The Ford Barn - View Single Post - Backfiring

Flathead Fever · 01-21-2023, 09:42 PM

I'm not a Model A Mechanic but I was modern mechanic for 30-years. Engine basics are all the same. The most common thing that goes wrong after a car has been sitting for a long time is fuel problems. You say you are pretty sure that's not the problem. Ignitions don't usually go bad while sitting but it could happen. For some reason flathead style engines are more prone to their valves sticking open, while sitting in that open position for long periods of time than overheads are. I did this every day for 30-years and never saw an overhead with a sticking valve unless a spring broke. One clue can be a loud valve train noise from the excessive gap. I have a '32 V8 roadster that was my dad's in the garage that sat for 15-years and now it has a stuck valve. It ran fine when it was parked and now it runs horrible, like it has all its plug wires crossed. About all I've done was give it new fuel and start it. I'm pretty sure from the single loud lifter noise that an intake valve is stuck open. If it was an exhaust valve it would only effect one cylinder. It runs way too bad to be one cylinder. With a stuck intake valve what happens is as the piston comes up on the compression stroke it forces that mixture back into the intake through that stuck open intake valve and it disrupts the flow to the other cylinders. Then the plug for that cylinder fires and it can ignite the outgoing mixture and the flame travels through the intake and backfires out the carb. An extremely lean condition can also cause a carb backfire as can timing that is off. It's unusual to have both an intake and exhaust backfire. Usually, an exhaust backfire is from excessive fuel and oxygen being ignited by the exhaust. Like if a cylinder did not fire and then the mixture was forced out the exhaust manifold and lit by the exhaust flame from another cylinder. I think the first thing I would be doing is a compression check. Most people are not going to have the large Model A spark plug adapter for a compression gauge. You can place each cylinder on top dead center and take a blow gun and a rag to make a seal and force air into each cylinder. If you hear it coming out the carb, an intake valve is stuck open or burned, and it comes out the exhaust it's the exhaust valve. You can also pull all four plugs and crank it while sticking something over the plug hole with a little pressure and see if the compression blows it back. If doesn't there no compression in that cylinder. Do the tests that cost anything.

01-21-2023, 09:42 PM	#13
Flathead Fever Senior Member Join Date: May 2010 Location: Yucaipa, CA Posts: 1,101	Re: Backfiring - getting frustrated I'm not a Model A Mechanic but I was modern mechanic for 30-years. Engine basics are all the same. The most common thing that goes wrong after a car has been sitting for a long time is fuel problems. You say you are pretty sure that's not the problem. Ignitions don't usually go bad while sitting but it could happen. For some reason flathead style engines are more prone to their valves sticking open, while sitting in that open position for long periods of time than overheads are. I did this every day for 30-years and never saw an overhead with a sticking valve unless a spring broke. One clue can be a loud valve train noise from the excessive gap. I have a '32 V8 roadster that was my dad's in the garage that sat for 15-years and now it has a stuck valve. It ran fine when it was parked and now it runs horrible, like it has all its plug wires crossed. About all I've done was give it new fuel and start it. I'm pretty sure from the single loud lifter noise that an intake valve is stuck open. If it was an exhaust valve it would only effect one cylinder. It runs way too bad to be one cylinder. With a stuck intake valve what happens is as the piston comes up on the compression stroke it forces that mixture back into the intake through that stuck open intake valve and it disrupts the flow to the other cylinders. Then the plug for that cylinder fires and it can ignite the outgoing mixture and the flame travels through the intake and backfires out the carb. An extremely lean condition can also cause a carb backfire as can timing that is off. It's unusual to have both an intake and exhaust backfire. Usually, an exhaust backfire is from excessive fuel and oxygen being ignited by the exhaust. Like if a cylinder did not fire and then the mixture was forced out the exhaust manifold and lit by the exhaust flame from another cylinder. I think the first thing I would be doing is a compression check. Most people are not going to have the large Model A spark plug adapter for a compression gauge. You can place each cylinder on top dead center and take a blow gun and a rag to make a seal and force air into each cylinder. If you hear it coming out the carb, an intake valve is stuck open or burned, and it comes out the exhaust it's the exhaust valve. You can also pull all four plugs and crank it while sticking something over the plug hole with a little pressure and see if the compression blows it back. If doesn't there no compression in that cylinder. Do the tests that cost anything.