I have a 48 farmall cub with a j4 magneto. I put points and condenser and did a tune up and it wants to run but won’t kick over. I have good spark and compression. Put a carb on from a cub that is running in case that was the problem. Still fires on one cylinder but won’t run. What do you people think? I’m baffled
1948 farmall cub
- Thread starter Jeffy231
- Start date
ricky racer
501 Club
Did you double check the correct wires were going to the correct spark plugs?
Yes I did. It won’t fire at all unless on idle.
When it did finally run it ran good and then stopped abruptly. Very hard to start. I’m sorta thinking the mag might be worn out. Has a lot of play at the rotor when it clicks.
staninlowerAL
501 Club
A worn rotor and or pinion will have the same effect as changing the timing slightly. Make sure that the rotor and pinion gear are correctly timed by the marks. Did you set the points gap to .013? JMHO Stan
Put new cap rotor wires plugs points set at 13 and condenser. It started very hard before all this and after this tune up it didn’t change. I’m going to check compression.
Hi,
Here is info for doing a compression test, if you need it.
Open the throttle fully to do the compression test, so the engine can get air.
Have all 4 spark plugs out of the engine at the same time.
Charge the battery fully before the test, and use the starter.
Charge a new battery also, they are not always fully charged, a low output charger is better, 6 amps or so, that automatically goes down as the battery charges. It may take several hours to charge.
The Cub service manual says a Cub engine before Cub serial number 192113 has 107 lbs of compression, when turning at 240 RPM. That is probably for a new engine.
Some less is ok, they have said on here 90 lbs or above is ok.
I would test the power of the spark the magneto is making.
Be sure the Cub doesn't roll while doing the spark test.
Remove the ignition wire from the center post on the magneto cap, and hold the wire by the insulation, with the metal end about 1/4" from bare metal of the engine block.
Be sure the transmission is in neutral, turn the ignition switch to on, and run the starter.
The spark should jump the 1/4" gap, and be blue or white. A yellow spark is weak, and needs improving.
The engine may not run, or run right, with a yellow spark.
Do the test in a building, or not in bright light, so you can see the spark well.
The battery should be fully charged for the spark test. Charge the battery if necessary.
They have said on here that some new points have a clear coating on them, power won't go through the coating, it has to be removed by filing the points. Wipe off all filing with a clean rag.
The magneto timing is important, the engine can start hard, or not have as much power as it should if the timing is wrong. Below is a page from the 1949 Cub owner's manual telling how to time the magneto.
There are pictures of the points and condenser, in the Cub owner's manual.
The coil may have quit, or partially quit, the spark it makes can be weak.
It is sort of common for the old original Cub coils to quit. Some of them work when cold, then quit when warm, then work again when the engine is cold.
TM Tractor Parts has a new magneto coil, below is the listing, you can look at the pictures.
http://www.tmtractor.com/new/el/713fp.htm
Below are their listings for a new rotor, and rotor drive gear, you can look at the pics of the gear teeth.
http://www.tmtractor.com/new/el/294fp.htm
Here is info for doing a compression test, if you need it.
Open the throttle fully to do the compression test, so the engine can get air.
Have all 4 spark plugs out of the engine at the same time.
Charge the battery fully before the test, and use the starter.
Charge a new battery also, they are not always fully charged, a low output charger is better, 6 amps or so, that automatically goes down as the battery charges. It may take several hours to charge.
The Cub service manual says a Cub engine before Cub serial number 192113 has 107 lbs of compression, when turning at 240 RPM. That is probably for a new engine.
Some less is ok, they have said on here 90 lbs or above is ok.
I would test the power of the spark the magneto is making.
Be sure the Cub doesn't roll while doing the spark test.
Remove the ignition wire from the center post on the magneto cap, and hold the wire by the insulation, with the metal end about 1/4" from bare metal of the engine block.
Be sure the transmission is in neutral, turn the ignition switch to on, and run the starter.
The spark should jump the 1/4" gap, and be blue or white. A yellow spark is weak, and needs improving.
The engine may not run, or run right, with a yellow spark.
Do the test in a building, or not in bright light, so you can see the spark well.
The battery should be fully charged for the spark test. Charge the battery if necessary.
They have said on here that some new points have a clear coating on them, power won't go through the coating, it has to be removed by filing the points. Wipe off all filing with a clean rag.
The magneto timing is important, the engine can start hard, or not have as much power as it should if the timing is wrong. Below is a page from the 1949 Cub owner's manual telling how to time the magneto.
There are pictures of the points and condenser, in the Cub owner's manual.
The coil may have quit, or partially quit, the spark it makes can be weak.
It is sort of common for the old original Cub coils to quit. Some of them work when cold, then quit when warm, then work again when the engine is cold.
TM Tractor Parts has a new magneto coil, below is the listing, you can look at the pictures.
http://www.tmtractor.com/new/el/713fp.htm
Below are their listings for a new rotor, and rotor drive gear, you can look at the pics of the gear teeth.
http://www.tmtractor.com/new/el/294fp.htm
Last edited:
Ok I’ll try this thanks
staninlowerAL
501 Club
Best to check the valve adjustment before doing the compression check to get the best results. JMHO Stan
k hutchins
501 Club
Definitely check the spark. Everything you're describing sounds like either a timing issue or a bad coil.
JohnMihevic
Well-known member
I finally solved my intermittent starting/running problem on my 1948 Cub. It was a bad coil. Ordered a new one from TM Tractor and my Cub runs like brand new.
At first I would check the spark by removing #1 spark plug and looking at the spark when I passed TDC on #1 Cylinder. I would get a spark every time. I took the Mag off to check it on the bench. I connected a spark plug to the Hi voltage output of the Mag and the spark plug threads to the Mag case for a path to ground for the Hi voltage. Then, I manually rotated the Mag impulse coupling. What I found was I would get a spark every 2 or 3 "clicks" of the coupling -- the HV coil was intermittent and would not produce a spark at each "trigger" of the impulse coupling. Replacing with a new coil produced a spark at every "trigger" of the coupling, as it should.
At first I would check the spark by removing #1 spark plug and looking at the spark when I passed TDC on #1 Cylinder. I would get a spark every time. I took the Mag off to check it on the bench. I connected a spark plug to the Hi voltage output of the Mag and the spark plug threads to the Mag case for a path to ground for the Hi voltage. Then, I manually rotated the Mag impulse coupling. What I found was I would get a spark every 2 or 3 "clicks" of the coupling -- the HV coil was intermittent and would not produce a spark at each "trigger" of the impulse coupling. Replacing with a new coil produced a spark at every "trigger" of the coupling, as it should.