Farmall Cub - Running but popping through air cleaner

Hotflashjr

Well-known Member
Location
Western MA
Back in the summer I posted my 1948 Cub, new to me, had stopped running and was backfiring. Then I was having timing issues. I ended up getting a rebuilt magneto and got the tractor running this weekend. However, I am low on power and I am getting a back pop through the air intake. I tried adjusting where the magneto was mounted on the engine block to see if that gave enough of a timing change to make a difference, and it didn't really do much. I am trying to decide if I re-do the timing procedure or, if I have a bigger issue such as a valve problem. Anyone have this before?
 
A little marvel mystery oil poured into the combustion chamber with the spark plug removed, can help with stuck valves in a flathead engine.

Ideally if you can "motor" the engine with the fanblade or crank pulley and drive the crankshaft you can get to other points in the cycle where other valves are open and it will do more good where it can run down thru the open valve and lubricate the stem.

Don't want to overdo and hydrolock the engine. Motoring the engine as above will all plugs out typically will blow out excess fluid that would cause a problem.

It does sound like 1 or more intake valves are not seating and the spark is backfiring out thru the intake.
 
Sounds like one or more intake valves sticking partially open. That would be my first guess. Otherwise spark could be jumping from one post to another through a crack in the cap on mag. But I would say that be unlikely since you have a rebuilt mag and all.
 
Well that was my thought was that I might be 180 out. However if I was I would be quite confused. I found a really good youtube video on how to time it and used that with the owners manual to set the timing. One thing I did notice was after installing the mag you rotate the engine to line up the pointer on the front of the block with the notch in the pulley on the crank. You would then rotate the mag until the impulse clicks. My cub catches the impulse right before the notch and pointer line up. I was thinking if I re-timed the engine I would stop before the notch and pointer line up, move the mag to catch the impulse, and see if that makes any difference. I think it would be such a small amount of timing increase or decrease though it wouldn't make a large difference. That's what led me to thinking there is a valve issue.
 
New cap and has no visible cracks but that means nothing this day and age. I kept the old cap and will clean it with some emery cloth and see if that makes a difference.
 
Sounds like he has the problem while running. If it was a 180 degrees off on the timing, it certainly wouldn't run, but may perhaps backfire while trying to start.
 
Pulling the valve cover should give me a good indication of which one is sticking and or broken. Just didn't get to that point yesterday. I had a John Deere LA which was a 2 cylinder upright flat head so I am a bit familiar with the overall design.
 
I agree, likely an intake valve sticking open.

I would try Marvel Mystery Oil in both the crankcase and gas. Sometimes it will loosen up sticky valves.
 
I do not know if it was 180 degrees out of time, but my F-30 did run with the timing in the wrong direction -- clockwise instead of counter-clockwise or vise versa.
Two cylinders were firing at the correct time and two were firing when the exhaust valve was open. It ran but was backfiring.
This might be your problem!
 
Put a quart of ATF in with 5 gal of gas and run it. Good chance you have a sticking valve. Yes you could do MMO but ATF is cheaper and will do the same thing
 
I have an opened gallon jug of ATF at home a buddy dropped off that I never used that is taking up space - Guess I found its use!
 
180 degrees advanced on mag input shaft would be piston down on intake/start of compression. The other way piston would be down on power stroke/start of exhaust. Would like to know how a piston getting fired at bottom of stroke would cause any rotation of engine at all??? Especially if intake or exhaust valve open on every piston that fires. Even if a piston did fire, how would it rotate engine the right direction, and with enough momentom for engine to run. I'm not getting how that would work. Just saying.
 
Something else you might try is this. Put a 1/4 cup of ATF in each cylinder put the plugs back in then with the hand crank turn the engine over. If while doing that it hydro locks then stop and wait an hour or so and then try turning it some more. That will force the ATF to push its way past the valves and rings which in turn will help free up sticking rings and valves. DO NOT use the starter to turn it with the ATF in the cylinders
 
By the way I forgot to say this. By trying to turn it over with ATF in the cylinders doing that may prove you have a sticking valve since if you do some ATF will be pushed into the manifold and then into the carb
 
Forgot to mention this before. Don't be too quick to blame a valve if it appears stuck.I have had several that appeared that way with all the normal checks. It turned out to be a STUCK lifter. They are splash lubricated and once in awhile when a motor sits awhile and a lifter is holding a valve open it may get some rust on the upper part of it and not go down like it should. Before I remove a head any more I pull the side cover behind the manifold and make sure that isn't the problem. I spray the lifters with PB blaster and make sure that both lifters on the bad cylinder are moving all the way up and down and not sticking. These motors don't have much spring pressure so it doesn't take much to make em stick. It's easier than pulling the head and finding out later. You'd have to pull that cover anyhow if you have to remove the valves. Good luck!
 
I looked at a cub low-boy a few months ago. Had a bent valve. It doesn't take too much to pull the side cover and look.

Do a compression test and go from there.
 

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