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Re: carb oil

To: "Mike Duvall" <>, <>
Subject: Re: carb oil
From: "Lawrie Alexander" <>
Date: Fri, 28 May 2004 07:14:24 -0700
I believe you have it the wrong way round, Mike. The thinner the oil, the
leaner the mixture. The faster the piston can rise, the more air can be
inhaled; the slower the piston rises, the more fuel is pulled out of the jet
as less air can be pulled through the less-open venturi.

That's why adding oil to an empty dashpot cures that lean misfire/stumble on

British Sportscar Center
----- Original Message ----- 
From: "Mike Duvall" <>
To: <>
Sent: Thursday, May 27, 2004 11:28 PM
Subject: carb oil

> 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
> well.
> 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!
> Mike
> 50 MGTD, 59 MGA, 65 MGB

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