Small Engines (Lawn Mowers, etc.): ENGINE BACKFIRING THROUGH CARB, compression stroke, knoweledge

Question
Michael,

I have a three year old well maintained lawn tractor with a 17hp OHV single cylinder B&S engine.

While driving it up the ramp into the shed it stalled. After a long crank the engine backfired through the carburetor. Two more attempts resulted in the dog running under the deck.

I removed the air cleaner and put my hand over the carb intake and cranked the engine. There was the normal suction pressure and then there was backpressure (as if the intake valve was open during the compression stroke).

I removed the carb and intake manifold and the valve cover.

The valves articulate and when cranked there is noticeable intake suction and them positive pressure coming out of the intake port.

What happened? It's not often that the intake valve warps since it's in the closed position during the combustion process. Is it the timing? How is the camshaft driven? Chain or gear?

I have a fair amount of engine knoweledge and before I tear the head off the engine I would appreciate your thoughts on the situation.

- David VW


Answer
Hi David,

I would say that either the intake valve is leaking or it is closing late.  You could check the clearance and test the leak with a compression test.  I'm not sure how your cam is driven,       it could be chain or gear.

Let me know how you make out.

Good Luck,
Michael