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ET: Engine (Twin) Annoying 289 Carburettor Leak

Rob H

Well Known and Active Website User
VOC Member
Just managed to get the shadow together again. All the complicated work seems to have gone OK but I have an annoying leak from the bottom of the rear cylinder 289 carb. It is not from the petrol pipe connection but seems to be coming out from the large retainer which holds all the insides of the carb (internals, needle jet etc.).

i have replaced the large fibre washer but still seems to be leaking. Any ideas on what the problem may be would be much appreciated.

My only thought is that there is a problem with the fuel level but it doesn not seem to be flooding and has not been to pieces since the bike was last used.


chankly bore

Well Known and Active Website User
Non-VOC Member
As I've mentioned before, toothpaste is good for lapping in needles. The brands made for sensitive teeth cure arthritic right legs, but Denture cleaners only work on Mikunis and Dellortos.

Rob H

Well Known and Active Website User
VOC Member
Getting frustrated now, Bike only supported on the Dave Hills centre stand. Put the carb together again and started the bike. Ran it for a couple of minutes to warm up then shut off fuel taps for a short while then stopped the motor. Left it for a about 2 hours and found petrol is still leaking from the bottom of the rear carb but maybe not as much as before.

Another idea - What about putting a thin washer under the clip on the needle to lower the float?

Would lowering the fuel level a little greatly weaken the mixture?

Don't understand why it is leaking as never had a problem with it before!!!!!!

greg brillus

Well Known and Active Website User
VOC Member
Make sure the large washer is the correct size, as this can block off the idle circuit. To align the carb, stand behind the machine and look to see if the bowl is as vertical as you can get it, check the front carb while your at it. The carb's on my Rapide never use to leak/drip, but then I found that the fuel levels were actually too low.


Well Known and Active Website User
VOC Member
I've been going through similar problems on my Rapide with (quite new) 276's. Issues mainly on the rear (lower) carb with petrol from the top of the float chamber and around the base of the float chamber holding bolt (below the main jet). I found a few particles of almost powder size rust in the bottom of both float chambers but this wasn't the problem. Also changed both float needles without improvement. The new ones supplied by the club included a 'wrong' rear (top closing) needle, quickly sorted by the Spares Team - wrong part bagged up by their supplier.
I suspect my problem centres around the carb / fuel lines set-up. On my bike the petrol is supplied from the LHS petrol tap to both pots via petrol pipes to an inverted 'T' plan. The RHS petrol tap is for reserve.
The petrol is fed to the base of the front carb and to the top of the rear carb. Hence the float needles are totally different with the front pot needle sealing at the base of the float chamber and the rear pot needle sealing at the top (or not). With the petrol turned off, there is nothing to stop the fuel in the front/upper float chamber and the contents of the fuel lines running down towards into the rear/lower float chamber other than the seal on that rear/lower float needle. On inspection (after a wait) I found the front float chamber empty, the rear full to overflowing. There is also a problem with the throttle closed on long downhill runs with popping & banging in the exhaust and eventually firing from the exhaust (big bangs); with the throttle reopened the bikes seems to run on one pot until it clears itself. Otherwise the bike is a good starter and ticks over/accelerates fine etc. I am unsure if I now proceed to try to:
(A) produce a mating surface of the needle seat / needle taper with fine paste (I can see a faint mark with a glass on the seat).
(B)Try to locate a needle that works. There used to be needles made with a hardened rubber (?) taper to improve the seal.
(C) Put up with this by managing symptoms rather than fixing the real problem, eg I now turn off the fuel a few hundred yards before stopping and have tried turning off the fuel for periods from the tops of downhill runs. Both plans work, no drips in the garage, and no pops & bangs on descents.
After talking to a few people and thinking about this, I'm still not full of confidence in the needle/seat set-up on the rear (top sealing) carb. The potential sealing face seems quite small and its very easy to imagine with vibration and road bumps etc that the 'seal' must often be allowing fuel to pass. It does seem possible to rig up separate fuel feeds to individual carbs from the Left & Right fuel taps, then the feed on the rear pot could be switched to bottom feed - which might help.
I guess I'm not the first to have this problem, and I havent searched this site for other threads yet.
Any thoughts?

greg brillus

Well Known and Active Website User
VOC Member
The pops/bangs in the exhaust are possible air leaks where the pipe seals in the exhaust port or even the "Y" joint were the two pipes join. Sounds like the mixture at idle is too rich, even one slide may be too low, this will cause the cylinder to draw more fuel and even oil down the guides, thus causing the rough running as the throttle is opened again. A stock twin with 276/289 carb's has a bottom feed front bowl and a top feed rear bowl. The left fuel tap should feed both carb's via a "T" piece and the right tap to a special banjo "T" piece atop the rear float bowl. I don't believe that reserve type taps work on a Vincent due to the shape of the tank, if the other tap with a stand pipe is used, the fuel bellow that pipe will never get used as that fuel cannot balance to the other side of the tank. Cheers.................Greg.

Bill Thomas

Well Known and Active Website User
VOC Member
This has my head spinning, We were trying to get a box of bits going, Some time ago, I didn't want to fit the P/tank, So left the front carb' top off, And filled with petrol, from a can, It worked OK doing this with a Comet !
But with the Twin it only got petrol to the rear, After holding the front float down !, I was not thinking right, But it worked, I was thinking the back pot was not as good as the front, Silly old ---.
So what I am saying is, it sounds like the front needle is not seating right ?.
Sorry , I type as I think, Slow !.
Cheers Bill.

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