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I have a B&S Vanguard V-Twin 23hp motor. It's mounted in my carpet cleaning machine. The other day after I was finishing with a job, I went out to shut it off and noticed that the choke was pulled all the way out. I'm not sure how long it had been running like that. Maybe an hour or so. I realized that the nut was loose and the vibration from the mower had pushed the choke out. When I got home, I tightened the nut and fixed that issue. When I started my machine, it would not start (it was still very hot from the job I was just at). I checked the spark plugs and they had turned grey/white. I went and bought new plugs and installed them. When I tried to start the machine again, it fired up immediately (it cooled down by then). I figured that I had just solved the whole problem. Today, after my carpet cleaning job, I went to start my machine after running it for 2 straight hours, and it wouldn't start. The plugs were grey/white again. Something's causing it to not start after when the motor is hot. It still starts fine when cold. Does anyone have any ideas on what the problem could be? My nearest B&S mechanic is 45 minutes away and would cost me 2-5 days of down time. I need to try and fix this issue myself if I can.

Is the plug firing when its not starting. A bad coil will not work when hot sometimes. When you first shut the engine down the temp of the coil will actually rise BC the air is no longer circulating around it from the flywheel fins.

RDA.Lawns said:

Is the plug firing when its not starting. A bad coil will not work when hot sometimes. When you first shut the engine down the temp of the coil will actually rise BC the air is no longer circulating around it from the flywheel fins.

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Yes, the plug is firing when it won't start.

Something odd about the spark plug color with the choke full on. You should have seen black roiling smoke, missing and dark sooty plugs when you discovered the choke problem. The white suggests a lean condition. Maybe an inlet air leak? Anyway spark coils are very sensitive animals once they heat up past their tolerance. The little chips will often work cold, but only intermittent at hot and the spark you see naked eye is not performing in the high compression condition of running engine. It would be good If you had a spare coil to interchange .

motoman said:

Something odd about the spark plug color with the choke full on. You should have seen black roiling smoke, missing and dark sooty plugs when you discovered the choke problem. The white suggests a lean condition. Maybe an inlet air leak? Anyway spark coils are very sensitive animals once they heat up past their tolerance. The little chips will often work cold, but only intermittent at hot and the spark you see naked eye is not performing in the high compression condition of running engine. It would be good If you had a spare coil to interchange .

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Today after I ran my machine for about 2 hours, I was able to start it back up while it was really hot. There was a slight backfire as it started though. Not sure what that means. When I checked the spark plugs, this time the right plug looked normal and the left one looked grey/white. Yesterday, it was the opposite. The right was grey/white and the left looks semi normal. Any ideas?

If it is running that hot, I would be taking off the shroud and make sure that the cooling fins around the cylinders are clean. Small engines mounted on cleaning and polishing equipment will have fins that will become coated with the fine dust generated in that environment.

Get a trigger pack of WD 40 or similar and when running spray liberally around the carb and the manifold.
If the engine changes speed & / or blows white smoke you have an air leak.
The fuel level in the float bowl cold also be a little low and the fuel flow might be a touch on the low side.
Usually the last 2 will cause the engine to surge if the governor is working properly.