Leaky Tillotson AJ30A carburetor

[bill-m]

My 1949 Mercury Super 5 is all ready to go except for a fuel system that refuses to stop leaking! I have rebuilt the carb twice, installing new gaskets and a new float. I put the carb on the motor and hooked up the fuel line. I put gas in the tank and without opening the fuel shut-off valve and the gas cap vent, I come back a day later and there is a puddle of gas on the top of the swivel bracket and gas laying the throat of the carb. Fuel is getting past the shut-off valve and the needle and seat. I can get a new needle but the seat is obsolete. I tried lapping the seat with an old needle and I bought an NOS shut-off valve but they both still leak. I have two of these carbs and they both leak. So far, I have bought 4 carb rebuild kits from 3 different sources and they all have at least one missing part. One kit will have a missing part and another source will will have that part but a different part will be missing from that kit! I have to buy 3 kits to get all the parts. I have way more into this motor than it is worth. I am about to buy another rebuild kit and try again. This motor is supposed to be a runner, not a wall hanger. I can use all the help I can get and would appreciate any advise. Thanks.

[garry-in-michigan]

I go to meets with other members.(Mostly Steve Woods) We never have a problem because we add fuel when we are ready to use the motors. Then they are drained when we are finished so they never sit with gas in the tank. That said, it is very rare to have multiple fuel shut-offs leaking like that. On a hot day vapor pressure can over power a carburetor float valve, but a good shut off valve should be able to stop it. . . . . . . 😉

[dave-bernard]

as Garry said, close the shutoff valve off and run it out of gas. have you cleaned and lapped the shut off valve

[billw]

I would concentrate on a good shut off valve. The metal-to-metal needle and seat simply cannot be trusted to hold back fuel indefinetly. Like Dave said, take one of your shut-off valves apart and see if there is corrosion, gunk or debris that you could clean up or lap out. And I, too, always empty the tank when not being used. That way, it can’t leak out in my basement and blow the house up, not to mention forming gunk. I am with you, that the shut off valve SHOULD work, though. The needle and seat just won’t. Not over night.

Long live American manufacturing!

[bill-m]

Thanks for the tips fellas. I will try to lap the shut-off valve but am not sure how to do it. I have lapped the needle and seat but they are easy to do. The seat on the shut-off valve is deep inside and I’m not sure what compound to use. Any suggestions? I was not planning to drain the tank after each use because I was planning to use this motor on a regular basis all summer. At least stopping the fuel at the shut-off valve seems to be the best option. Then, I would only have to be concerned with the fuel in the float bowl. Thanks again!

[Mumbles]

You wont be able to lap the shut-off valve if the stem is threaded.

The best you can do is chuck it up and resurface the taper on the end.

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[billw]

Very true! Maybe you can clean it up, though.

Long live American manufacturing!

[Mumbles]

Some shutoffs I’ve seen don’t have the threaded stem but have a cone shaped piece with a hole in it for the valve. This style could be lapped.

[bill-m]

Thanks for the lapping tips. I can chuck the valve shaft in my drill press as I do not have access to a lathe. You also gave me an idea to find or create an object with a rounded snout and no threads that I can use as a lapping tool along with a slurry of rotten stone and water. THANKS AGAIN!

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