Farmall M running issues?!

[H fanatic]

Hello,

I have a 1946 farmall M and it has a miss to it. It misses on the last two cylinders (3 & 4), most of the time, but will fire sometimes. (I think) I have rebuilt the carb, put in new plugs, plug wires, and points and stuff in the mag. I know I have gas and spark to the cylinders but it doesn't seem to fire on them. What could cause this? But one thing I did notice is that the carb has a super M part number.

Thanks
Nick

 

[Will Sick]

It could be a leak in the intake manifold where i attaches to the head. Spray some WD 40 around the gasket area for 3 and 4 and see if it hits then.

 

[Andy Motteberg]

Make sure that the carburetor to manifold gasket is good and that BOTH bolts are kept tight so it does not leak.

 

[JOSSETTE]

are they switched around???

 

[old]

Have you tried to switch #3 and #4 and see what happens?? If could also be bad bushings in the mag or a bad rotor and or cap
Hobby farm

 

[Daniel Bear Kelley]

Misset valves would cause misfires.

 

[Bob Kerr]

As Will said an intake can cause that. Look close at the intake runner going to number 3-4 cyl for leaks or loseness. Also check it for a small rust hole near where the exhaust wraps around the runner and even inside where the exasut pipe lets out. Carb cleaner or a propane torch with the torch head removed will work also to find leaks. To check the inside part in the exhaust for leaks use brake cleaner and with the manifold removed, spray the inside intake runners completly and see if any wetness appears in the exhaust side. It may just be a small area so you might have to get a small mirror to look down inside. They can get rusty in there from use or from sitting with a plugged up rain drain hole and crud sitting in the bottom of the exhaust part above the carb. Another thing to do and I would do it first, would be pull all the plugs and look at them good. It may just be those 2 plugs flooded and are shorting and that could save a bunch of searching for the leak in the manifold.

 

[Janicholson]

My first guess is a blown head gasket between the cylinders (common ailment) Putting diagnosis first is the best practice. Second guess the firing order, then do a compression check. If the compression is good, then worry about intake leaks. If the compression is miserable and similar on those two cylinders, the gasket is blown. If down but not equal, I would then do a leak down test. Pull the valve cover and Check adjustment first (could be a pushrod or two off). Then put #3 on its TDC compression (both valves closed)and set the trans in High gear with brakes locked to prevent the engine from turning. apply 20 PSI of air to the cylinder with a rubber hose in the spark plug hole to make a easy adapter. Listen to where the air escapes. Oil filler is a hole in a piston, or broken rings. #4 spark plug hole is blown head gasket to #3. Air cleaner is an intake valve, Exhaust stack is an exhaust valve, Bubbles in the radiator is head gasket, or crack to water jacket. Pretty definite indicator of the problem. JimN

 

[dougie94]

Pull the mag off,set the m to tdc number 1and put the rotor on number 1,firing order may be off happened too me,on my c