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MG MGA - SU Problem?

My 1500 started sputtering yesterday, barely making it up my drive. I left the engine idling and noted the engine was alternating between 800rpm and 1300rpm in 5 second or so cycles. Then I saw the puddle of fuel under the car and immediately shut the engine off. Fuel appeared to be coming from the overflow tube on the rear SU. I removed the lid from the rear carb float chamber and the float looked okay. I reassembled, started the car up and it idled fine and showed no leaks nor dripping. I'm obviously concerned that since I did not fix the problem, it will recur. Anyone have any suggestions as to the likely cause?
Gary Poe

Dirt! Now gone.
Neil McGurk

Gary, I had the similar symptoms on my "A" when I pulled it out this spring. In my case it was a small amount of fuel inside the float in the rear carb. That was enough for fuel to be pumping out the overflow in a steady stream. You would have thought that it was a fire hose. If you have checked the float and you are sure that it is dry, my next question is do you have an in line fuel filter in the fule line ahead of the carbs? The little screen at the bowl intake does not do much. A small bit of crud can cause the metering valve to stick open. Check the metering valves for wear, the plastic ones are known for sticking and make sure that the float lever is correctly set. A 7/16" drill bit can be used as a test rod if you don't have an SU tool kit. (P.S. these kits are not expense and worth having.)
Keith Lowman

Gary - If your MGA had bee sitting awhile since last running, the problem you experienced could be that the fuel in the float bowl evaporated allowing the float to go all the way to the bottom of the bowl, thus allowing the float lever to drop far enough to trap the needle. For information on how to check for and correct the situation can be found in the article, Float Lever Drop in the Other Tech Articles on my web site at: http://homepages.donobi.net/sufuelpumps/

Keith - It sounds like you have exactly the situation described in the above article. Cheers - Dave
David DuBois

Dave, could you also use a spacer under the float to limit the downward travel?
Art Pearse

Could have been dirt, but I had replaced the in-line filter at the fuel pump only a couple of weeks ago. I saw no dirt on the screen at the float bowl, but I suppose that could have been washed away when I took it apart. I drive the car weekly and doubt that evaporation would have caused low fuel in the float bowl. I am going to take another look at the float lever drop as per Dave's article. Thanks for your responses.
Gary Poe

Art - "...could you also use a spacer under the float to limit the downward travel?"

I suppose that would work, but I don't know what other implications one would have to consider when doing that. I went with the method in my article because float drop (or in our case, float lever drop) is a specification given in modern carburetors (which are now obsolete anyway). Also, the float lids have a provision for the later float lever tang to ride against to adjust the drop. I seem to remember seeing something in a later publication that talked to this adjustment, but I don't remember what it was that I saw it in. Cheers - Dave
David DuBois

This thread was discussed between 16/08/2009 and 17/08/2009

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