I too have tried a variety of oils in my carbs. My Haynes SU workshop
manual says to "top up the piston damper with thin engine oil grade SAE
20w/50 until the level is .5 in above the top of the hollow piston rod.
On dust -proofed carburettors, identified by no vent hole in the
damper cap, the oil level should be .5 inch below the top of the hollow
piston rod." I think all B equipped SUs are dust proofed.
I just fill the piston rod to the top and when I stick the damper in,
it overflows and is amazingly about .5 inches from the top! I also
have been using pneumatic tool oil. It has additives to keep the tools
clean so I figure the slight overflow might help keep the piston moving
freely up and down in the suction chamber.
The way I read how the SU carb operates ( I imagine the Zenith is the
same) , the thinner the oil, the richer the mixture will be upon
acceleration. Just guessing, I would say that Marvel Mystery oil is
the thinest followed by ATF and pneumatic tool oil. I think ATF is
about 7.5 to 35 weight. The newer 5w/30 engine oils might work very
I'm always running into people who say the used to own an MG but they
could never get it to run right. I had one guy tell me he adjusted and
adjusted the carbs and never could get them right. The manual says not
to fiddle with the carbs. My guess is people would never put oil in
their carbs.....any oil is better than none at all!
50 MGTD, 59 MGA, 65 MGB