Export thread

I have a 2011 Craftsman 42" rider with a 21hp 540cc Platinum XRD Briggs and Stratton OHV engine. Five minutes after starting mowing with it, I got off to move something out of the way. While I was off the mower, it started running very rough. I turned it off. It wouldn't start again. The next night, I replaced the spark plug. It started but continued running very rough and backfiring. I then drained the gas (even though it was only about 10 days old) and replaced it with fresh. When I started it, it continued running rough and backfiring and the exhaust was nearly black. I let it run thinking it still had some "old" gas in the lines. No luck. After a few minutes, I shut it off. It would not start again. I pulled the plug. It is completely fouled. Any ideas of what has malfunctioned?

Sounds like an electrical problem.
But before we jump put a catch bowl under the carb and drop the float bowl.
You are looking for water. If it looks clean pop the bowl back on.
Next
Check for a spark at the plug . You should use a known good plug or an in line tester.
If no spark
Disconnect the cut out wire from the coil and try again
No spark = duff coil
good spark = safety switch problem.

If you got a good spark with the coil connected then check the flywheel key

I think we may be dealing with a partially closing needle valve in the carb in combination with a fuel pump. The carb is overfilling and trying to flood the engine, hence the black smoke and fouling plug.

remove and clean the carb and see what happens.

Practically same problem. Engine starts but backfires constantly and has significant power loss. I am researching carb solenoid. Could this be the problem?
I did remove the air filter and ran engine. It runs but is like a rapid fire shotgun. This problem started while operating. Everything was just great up to that point. Engine is well maintained and sparkplug has been changed. Problem continues. Due to loss of power, mower is unusable. Any ideas / fixes??

LLB Tractor-SC said:

Practically same problem. Engine starts but backfires constantly and has significant power loss. I am researching carb solenoid. Could this be the problem?
I did remove the air filter and ran engine. It runs but is like a rapid fire shotgun. This problem started while operating. Everything was just great up to that point. Engine is well maintained and sparkplug has been changed. Problem continues. Due to loss of power, mower is unusable. Any ideas / fixes??

Click to expand...

LLB Tractor,
Why don't you start a new thread with your problem? It is hard to follow two different peoples' tractor problems on the same thread. There are people here who will try and help you figure it out. Need make/model engine numbers and such.

Thanks for the suggestions. Carburetor/needle valve check out ok. Flywheel key ok. Mower starts but backfires in a very rapid, nearly rhythmic pattern.....sounds like it is out of time. Any additional ideas?

It very well could be.
whip off the cover & check the flywheel key

Flywheel key checks out with no problem. Anything else that would affect timing?

They can skip a tooth on the cam as most are plastic,
you diagnose that by turning the flywheel slowly and watch for a little "jump" as the cam get to the broken tooth.
Bad valve lash & sticky valves can also give you similar problems as can a faulty plug which misses a few times
so the exhaust get filled up with unburned fuel that burns every now and then.

Look at position of valve guides. All the same height above the head (remove valve covers)?