• Q. 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), 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. •

RAY: Well, we definitely can help you narrow it down to one of two things. 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 little, 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. 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.

• Q. I have a 2007 Toyota Prius. I was hoping you could help me with what I suspect is a cheap sales tactic by my Toyota dealer. The dealer says that if I buy tires from anyone else, I will adversely affect my gas mileage. He says that the Prius takes a special low-rolling-resistance tire that is available ONLY through the dealer. Like you guys, I have been around awhile, and this seems like a big, fat load of you-know-what. What do you think — is this a sales pitch, or is it really the case on the Prius? Of course the dealer wants about twice as much for the special tires as the local tire shop. I love your column and read it first each week! Thanks for your help.

RAY: The 2007 Prius came with either Goodyear Integrity or Bridgestone Turanza tires. Those tires are available at lots of places other than your dealership. We even found them at Sears. So your dealer is — what’s the word? — lying!

TOM: He’s right that the Prius uses low-rolling-resistance tires to improve gas mileage. But there are a lot of low-rolling-resistance tires on the market these days. In fact, some will give you even higher mileage than the original-equipment tires.

RAY: The website Tire Rack tested a bunch of low-rolling-resistance tires in your size (185/65R15). The Michelin Energy Saver A/S, the Bridgestone Ecopia EP100, the Yokohama dB Super E-Spec and the Goodyear Assurance Fuel Max all beat your original tires for mileage (“When Round and Black Becomes Lean and Green” at www.tirerack.com).

TOM: Of course, mileage isn’t the only thing to consider when buying tires. There’s dry handling, wet handling, snow-and-ice traction, comfort and tread life. So, depending on your needs and desires, and the weather conditions where you live, you should look at the research and pick the tire that best balances all of your needs.

RAY: For instance, if you live in Portland, Ore., you may be willing to give up a mile or two per gallon to be able to stop better in the rain.

TOM: If you live in Minnesota, it may be worth a couple of mpg to get better traction in the snow.

RAY: Or, if you live near us, it may be worth a small reduction in mileage to soften up the potholes and reduce the number of welts on your head from bouncing up and hitting the roof.

TOM: But in any case, you’ve got plenty of choices, and plenty of competition for your business. Good luck.

Got a question about cars? Write to Click and Clack in care of this newspaper, or email them by visiting the Car Talk website at www.cartalk.com.