Farmall Cub

...my brother has been pulling his hair out trying to figure out why his Cub will run like a champ, cutting grass, for 15-20 minutes, then all of a sudden it dies "like someone turned the switch off". Then, it won't start until it cools down, then runs again well for 15-20 minutes, then dies again. He has changed the points/plugs/condenser/coil/regulator/etc., and has good battery. What could be causing it to die when it is hot?

Could be a plugged vent in the gas cap. But we really need more information. When it dies, you need to check to see if you still have spark to the plugs. If the spark is still nice and hot you need to check the fuel system. If the spark is weak or absent you need to re-check the ignition system. The most common cause of this type of problem is a bad coil. You might also check to assure that your replacement coil was a 12 volt coil and see if it requires an external resistor for operation.

...nope, there is no vacuum in gas tank.
...he replaced his coil, said the clerk told him it had a resistor inside it.

HOWEVER, he said that the coil gets very, very hot....he does not know if there is any internal safeguards to shut it down when it gets so hot.

...he thought he had spark when it died, but when he gets off it to problem solve when it dies, he does not have a means to check to make sure there is a spark. We'll have to work on that.

...thanks for your thoughts.

rla,

My experience has been with my '52, which I converted to 12v and didn't use a resistor before my coil. It would get VERY hot, and then the tractor wouldn't start until it cooled almost all the way down. I have since replaced the coil and put a resistor inline with the power supply to the coil, with no problems since.

There are many parts that just don't function when they get too hot. I think that is one of the reasons ignition coils are filled with oil - to dissipate heat.

One other thing I have done is have the coil hooked up with the wrong polarity, which has a similar effect.

Just a couple of points from my own "duh" moments.

Long shot here, how about a bad radiator which inturn allows the motor to get so hot that Vapor lock occurs

Check your rad , blow some air thru the cooling fins and flush the system

Jackman.... Oooooh! Vapor Lock I have had a similar problem (Cub dies when engine is hot + won't start until it cools down). Gosh, you should be on Car Talk with Klick & Klack!

I'm thinking that, in Lil's case, the starter motor seized because of the engine heat. It's got to the point where we've have to jump-start at the battery-starter wire connection and that seems to un-freeze it. Although my Cub's radiator doesn't look corroded and there are no leaks that I can see, the engine does run hot and I keep having to check the coolant. I wonder if it's losing fluid when I'm running it. On the other hand, I too use it to mow a LOTTA grass and I run it a lot longer than 20 minutes at a time, over thick, lush grass. I stop when the gas runs out.

I got good advice here, which was to take the starter motor to someone and have it checked out to isolate the problem. Fortunately, just down the road from me there is a mechanic who specializes in auto electrical systems and actually advertises for 'antiques & agricultural' What a find, and what a great thing it is to live in a farm-oriented community where somebody like that is in business.

Jocelyn

FarmLadyWannaB wrote: Oooooh! Vapor Lock I have had a similar problem (Cub dies when engine is hot + won't start until it cools down).
Although my Cub's radiator doesn't look corroded and there are no leaks that I can see, the engine does run hot and I keep having to check the coolant. I wonder if it's losing fluid when I'm running it.Jocelyn

Vaper lock. Not on a gravity fed fuel system such as found on most farm tractors including the Cubs.

Coolant. The cooling system does not have a water pump. The water circulates by thermosyphon. If you fill the radiator to the top, as the engine warms and the water expands - excess water is expelled out of the overflow.

If you can see water above the core - If you can see water in the radiator - it's ok. I live in hill country. Every so often I have to add a quart to the radiator. It spills out when going down hill - radiator gets lower than the engine.

Engine dies when hot. There are several causes. The most common is a bad ignition coil.

Suggestion. Flush out the cooling system. The thermosyphon cooling system has a tendency to accumulate crud around the bottom of the cylinders in the water jacket and in the lower radiator housing. This crud around the piston cylinders will cause the pistons and cylinders not to dissipate heat.

Eugene

Gotta agree with Eugene , possible bad coil...... After I posted the vapor lock thery I was thinkin hey is that even possible with a gravity feed carb .....

So much for Click and Klack

...thanks to all on your suggestions for my brothers Cub. The suggestion about the hot coil needing a resistor appears to have solved the problem. My brother did replace the coil, but for some reason there was no resistor on the old one (as there may have been on the previous one), or the previous owner took the resistor off, or who knows what. Regardless, he went to town and got a small resistor, put in on with the new coil, now it runs and does not die. He is greatly appreciative of all the suggestions and input, you guys are great.

rla - thanks for the feedback. It really helps to know what the problem was. We're glad he's up and running again.