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MG MGA - S.U. lifting pin

Hello people;
During the procedure for mixture adjustment on the S.U.s, they recommend pushing up on the lifting pin under the carbs 8mm and noting the results in rpm decrease/increase, etc, etc.
Unfortunately, I don't have a micrometer grafted to my finger, so would it be reasonable to assume that the 8 mm is the travel that the piston rises when the pin is pushed all the way up? (The pin doesn't come into contact with the piston until the pin itself has been pushed about halfway up.)
Thanks.
L.R. deOlazarra

Should be no more than 1/16" and that is the amount the piston is lifted with the pin. That works out at about 1.6mm
Neil McGurk

My WSM says .8mm, which is 1/32. You can go to 1/16 as Neil says, but that's more than enough.

FRM
FR Millmore

Neil, FRM;
Thanks for the input, and pointing out the missing decimal point.
Still looks like lifting the piston the .8mm or 1/32 inch is a best guess affair. It's odd that they didn't design the lifting pin to raise the piston the specified amount when pushed all the way up; seems like it would take the guesswork out of it. Maybe, like you said, Neil, the 1/16" lift is within the upper limit to do the testing. Well, I know more now than when I started. Thanks. :)
L.R. deOlazarra

It's worse than that, since the piston does not in fact sit hard against the bridge when it is down. There is a small gap set by a brass or plastic pin on the piston bottom to ensure that it is never fully closed, but I don't remember what the protuberance is. I can tell you that if the pin is missing the vacuum will suck the piston hard against the bridge, causing it to not rise at all = no run; I had this happen on a car with the plastic pins that had been melted in a carb fire.
As an easy reference, you can stick a piece of 1/16 wire in under the piston (at the side so you do not interfere with airflow past the needle), which will give you about the correct total gap. You'll be a lot closer than most people, who raise the piston as far as the pin allows (which may not be a constant across carbs).

FRM
FR Millmore

This thread was discussed between 21/01/2011 and 22/01/2011

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