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MG MGB Technical - dashpot oil

I took Mr. Twist's advice in re. using 90w in the dashpots of my '73B...and wow!...The carbs are
original hif4, although finely refurbished...and are not eating oil anymore...and it runs very nice......

Two things with eating (or rather drinking) oil. First is that HSs have a blind hole drilled for the reservoir so it cannot leak. HIFs have a through hole that is plugged after drilling, so there is the possibility of it leaking. Second I think some diagrams of the oil level are bound to result in high consumption and continual refilling, as it shows it *above* the top of the reservoir. The damper piston never gets this high, as long as it is in oil you have enough. The way to check is to unscrew the damper cap, lift it up, press it back down, and if you can feel the resistance of the oil before the cap reaches the piston cover, you have enough. How much before tells you how much of a 'reserve' you have. I've been doing my HS and HIF carbs like this for 19 years and 14 years respectively - with engine oil - and don't have to top up from one years end to the next. The dampers are only used for the very initial part of acceleration anyway, if you have a damping problem it shows itself as stumbling or flat spots on hard acceleration but not gradual acceleration.
Paul Hunt

The damper also performs a second function: Its purpose is to prevent the pressure fluctuations occurring in the airflow of the incoming fuel-air charge from causing the vacuum piston to rapidly oscillate inside the vacuum chamber (dashpot), playing havoc with accurate fuel metering. The reason that the factory diagrams show the oil level above the hollow damper tube? If you invert the vacuum chamber (dashpot) and examine it carefully, you notice a bushing inside of its neck. The damper tube of the piston is a precision fit inside of this bushing. When the piston rises, the air trapped inside the damper mechanism forces oil downward around the neck of the piston in order to supply lubrication to this bushing. Should the vacuum chamber (dashpot) bushing and/or piston damper tube become badly worn from lack of lubrication, the vacuum chamber will quickly duct the oil above the damper tube into the intake manifold. In addition, air will leak past the vacuum chamber (dashpot) bushing into the vacuum chamber (dashpot), decreasing the pressure differential and thus causing the piston to rise less than it should. An otherwise unexplainable lean running condition will result.
Steve S.

I'm still not sure why Mr. Twist recommends 90W oil in the dashpots...what is the advantage?
Bob Dougherty


I have been using Castrol 20W/50 for engine, and carb dashpots for nearly forty years. I add a few drops to each dashpot perhaps every six months, or before a long trip. I have yet to try Mr. Twist's 90W suggestion, even in summer.

I guess you've already heard the sad news about Roy passing away on the 27th?


Larry C. '74 B/GT & '69 midget

Larry C '69 Midget

Roy Dougherty?
Bob Dougherty


Sorry to interrupt this thread with sad news about a fellow M.G. enthusiast, but I had lost contact with Bob durning a recent move.


Yes, Roy Dougherty passed on the 27th. Last time I saw him was on August 2nd at the Pennypacker Mills after party at Ripperts. This, almost a year to the day when we lost Rocco Grillo. You can contact me off line to discuss further if you like?


Larry C. '74 B/GT & '69 Midget

Larry C '69 Midget

Thanks Larry...sad news indeed...I'll send a card off to Sue...he was a fine gentleman...
Bob Dougherty

This thread was discussed between 08/09/2008 and 11/09/2008

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