dangerous fuel leak

[JonathanT]

Zenith 28iF

There is an occasional petrol drip from the throttle spindle on the carb directly onto the intake manifold below. This seems to occur only when the engine has stopped and still hot.

I did have a significant problem with a large quantity of petrol discharging over the manifold the moment the engine stopped. (From the overflow on top of the carb) I had fitted a new float valve. Refitting the original cured the problem. But left with this small drip

Not sure how the petrol can get to this point and wondering how I might effect a cure. Thinking I will have to refinish and reseat the butterfly bearings?

 

[JonathanT]

So how can it leak on the spindle when sitting still and no engine running? Could the float chamber be overflowing enough to discharge fuel down the 'throat' in the carb and then it gets out by capillary action ?
I'm guessing the butterfly spindle must be worn but I'm puzzled as to why I have a leak anyway ?

 

[uaz04]

Mine had the same problem, which was fixed by replacing the needle valve in the float chamber. When the car is shut off, the remaining heat from the exhaust manifold causes the fuel to expand, and then fill into the carb if the needle valve does not sit tight enough to contain the extra pressure.

 

[petak]

JonathanT....How did you find out that you have leakage?

uaz04....you mean this needle valve

 

[mojobaby]

Hello Jonathan, I recently had the same problem with fuel escaping from the little dome on top of the carb. Gave me a bit of a fright but the next time I started the car, the problem didn't reoccur, thankfully.
In the meantime I have dismantled my carb to give it a good clean and to see how everything works.

Uaz04 is completely correct when he says that your problem has to do with fuel getting hot and a high pressure building up and needing a place to escape. However I don't think it's escaping from the above needle valve (Petak's photo) as its passageway only leads back into the fuel filter and then back to the petrol pump.

However, it may still be the cause of your problem in that it may be letting too much fuel into the carburetor before the float closes the valve. I just think of a boiling half empty kettle as opposed to a completely full boiling kettle. Big difference in pressure build up!

So it might be worth checking the recommended level of fuel in the chamber. I'm not exactly sure how to do that but I know some members have done it already.

When the float (mine is plastic) is raised up, it pushes the needle on the valve to close the opening and stop more fuel arriving. Perhaps the top of your float is worn down from rubbing against the valve needle. Even 0,10mm would cause more petrol to enter.

I think that the reason why your petrol is coming out from the butterfly spindle is that the butterfly is closed when you stop the engine. Excess petrol is probably pooling there and can only escape through the spindle. Maybe.

Do you have the correct gaskets beneath your carb? 2 thin gaskets with a thick plastic one in between? it should keep the temperatures down.

 

[mr-reno-139]

It's modern petrol partly to blame it vaporise at lower temperature as it's designed for fuel injection where it is constantly circulating under pressure and as you know from how a pressure cooker works it raises the boiling point
I have been fitting 2 or more plastic spacers and gaskets on base of carb to help stop heat transfer from hot manifold
Not so bad on 845 with fan running constant to keep air circulation under bonnet
Presume you have got air intake pipe set on summer position and the flexible air hose running as far forward into a cold air pickup area away from exhaust and hot radiator

 

[JonathanT]

Hello everyone this is very thought-provoking conversation. Searching the Internet has revealed are a number of people with similar problems most noticeably a video made by a triumph owner on YouTube:

I

noticed the problem when parked up at my mum's house where the nose of the car is raised on a slope . The most obvious explanation is that that the spindle is worn. However there seems to be no detectable play in the fitting. I'm curious as to how the fuel from the bowl can get to this point? I certainly did have a problem with the fuel overspilling the carb when the engine stopped. Replacing the needle valve as in Petaks photo has sorted that problem.

I have one thick plastic gasket and two thin ones below the carb. The photo is taken a few weeks back the pipe is a bit tidier now but yes is in the summer position Paul

 

[mr-reno-139]

If you extend that pipe right to front of car it will pick up colder air mine drops down to side of gearbox and I've got no undertray so gets low down air away from exhaust and radiator

 

[Grenuj]

I have a cold start issue.. after a few days of sitting, R4 taking longer than usual to start.
Every next...normally the engine should go "boom(on)".
I discovered the cause..i have a leak on jet head!
petrol flow out from carb body... wet on exhaust manifold..
I was recently cleaning my carb and check obvious problem...