I searched the forum for info on my topic but did not find any good answers. My issue is the number one cylinder on my 25 coupe is backfiring through the carb. I assumed this meant a problem with the intake valve but a compression check comes in at 50 psi. With the plug out I can see both valves opening and closing. What else could possibly cause #1 to backfire through the carb? Your help is greatly appreciated.
It may just be a sticky valve that isn't closing quick enough. This happens from time to time as gunk builds on the stem. I would pour some marvel mystery oil in that cylinder with that valve open and let it soak overnight. It will smoke like crazy when you start it and if it was gunk on the stem it will help to get it off.
I did a valve job this spring, lapped each one, checked the clearance and installed new springs. Engine ran great until recently. Valves are not tight in the guides and I can't imagine that gunk has built up that quick.
Something that is interesting is that I did a compression check with the engine running and got only 20 psi as opposed to 50 psi when hand cranking.
How do you know it is #1?
Bad old gas or even good fresh gas and an old gas tank with residue in it can gum up valves in an instant.
I think Gary is referring to the buildup of deposits left behind by ethanol gas, which can occur from one tankful to another, esp if the gas has gotten a few months old.
You will be time ahead to heed Gary's advice.
Sounds to me that a valve is sticking and it moves slowly in the guide. When you hand crank the engine is moving so slowly that the valve has time to close, but when it is running, the piston comes up on compression stroke before the valve is completely closed. When the spark comes, it fires into the intake manifold. Perhaps some type of solvent mixed in with the gas will help unstick it. You also might have a weak spring compounding the problem. Good luck. If that doesn't work, remove the manifold and put some carburetor cleaner around the valve stem when the valve is up and let it trickle down into the guide as you turn over the crank. If those attempts don't work, you might need to do a valve job.
I have the same problem only it backfires through the #3 exhaust, after trying everything we discovered that the lifter is not moving fast enough so MMM in the gas and drive it more ,it only has about 200 miles on the engine.
Jim- I know it's number 1 because when I short it out the backfire goes away (or am I wrong?). I guess it could be sticking while running but appears ok with the plug out while running. Also, the valve is somewhat loose in the guide.