Help with 22 HP Briggs ELS motor. Backfiring under load.

[dirtyb15]

I am stumped trying to figure out what is going on with my zero turn. Its a 2006 Dixon ram 50, it has a 22HP els Briggs motor. Up until Saturday i have had zero problems with it. This past Saturday I was mowing some overgrown grass and it started to back fire and lose power especially going up hill. It progressively got worse by the time i had finished mowing. First I replaced the fuel filter, no change. Then I changed out the plugs, no change. Next checked the valve clearances, They were all around .006. I adjusted them to spec,at 1/4" past TDC but still no change. Then i pulled the push rods, none were bent. Went through the carb and everything seemed okay. Did a compression check and one cylinder came in around 100, the other 110. Tried unplugging wire on each cylinder when running and each time motor would run noticeably worse. Not sure what to try next. Starts great, idles great and runs pretty good until under load. The exhaust seems a little louder to me, but not positive on that.
Anyone have any ideas? Maybe cam lobes??
Thanks!

 

[Boobala]

Backfire is USUALLY a sign of incorrect timing , have you checked the keyway under the flywheel for slippage, or bent/sheared .

 

[dirtyb15]

Backfire is USUALLY a sign of incorrect timing , have you checked the keyway under the flywheel for slippage, or bent/sheared .

Forgot to put that in my first post. Yes, i pulled the flywheel and the key looked good.

 

[bertsmobile1]

Remove the blower housing and then remove one of the kill wires from the coils.
Start the engine and move the governor arm by hand slowly.
If engine runs fine the diode between the 2 coils is duff.
If no change then repeat with the choke slightly closed
Engine runs fine = low float , slow fuel delivery partially blocked main jet blocked bowl vent.

Don't do either test for too long as the engine will overheat.

 

[dirtyb15]

Remove the blower housing and then remove one of the kill wires from the coils.
Start the engine and move the governor arm by hand slowly.
If engine runs fine the diode between the 2 coils is duff.
If no change then repeat with the choke slightly closed
Engine runs fine = low float , slow fuel delivery partially blocked main jet blocked bowl vent.

Don't do either test for too long as the engine will overheat.

Thanks for the reply. Tried both tests and neither made a difference. The motor actually runs and sound great until its under a good load. (Fast acceleration, or blades on and on incline.)
Any chance the muffler could be plugged up? I thought maybe fuel pump, but since choking it makes it worse, i was thinking it was already running rich.
EDIT: Cleaned out the muffler, did not look bad, made no difference. Pulled plugs and definitely indicate a rich condition.

 

[dirtyb15]

After some help over at tractorbynet finally think i have it narrowed down. The exhaust valve on one of the cylinders is barely moving so looks like i need a new camshaft. Not looking forward to taking the motor out but glad i found the problem. Thanks everyone for the replies!