Don't you just hate it when your tractor dies in the middle of a field of dry grass, and when you go to investigate, gasoline is gushing over the hot engine? I know I do!
Gas was pouring out of the air cleaner side of the carburetor and out of the the fitting of the bottom of the gas tank as well. Not good!
I took off the gas cap to see what the deal was, and there an enormous "whoosh" and the cap shot up ten feet into the air.
After finding the cap again, I verified that it said "Vented" on it. You couldn't prove it by me. What the heck?
The gas continued to leak out the carburetor after the pressure was relieved, but banging on the carburetor with a wrench recalled it to its duty. The pressure must have jammed the carburetor floats temporarily.
Here's a picture of my tractor during an outbreak of teenagers a while back:
Now I'm down to an annoying slow leak at the outlet of the gas tank, of maybe a drop a minute. The pressure must have distorted the O-ring. This is not an easy part to get at. I may have to take the top cowling off. Grumble, grumble.
So what the heck happened? I thought all vented gas caps were the same -- basically a gas cap with a hole in it to keep the tank from building up any pressure (or vacuum). But unless the cap I had was simply defective, this is clearly not the case.
The cap wasn't specifically recommended for a Ford 600-series tractor, it's just that I noticed that a cap for my 1972 Ford F100 pickup also fit my tractor. I needed a new gas cap because I lost mine and Ford/New Holland no longer carries them. On the theory that all unvented gas caps were the same, I got the vented version of the one for my pickup. This clearly was a mistake.
The smart thing to do would have been to go to Yesterday's Tractors and order the right gas cap. They've got everything, including forums with good advice. Check 'em out. I ordered the right gas cap and that should be that.
So the take-away here is that gas caps contain mysteries that are beyond mortal ken. Buy an exact replacement.
[Later:] I thought I had the gas tank fixed, but it turned out it still had a slow leak. After considerable fiddling around, it turned out to be a leak in the tank itself, rather than at the valve. I have ordered a new gas tank.
[Later still:] This is a serious problem! I've discovered several things:
- Cheap Chinese gas caps sometimes contain parts that dissolve in gasoline! I am not making this up. Buy a name-brand cap, like Stant.
- Even with the recommended Stant gas cap, the tank would over-pressurize and leak. The problem seems to be that putting a gas tank directly above the engine isn't the smartest thing in the world, and the hot engine pressurizes the tank beyond what the gas cap's vent can deal with. In the end, I used the trick I read about on Yesterday's Tractors: there's a spring-loaded plastic button on the inside of the gas cap in the center. That's the vent. Drill a teeny-tiny hole in it. This gives you a non-pressurized gas tank.
Earlier tractors like the Ford N series had non-pressurized tanks, with a dome built into the top of the tank with a pinhole in the top and bottom to vent off gases. I'd rather have a proper spring-loaded vent (since it reduces emissions and minimizes the amount of gas that dribbles out if the tractor turns over), but I have to select something that works over something that doesn't.
The safety issue, by the way, is why old tractors have metal gas lines rather than rubber ones. Because they use gravity feed rather than a fuel pump, turning off the ignition does nothing to stop the flow of gasoline. If you ever feel moved to use a rubber fuel line, you need to put a fuel shutoff solenoid between the tank and the rubber, and have it turn on and off with the ignition. Such shutoffs are available.
P.S. Check out my other tractor pages.