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ET: Engine (Twin) Annoying 289 carb petrol leak- help required


Sakura

Well Known and Active Website User
VOC Member
I think all your problems stem from the the rear float needle not seating properly, or the rear float level being too high. The seat should be as narrow as possible, giving a high seat pressure. Try lapping the needle and seat with Brasso or similar metal polish. Banging in exhaust is from an overly rich mixture firing in the hot exhaust. Once a float chamber starts to flood it does so continuously by siphon action. Try lowering the chamber by rotating the carb.
 

druridge

Well Known and Active Website User
VOC Member
Thanks all.
As I understand it, the front float needle lifts up from below, any lowering in the fuel level will allow fuel from the float chamber back out the bottom (the same way it came in).
It sounds like my carbs are plumbed as standard, I did think the RHS tap would need changed if I altered things and went over to feed the carbs individually . I've looked for exhaust system leaks and cant find any, but I also suspect the mixture is too rich at the bottom, the pilot mixture screws are set to standard but i think these would be better screwed out a tad more.
Rob, I hope this doesnt appear as a thread hi-jack, I thought our problems seemed very similar?
 

Rob H

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VOC Member
Rob, I hope this doesn't appear as a thread hi-jack, I thought our problems seemed very similar?
No worries a related issue. I added a small washer under the clip on the needle to lower the float. On running the bike and switching off the petrol the last 100 yards before stopping the engine seems to have worked. Will try the bike like this for a while and then remove the washer and try again to see if it starts leaking.
 
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SteveO

Active Website User
VOC Member
I had a similar problem, but on the front carb. I tried replacing the needle but this didn't help. I thought I might be in for a new float bowl, however lapping the needle in with Autosol and canting the carb slightly anti clockwise fixed the problem, even if the bike is left on the side stand (taps off of course). However I still have to be careful flooding the carbs for a cold start. Do not tickle the rear carb. Because it is lower than the front, it will fill automatically.
 

druridge

Well Known and Active Website User
VOC Member
Tried to lap a new float needle into the face within the float chamber cap (of the top feed rear 276) today and made a real mess of the needle taper. Although hardly visible (even with a glass) the face has obviously got a bad mark on it; confirmed with impression into plastercene. So a new needle and float chamber cap pair on order from the club.
 

SteveW

Active Website User
VOC Member
Directly from Amal, about my inquires of fuel leaking from my 289 Amals.


Hello Mr Walsh,
Traditionally with these type of carburettors you would switch the fuel off just before you reached your destination.

Only turning the tap again to start, tickling the carburettor raises the fuel lever above that of the overflow on the body. Without the engine running this can take ten minutes to re-set itself albeit by leaking fuel from the overflow above the chamber nut.

The starting procedure should be done with the bike upright on its wheels.

If the bike runs well in all other scenarios then it can only be because of the angle of the bike.



We hope this is of help.



Yes, "…..turn the petrol taps off before reaching your destination".
When on it's left side stand (probably the right, too.) you need to turn off your taps otherwise fuel leaks out the tickler tell-tale hole on the mixing chamber. Good luck.
 

Chris Launders

Well Known and Active Website User
VOC Member
A friend had the constant leaking on a Comet and he asked me to have a look at it, the carb was tilted raising the level too much and it was seeping through the pilot screw threads, after I'd levelled up the carb it was still seeping a tiny amount, I made up a bottom banjo bolt with a clear pipe which I ran up the side of the carb, it needed the float chamber lowering, it's no good getting the level right in the float chamber it needs to be right in relation to the carb.
 

Roslyn

Well Known and Active Website User
VOC Member
I changed the float on the front carb to stop flooding/leaks and put on a bottom feed float chamber on the rear carb and it did work as the top feed always seems to leak as the float needle wears a groove in after a short time and ceases to be an effective seal
 

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