Dear Tom & Ray,
Do you ever address questions about old farm tractors? Our 1945 Farmall-A starts well and runs well for about 10-20 minutes, but then it starts to miss and then stalls out, typically while going up a slope — even a small one. Along with our local mechanic, we checked out the carburetor (the original Schebler) and the fuel lines, put in new plugs and added B-12 Chemtool and STP Lead Substitute additives to the gasoline. The problem will not go away, and we really need this little old tractor to mow our fields and haul logs. Any suggestions? We have a new Zenith-type carburetor on the shelf, but I don't want to put that in unless that's the problem for sure. I will be grateful for any words of wisdom on this. Thanks. — Will
Ray: Well, we definitely can help you narrow it down to one of two things, Will. It's either an ignition problem or a carburetor problem. How did we come to that conclusion? That's about all there is to this engine!
Tom: You might have a classic case of float sink. Carburetors regulate the gasoline flow with a float — just like the one in your toilet tank. Most of them are made of plastic, but yours may be old enough to be made out of copper. Or maybe granite!
Ray: Over time, what happens is that the float develops tiny pinholes in it and becomes porous. And it gets to the point where, basically, the float barely floats!
Tom: Then, when you change the geometry of the tractor — like by heading up a hill — gasoline overwhelms the float and it sinks. That causes the carburetor to deliver more gas than is needed, which floods the engine and stalls it out.
Ray: In that case, that new Zenith carburetor will solve the problem.
Tom: But the stalling is not necessarily related to the geometry of the hills. Old engines often run just fine until you ask them to actually do something — like work.
Ray: In that way, they're very much like my brother.
Tom: So if your spark is weak, it could be strong enough to run the engine at idle, but then as soon as you need to give it gas — to climb a hill, for instance — the gasoline washes over and extinguishes the weak spark, causing the engine to stall.
Ray: So, while you said you've put in new plugs, you really need to put in points and a condenser, too. And you even might want to test the resistance of the coil, to be sure it's still putting out sufficient voltage, because a weak coil can cause the same problem.
Tom: I'd start with the points and condenser. They're cheap and easy. If they don't fix it, ratchet up to testing the coil. If the coil is fine, put in the Zenith carburetor.
Ray: I'm confident one of those things will fix it, Will. But if not, don't write to us again. Remember, the reason you never see questions about old farm tractors in our column is because we don't know anything about them! Good luck, Will.
(Car Talk is a nationally syndicated column by automotive experts (and brothers) Tom and Ray Magliozzi. Write to them at the Car Talk website.)