!8.5 hp Briggs backfires when throttle is open
This is my first post and I hope this is the right place, so if not please suggest where. I am a diesel mechanic but this small engine stuff is different and I am stuck. Thanks in advance for any help.
My 18.5 hp Inteck Briggs 31P777 0293E1 03 112ZD
is backfiring through the carburetor if I open the throttle any at all. The engine is in good solid shape and has no wear to it. It will start up ok and idles smooth. If you open the throttle the engine does not speed up it instead will start a loud backfiring out of the carburetor. You would think that the intake valve is sticking but after disassembling the head and going through it, itÃ¯Â¿Â½s not the head. The tappets are not sticking and it is not that the valve clearance is wrong. It is not the ACR opening the valve during the compression stroke. The flywheel key is not sheered and the flywheel is correct with the crank. In short everything it should be causing this is not what is causing this but the engine is real sick. I have worked quite hard at checking the above mentioned things but am open to second guessing if anyone has any ideas.
Details: Engine was running fine and instantly developed this condition while at full power, there is fire and burned fuel in what is backfiring from the carburetor so there has to be spark, fuel and an open intake valve to explain this. I am concluding that somehow the plug is firing during the intake stroke rather than the valve is open at the wrong time.
Does any one have any ideas on this? Has any one ever heard of a coil causing spark at the wrong time?
Thanks , Bruce Buchanan