Hi,
Both carbs of my XK140 FHC are leaking fuel. It’s dripping (see attached pictures)
The needle and seat (controlling the petrol flow are ok). Both floats as well.
What could be the root cause for my leaking carburators?
Thanks,
Harrie
There are cork and fiber seals in there.
You need to replace them all.
Moss, Burlens, SNG et al should have those seals.
The one down inside the lower hex cap that you have already removed is a special size, ID and OD carefully manufactured, so don’t try to substitute something else in there or it will drag on the mixture adjustment screw.
Harrie:
When replacing those cork seals I was advised to soak them in oil overnight prior to fitting them, this from the company who at that time held the S.U. distributorship in my neck of the woods.
Chris.
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I never fully stopped those leaking… despite the carbs being completely rebuilt.
Check the needle seat diaphragms. Mine were brittle and cracking. These things only move when the mixture is adjusted.
Harry has the H6 carbs, no diaphragms to worry about.
It took me a couple of tries to get the cork seals right.
This job is best done on the work bench, not on the car.
I often have a black drop of oil on one of them, at the same spot
Some people including myself have replaced the cork seals with Teflon O-rings. H4s have 1/4" jets, so they need O-rings with a 1/4" (nominal) I.D. The actual I.D. is very slightly smaller. Not sure about H6s. Teflon O-rings will NEVER leak.
Bob Frisby
Boise Idaho
Thanks all for your reply.
Harrie
Hi,
I need to come back on this issue.
After replacement of the cork seals internally and the fibre washer in the main yet cap nut, the carbs are still leaking.
It’s leaking from the inner side of the jet sealing nut.
What more is needed to make them free from leaking?
Did anyone succeed in 2 leaking free H6 carbs?
Thanks,
Harrie
Harrie,
Have a look at the latest replies in this Item. You’re not alone…
Bob K.
I read somewhere, and to be honest I have not tried it-
You are supposed to soak the cork washers in oil for a day, THEN use them.
Feel free to debate and disagree, but it makes sense to me.
I did that several times and it (most of the time) helps with the larger cork washers, but not with the tiny ones at the bottom of the Main Jet Cap Nut.
I also have the impression that the latter cork washer is always completely submerged in fuel and the oil doesn’t really help in that case.
Bob K.
Bob’s right - my cork washers were all fitted by the late David Lonsdale of Classic Carburettors, who told me he always soaks the cork washers in oil. He kept a few in a tray of oil, and when one was used he replaced it with a fresh one, so most of those he fitted had days, if not weeks, soaking. As Bob says, this is no use for the tiny, thin washer in the jet cap nut. And in normal running, assuming properly rebuilt carbs, the petrol dripping into the cap nut will have come down the thread of the jet adjusting nut, not through any seal. If you look at the SU diagrams, assuming the float level is correctly set, there is no pathway down except through that thread, so the cap nut seal is essential.
Many years ago I tried rubber o-rings in my jet cap nuts. They squirmed and interfered with the jet adjusting screw heads and eventually leaked.
So I went back to the fiber washers. They don’t leak.
The key to using fiber washers here is the same as using them anywhere else on the carbs, and that is clean smooth flat contact faces and getting them tight.
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The Viton rings I have used are only 1mm cross section Rob, and a tight fit inside the cap so they need to be gently worked down the female thread. They’re nowhere near the jet adjusting screw, and not squirming anywhere, so seem to be working OK. No hint of fuel on the underside of the carbs.
I once read a story here on the Forum that someone recommended using Teflon tape on the mixture adjustment screw. I tried it once and it works well indeed. So leaking via the mixture screw is a for me a proven fact.
But especially after an engine rebuild you have to adjust these screws (too) often and replacing the Teflon tape everytime is taking a lot of time.
Bob K.
Hi,
I did the following as an experiment:
- first I polished the underside of the Bearing for Jet (Bottum Half).
This to be sure that it is flat for the fibre washer in the cap nut. - I placed an extra tiny rubber ring under the Copper washer for Bottum Bearing.
- Replace the “big” Cork ring sealing (36) by a rubber ring.
After a few rides the carbs are still dry.
These are normal rubber rings (not fuel resistant). So I will replace these by use fuel resistant rings.
Harrie
Check the float levels, make sure the level is just below the jet hole (never had a good answer on how far). If it spills overs the jet bearing
it has to flow down. Trick to check this is to take the dashpots and pistons out and turn on your fuel pump. When it stops clicking you shouldn’t see any fuel between the jet and get bearing forming a pool above the jet. It should be contained with the jet.
That being said, I’ve never been able to completely stop leaking. I think what’s important is it doesn’t leak while running. I get about 5 to 10 drips after driving which I equate to fuel system settlement. Whatever excess was in the jet cant manage its way back to the float bowl.
Just my two cents.
Bill