Backfire

I have what should be an easy question for many forum members.

When an engine leans or starves out, why does it fire through the carburetor?

I can’t see any reason that a lean mixture should cause an intake valve to not fully close or ignition or valve timing to suddenly change. So what causes this?

I’ll take a swing at your question. The rough ratio at which the mixture will not burn is about 8 to 1 on the rich side and 16.5 to 1 on the lean side. When the “lights go out” on the lean side you still have raw unburned fuel passing in and out of the cylinder and into the exhaust where the mixture is then ignited on the exhaust manifold. If the mixture happens to light on the exhaust manifold when the intake and exhaust valves are open during overlap, then you get the backfire thru the intake. Lots of things have to happen for this to occur, but very possible.

Generally a backfire through the carburettor is bad ignition timing.

A lean fuel mixture will more likely miss (misfire).

A extremely lean fuel mixture when used over a long period of time, it will cause burnt valves and ruin the valve seats. When the valve seats do not have good seating, there is no heat transfer between the valve and the head. This will cause the exhaust valve to glow red and in turn ignite any income fuel mixture entering the combustion chamber on the intake stroke.

But this is to the extreme when the engine is very hot, I would not consider it if poping or backfiring through the carburettor, I would always go for timing is incorrect.

Another possibility is the inlet valve is not closing, that is when the valve lash is too tight, thus the valve is not seating. Maybe? But in this case the engine generally will not start or run.

The most common reason is distributor is in wrong position, I have even seen Mechanics get this wrong and end up chasing their tail.

Peter :slight_smile: