Vintage Dirt Bike Q & A

I'm a new owner of '74 MT250. My brother had one when I was a kid. I recently saw one for sale and bought it. I've now ridden two motorcycles in my life and they're both MT250's. I paid $2500usd and it had approx 2500 miles. Was that an okay price? It seems to be in good shape except for a gas leak, the headlight and front left turn signal don't work. Anyhow, I'm no mechanical genius so I need help with the gas leak. I've noticed a couple things. When I filled the tank to the top, gas leaks out from the very top, underside of the tank, near the frame. Any ideas? Also, when the bike is sitting, gas leaks out of the vent tube (in front of the rear tire). Any idea why gas would leak out of that tube? I haven't ridden the bike in 3+ weeks (rain) and 3/4 of a gallon has leaked out the vent tube. I could use some help. Thanks

Sounds like you have two problems to me. The first is a pin hole in the underside of the tank. The quick fix, but really only temporary, would be to find the hole, if only a pin hole, clean up the area, and put a little JB Weld on it. That may seal it for quite awhile. The better solution would be to carefully inspect the inside of the tank for rust (MTs had steel tanks, didn't they?) and cure that problem. Rust in the tank, could be causing your second problem. It sounds like what is leaking is an overflow from the carb float bowl. The float needle isn't seating to shut off the flow of fuel to the carb. Could be caused by bits of rust from the tank. The petcock should be shut off when the bike isn't running, to prevent the fuel overflow you are having. But, that isn't a fix.

I'm only speculating here, but it sounds as though you need to clean out and treat your tank, clean and possibly rebuild the petcock, then rebuild your carb. There are some good threads here on this forum with different ways to do that.

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Steve Amling

1976 CR 250M
1980 Yamaha DT100G
1980 XR 200
1981 CSR650

Price was a bit high for having problems like [lights, signals, gas leak]. As long as all the bits are there it shouldn't be to hard to fix. IIRC it should be 6V bulbs, 35 Watt headlight. Standard for Honda was 35 high and low beam with an external resistor for dropping low beam but I don't know where my manual is to double check.

You may have three problems with your gas leak.

1) If you had shut the gas off at the petcock and it still drained, then you need to rebuilt or replace it.

2) The float needle and seat in the carb probably should be replaced. Even with the petcock letting fuel flow, the float set up should have stopped flow. That you see gas coming out of the over flow tube while parked is not good. That it runs OK may mean you can put up with it if you fix the petcock.

3) The gas cap is supposed to be vented and the vent tube runs up to the nut that holds the fork assembly on the frame. If it isn't vented, this is very annoying to say the least. Tank can build up pressure on hot days and squirt out gas where isn't supposed to and pull a vacumn on cold days and shut the engine down from lack of gas.

The JB Weld thing will work but you may want a permanant solution. If you can find a radiator shop or solder yourself, you can make a permanant repair. There are also products like "Kreem" that reline the inside of your tank if it is rusted out.

Rick

...an in-line fuel filter between the petcock and the carb wouldn't be a bad idea.

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

Thanks for the info. I'll investigate and fix her up by next spring. I noticed an error in my original post. I paid $600 not $2500. Thanks again.