned the carb, switched carbs, cleaned again, and switched back again (cleaning with every switch), and tried 3 different coils (one was bought new). When you pull the gas line off there is good fuel flow so the line/filter isn't clogged (he had tried taking the in-line filter off though, in case that was the problem). When he starts it, he can usually drive around the yard perfectly fine for a while at high speed, but if he turned the blades on, it will start to surge within a few minutes (not an immediate response). Going up hill seems to be worse than level/down hill. They've been tinkering with this thing for several weeks now and just can't figure it out...

Thoughts?

When it starts to sputter, try loosening the fuel cap. Sometimes the caps vent gets plugged with debris. If the engine runs smooth after you loosen the fuel cap, simply replace the cap

chance123 said:

When it starts to sputter, try loosening the fuel cap. Sometimes the caps vent gets plugged with debris. If the engine runs smooth after you loosen the fuel cap, simply replace the cap

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Yeah, we tried that too...

BrokenTiller said:

Yeah, we tried that too...

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If the carb has a solenoid/wire mounted in the bottom of the float bowl, check its operation by turning the key to on and listening/feeling a click from the solenoid. This is a 12 volt solenoid with a plunger that covers the carb pickup tube when the unit is shut off to reduce engine back fire through the exhaust. It is spring loaded and is normally extended to cover the carb pickup tube when the unit is shutdown.
Sometimes these can cause the problems you are experiencing.
Next time it acts up, shut it down and do the above test.
Some folks remove the bowl, remove the plunger, and run w/o it. The back firing can be kept to a minimum by reducing engine speed to an idle before shutting off the engine.
Hope this helps.

Thanks for the suggestions. I talked to my dad again today, and he had already verified that the solenoid on the carb was working properly, and just to make sure he had cut the plunger off so that it couldn't close (since he has another carb, he wasn't worried about it). He finally ended up taking the carb in to a shop and they were able to get a get clean that dad thought he had gotten, but apparently the bit of gunk was more stubborn than him. Has the tractor running great! Thanks again for the suggestions, and I just figured I'd close the loop on this thread in case someone else stumbles upon it through a search later on with a similar problem.

BrokenTiller said:

Thanks for the suggestions. I talked to my dad again today, and he had already verified that the solenoid on the carb was working properly, and just to make sure he had cut the plunger off so that it couldn't close (since he has another carb, he wasn't worried about it). He finally ended up taking the carb in to a shop and they were able to get a get clean that dad thought he had gotten, but apparently the bit of gunk was more stubborn than him. Has the tractor running great! Thanks again for the suggestions, and I just figured I'd close the loop on this thread in case someone else stumbles upon it through a search later on with a similar problem.

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Thanks for the update.

Toro1