Farmall Cub

Well my cub won't start. Thought it was a carburetor issue but have now ruled that out. My cub is a 1948 and has a magneto. I think I have an electrical problem. Or could be that my valve clearances need adjusting. I am just guessing. I am considering rebuilding the magneto and checking the timing in the process. I am also thinking about checking the valve clearances. The coil is creating a spark cause i tested it by grounding it. Not sure how strong however. I changed the plugs and have rebuilt the carburetor. no change. engine turns over and wants to catch but never really starts good. My question is should I do the magneto rebuild before the valve clearances or should I start with the valves? I am open to suggestions on other possible fixes as well. As a side note the tractor has for years been loosing power as it heats up it becomes quite weak after running for 45 minutes or so. even worse in the heat. Advice would be greatly appreciated.

I'd go with valves first. Valves probably were not touched, since Eisenhower was president!
Ed

First is the valve tappet adjustment. That's a not cost procedure.

Spark. You are looking for a nice blue spark about 1/4" long.

Timing light, check the ignition timing while cranking the engine over.

My guess, you magneto's coil and/or condenser is failing.

Hi,
The spark needs to be strong and blue when you check it. A yellow spark is weak, and the engine may not run.
The ignition point surfaces should be flat and in good condition. They get old and burned from use.
Below is a listing for new points and condenser at TM Tractor, you can look at the pics.

http://www.tmtractor.com/new/el/367fp.htm

It is common for the coils in Cub magnetos to quit. Some of them work right when the engine is cold, then quit working when the engine gets warmed up, after 20 minutes or so.
Some of them work again when the engine cools down.

Below is a listing for a new magneto coil.
There is a rectangular bar that goes through the coil, keep it and reuse it with the new coil, if you replace the coil.

http://www.tmtractor.com/new/el/713fp.htm

Below are pages from the 1949 Cub operator's manual, showing the magneto, and how to get to the points.

http://farmallcub.com/rudi_cub/www.clea ... age-30.jpg

http://farmallcub.com/rudi_cub/www.clea ... age-31.jpg

While you have the plate that holds the distributor cap off, you could check the timing marks for the rotor and it's drive gear. You remove the 2 screws and the cover holding the rotor on, to get to the gears.
Clean the area under the cover, and grease the gears while you are there.
Below is a pic of under the cover.

Eugene wrote: Spark. You are looking for a nice blue spark about 1/4" long.

From where to where? Spark plug, spark plug wire, coil wire? I would assume from one of those to a good grounding point. Have a similar problem.
I don't think the spark plug would give a 1/4" spark but I don't know what I don't know, better to ask. Thanks Walter

Hi,
The 1/4" long spark means it is able to jump that far from the end of a spark plug wire, to a bare place on the engine.
It should be strong and blue colored.
It should look strong and blue jumping the electrode in a spark plug also, when the spark plug is out of the engine, and the bottom or side of it grounded to bare metal on the engine.

Thanks Glenn that's what I thought. I'm just tired of making so many mistakes. Thanks again. Walter

You said it loses power when it heats up, or itxs hot outside. Did you have trouble starting it after it stalled? Had to wait for it to cool down before it would start?
I'm with Eugene., l believe your coil has gotten weak. I would change that before l got into valves.