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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.

Regs
Rob
 

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

Well Known and Active Website User
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.
 
Last edited by a moderator:

SteveO

Well Known and 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

Well Known and 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
Non-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
 

poor1

Well Known and Active Website User
VOC Member
Comet with brass/bronze carb body which appears to be all original.. Same problem. Not cured by all (three) new fibre washers on the carburettor., including the large on directly i contact with the carburettor body.
Petrol weeps from the very top of the large flange which screws into the bottom of the carburettor body. I gather that the thinking is that the float chamber level could be too high. Fitted a new tap because the tap itself was leaking, so that possible reason is eliminated..

Any updates on a cure please.
 

Bill Thomas

Well Known and Active Website User
VOC Member
Comet with brass/bronze carb body which appears to be all original.. Same problem. Not cured by all (three) new fibre washers on the carburettor., including the large on directly i contact with the carburettor body.
Petrol weeps from the very top of the large flange which screws into the bottom of the carburettor body. I gather that the thinking is that the float chamber level could be too high. Fitted a new tap because the tap itself was leaking, so that possible reason is eliminated..

Any updates on a cure please.
Have you tried twisting the carb on the stub. Just a little to alter the float height.
 

LoneStar

Well Known and Active Website User
VOC Member
If the leak is at the upper edge of the union nut (E on illustration), it's either

- escaping from the opening in the carb body just above that edge
- escaping past the large fiber washer between the union nut and the jet block

The first indicates the fuel level in the float bowl is too high (leaking float valve, tilted carb) as the normal level is below the hole

The second would be a poor seal at the washer, despite it being new. Are the surfaces on which it rides (jet block and union nut) undamaged? As an earlier post suggests, a smear of Hylomar on the washer might help.
 

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nigsey

Well Known and Active Website User
VOC Member
Had the same problem on my twin 276 rear carb, it flooded every time I put the taps on, so on advice I fitted an additional fibre washer between the banjo connection on the float bowl and the Mixing Chamber Union Nut. This lowered the float bowl level by a few though and seemed to cure the problem. Curiously though, the front carb leaks fuel significantly when I tickle it and keeps running until I start the bike whereupon it stops and stays that way even when I stop the bike and leave the fuel on. I think the carbs are a law unto themselves.
 

Bill Thomas

Well Known and Active Website User
VOC Member
We built a Comet for a friend, And rather than put the petrol tank on,
I just left the top off the float chamber and filled it by hand, Just to get it going and check things.

Then we built a Twin for another friend and I tried to do the same !!, Didn't work right this time !.
After a few start ups, I found the feed to the rear carb' would only work if I flooded the front carb',
Because of the way the pipes run, These were the old standard Carb's.

Another thing that has already been said, For "Poor 1 " Best to get a centre stand,
Because when put on the side stand the float is upset !, On Old Standard carb's.
 

poor1

Well Known and Active Website User
VOC Member
I think I've fixed it thanks to all the suggestions received.
A combination of tilting the float chamber and a gentle bedding in of the float valve with silver polish seems to have cured it. I would put my money on the lowering of the float chamber.
It will still weep when the float chamber if flooded but that is to be expected, but it does not weep when the engine is running.
It was a first time start up after a lot of work on the engine for about three years and it fired first time. Need to sort the wiring out now because although the lights are working and the ammeter is showing a discharge when the lights are on and there is no movement of the needle with the engine running so I am not getting a charge, but I'm not surprised because I have rewired it and I may have the polarity the wrong way round.
On the subject of petrol leaks it would not be tolerated in todays motor vehicles. Ask yourself whether you have ever had a fuel leak on your car or modern motor cycle other than on the lines maybe. It's a wonder more of these old bikes have not gone up in smoke.
PS: I do have one of those excellent Dave Hills stainless steel centre stands. Bless his heart he was so helpful..
The only other observation I have on these Vincents is the uncomfortable riding position for anyone six foot tall. The footrests are far too high and my knees are way above the tank and the legs bent at far too acute an angle and no adjustment for height.
 

Bill Thomas

Well Known and Active Website User
VOC Member
Yes I bought a house once, And the old owner said how Warm it is in the Garage !!!,
The House Boiler was in the Garage !, It did worry me , But I always checked things were right,
After parking in there,
This went on for some years, Then a knock on the door !,
The Gas man wanted to check my boiler, It's in the garage I said,
Wrong thing to say !,
The Bastard shut my boiler OFF,
And yet I still see new Houses having Boilers in the Garage, Strange.
 

poor1

Well Known and Active Website User
VOC Member
Lots of gas boilers are installed in garages attached to houses. Not quite the same, but at the Gas Boards insistence when I moved my gas meter, they insisted it should be installed in the garage because it was the nearest point to the outside of the property.
Some of these guys are a law unto themselves. With apologies to any VOC members in this profession.
 

Bill Thomas

Well Known and Active Website User
VOC Member
Lots of gas boilers are installed in garages attached to houses. Not quite the same, but at the Gas Boards insistence when I moved my gas meter, they insisted it should be installed in the garage because it was the nearest point to the outside of the property.
Some of these guys are a law unto themselves. With apologies to any VOC members in this profession.
Funny thing was. He never came back to check.
Cost me a new boiler. Which was put in the house.
I should have just turned it back on.
And took his silly sticker Off.
 

nigsey

Well Known and Active Website User
VOC Member
I think I've fixed it thanks to all the suggestions received.
A combination of tilting the float chamber and a gentle bedding in of the float valve with silver polish seems to have cured it. I would put my money on the lowering of the float chamber.
It will still weep when the float chamber if flooded but that is to be expected, but it does not weep when the engine is running.
It was a first time start up after a lot of work on the engine for about three years and it fired first time. Need to sort the wiring out now because although the lights are working and the ammeter is showing a discharge when the lights are on and there is no movement of the needle with the engine running so I am not getting a charge, but I'm not surprised because I have rewired it and I may have the polarity the wrong way round.
On the subject of petrol leaks it would not be tolerated in todays motor vehicles. Ask yourself whether you have ever had a fuel leak on your car or modern motor cycle other than on the lines maybe. It's a wonder more of these old bikes have not gone up in smoke.
PS: I do have one of those excellent Dave Hills stainless steel centre stands. Bless his heart he was so helpful..
The only other observation I have on these Vincents is the uncomfortable riding position for anyone six foot tall. The footrests are far too high and my knees are way above the tank and the legs bent at far too acute an angle and no adjustment for height.
You won’t hear me complain about the height of the footrests, I’m only 5ft 6” and shrinking and so they’re perfect for me, I just need a stepladder to get on the bike.
 

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