View Full Version : SU Carbs

03-07-2012, 06:58 AM
So...my 60 TR3A is progressing nicely, except for a puzzling problem. We removed, cleaned, and rebuilt both SU carbs. Adjusted, and the engine runs fine. There is a persistent leak from both jets. Initially, only the back carb leaked and we decided to try it again, replacing all washers, etc with a fresh rebuild kit. On the original back carb, the prior owner had broken the lever attachment site...we fashioned a fix. With the second rebuild, we found a local TR3 enthusiast with a spare carb body and replaced it as well. Drip, drip, drip... Any suggestions??

03-07-2012, 07:09 AM
Al - are you saying the jets leak at the bottom, where they enter the carb base?

Sounds like the jet seals (cork? rubber?) aren't tight - or were tightened way too much.

Where did you buy the new seals for the rebuild?


03-07-2012, 07:39 AM
Did you replace the spring that loads the jet seals? Although not included in the rebuild kits, they are a fairly critical item that I feel should be replaced, especially if you don't know the carbs history.

Are the little cupped washers between the seals & springs installed properly?

Which kind of seals?

Were the jets replaced? I have seen poorly made replacements where the jet itself would leak. And it seems that all of the replacements are rougher than they should be (dull) in the seal area. Good idea to polish them before installation; ordinary toothpaste and your fingers will do.

03-07-2012, 08:01 AM
Did you soak the cork seals in oil before installing them?

03-07-2012, 09:38 AM
Thanks for all the responses.
Yes, the leak is from the carb base.
The cork seals were not soaked before installation.
The kit was from VB. Did not replace jets, or the spring that loads the jet seals. Cupped washers installed correctly.

03-07-2012, 09:41 AM
The corks are supposed to be soaked for 24 hours in oil before installation, so that may be your trouble.

This assumes that float heights are correct and needles in float chambers are sealing.


03-07-2012, 09:48 AM
Thanks...that could be it. Sounds like new cork and spring may be in the SUs future.

03-07-2012, 12:29 PM
The corks are supposed to be soaked for 24 hours in oil before installation, so that may be your trouble.

Just curious, do you have a reference for that? Doesn't seem to be mentioned in the workshop manual, and I've never done it.

OTOH, I was just going to mention that the rubber O-ring seals from Moss seem to last longer, so maybe the oil would help (in spite of the fact that the seals are sitting in gasoline all the time, which I think would wash away the oil fairly quickly).

03-07-2012, 04:51 PM
I've just rebuilt two sets of carbs. No leaks.

According to a notice that comes with the SU jet rebuild kits supplied by Moss, the corks should be soaked for 24 hours before installation. The reason for this is to make them more pliable. If you put them in dry, there is a chance of breaking or cracking them. Moreover, its possible that they may not settle properly into the little brass cupped washers. I certainly would recommend that you redo that part of the rebuild and soak the corks in 3-in-1 oil overnight. If that doesn't work, then try other solutions.

Randall is absolutely correct that the gas would eventually wash away the oil, but the reason for the oil is to make the corks initially pliable so they don't split or break on you.

One other observation. The corks that are supplied these days, according the SU manual, are thicker than they used to be, and this sometimes presents problems.

And one more thought: be sure that the overflow pipe from the float bowl is clear. Sometimes, if you soak carb components in chemical cleaner while rebuilding them, these little pipes get clogged and plugged. The plug creates pressure that can cause leaks in the jet area.

Just some thoughts...

Just a thought...

03-07-2012, 05:34 PM
If you put them in dry, there is a chance of breaking or cracking them. Moreover, its possible that they may not settle properly into the little brass cupped washers.

I see. Thanks, Ed.

My experience (some 20 years of driving almost every day) was that the cork seals would only last a year or two, then start leaking again. I don't recall ever having them leak immediately, but I did get tired of having to redo them so often. So when Moss came out with the rubber O-ring seals, I jumped right on those. They lasted a lot longer; actually outlasted the car (which got wrecked in 2005 I think it was).

03-07-2012, 05:40 PM

Good advice. I put in cork seals that came with the rebuild kit, but I have a supply of those Moss rubber 0-rings and will substitute them when, and if, the cork seals begin to leak.

03-07-2012, 06:57 PM
Hey Al,
Soaking the cork seals in oil overnight is to make it so the jets will slide up and down when you pull the choke cable. I think they should seal OK without the oil and you likely have a different problem. Are you sure the gas isn't coming out from around the bowl-to-body seal and dripping down? Good luck. Pat

03-08-2012, 06:19 AM
No, the bowl to body seal is dry. Definitely a jet source of leakage. Appreciate all the input.

03-08-2012, 08:53 AM
I will second Randall's suggestion, I have tried both the cork and the O-rings, getting a good seal is finicky, I have had more luck with the O-rings.

Also doesn't hurt to carefully clean and polish the jet surface that the o-ring contacts, I have heard people suggest jewelers rouge, but I think any old polisng or rubbing compound works fine, smooth surface for better seal and less wear.

03-08-2012, 10:48 AM
Thanks. I like the O-ring approach and have checked the Moss catalogue, referred to as the leak-free option to the cork in that location.