One Possible Fix for SU Leaks (Bowl and Dashpot)

by Don Locker

The jet tube to the bowl fitting uses a nut that compresses a seal (like an O-ring, but not quite) into the female part on the bowl. The first thing to do is to loosen the jet tube nut and remove the nut and tube from the bowl. If you are lucky, the O-rings will all come out with the tube. I've never been lucky, though. (And note I said O-rings -- the reason you have a leak, I suspect, is that there are old O-rings left behind from a previous repair that never were removed, preventing any new O-ring from finding a sealing surface.) So, using a small crochet hook and flashlight, search and probe in the bowl fitting for all O-rings and O-ring parts, clean the inside of the fitting, and refit a new O-ring. It should seal easily if the surfaces are clean and the O-ring is good. If the carb dashpot has started losing its oil, obviously it is leaking past the o-ring that seals the needle adjusting screw. Below is the advice from Charlie Brown cb1500@erols.com for replacement.

Q: The Zenith-Stromberg carb on my '79 Spitfire will not retain its oil inside the dashpot (or more precisely, the "air valve guide rod"). If I lift the air valve it just pumps the oil into my intake.

I would like to replace the o-ring over the metering needle adjusting screw. But I can't figure out how to remove the retaining clip(s) without possibly damaging them. A picture of what I am referring to is on page 15-4 of the Haynes Carb manual.

Can anyone help me? The last sentence of the above-referenced page says "The [needle adjusting] screw is not removable."

A: Take out the piston, remove the needle, and using a 1/4" wooden dowel [or unsharpened wooden pencil], tap the adjuster back up the bore and out the top. Be careful not to lose the star washer that will come out first. It's small, and probably near impossible to replace (just a guess).

Replace the O-ring, reinsert the adjuster in the top of the bore, set the star washer back on top and drive the whole assembly back down the bore--the adjuster will keep the star washer square to the bore.

Doug Braun also performed this work on his carb and had this comment:

I got the o-ring and the retaining clip from TRF. They were about dollar apiece.

The O-ring size is:

Outer dia: 9.5 mm
Inner dia: 6.0 mm
Thickness: 1.75 mm

I recommend replacing the retaining clip. If it is weak, the needle holder can push it up, and then the needle will move up and down in the piston. Also, beware that if you over-adjust the needle downwards (lean), it can force the retaining clip up out of position.

Also make sure that you clean out any crud from the place where the O-ring and needle holder go.

I got the clip out with a dentist's pick, with a tiny hook bent in the end. The job took about 5 minutes, once I removed the piston from the carb. The leak went away, and the car runs better now that the needle is not loose.