petrol leaks

[Deleted]

Decided to overhaul carb on my 3l, due to problems with cold starting etc. Everything looked fine, so put it back, only to have petrol dripping out from jet area. Pulled it all to bits again. Diaphram on jet seemed flexible and ok, but on further investigation it had gone hard at the jet/ diaphram joint, and leaking petrol from bottom to top even with ignition off. Replacement jet cured problem. The point of this blog is to say- be aware that petrol can leak even with the ignition off, because with the high mounted tank, when it is full, the level is higher than the carb so the fuel can syphon out causing a petrol smell that may be wrongly assumed to be coming from the tank area.

[dorsetflyer]

Thanks for the info. Glad you sorted it out OK. There is nothing quite as bad as a smell of leaking petrol. Even worse when its coming from that area and dripping on to a hot engine.

[Ken Nelson]

Hello all. I have noticed petrol leaking from the throat of the carburetor on my Mk IIC 3 litre coupe and need some advice. I first thought that the fuel float bowl shut off valve might be leaking, so I replaced it with a new one and set the float bowl height by using a 7/16 inch rod between the top of the reserve bowl and the float lever as indicated in the manual. It started OK and ran fine. When I stopped the car, the carburetor throat was dry and I assumed all was well. But returning 30 minutes later the slow leak had started again, running out along the carb intake throat. I don't see petrol dripping out from the bottom of the carb. The jet and diaphragm assembly were replaced about 1-1 1/2 years ago. Even if the diaphragm had torn, I won't think fuel would leak out of the throat of the carburetor, would it? Is the float bowl height setting the problem perhaps? I hate to tear the carb apart again, but could this indicate another torn diaphragm? Any and all sage advice again is warmly solicited. Thanks,
Ken

[Phil Nottingham]

The diaphragms are very long lasting unless NOS especially these dissolve in unleaded fuel. if you are certain the needle valve is sealing properly and float is not punctured it can only be the diapragm (apart from a cracked/porous carb body which I doubt.

The carb is below the fuel line and will as you said siphon through

[Ken Nelson]

Thanks Phil. I just replaced the shutoff needle and seat with brand new SU parts and that didn't fix it. The float was bobbing up and down in the float bowl, but I'll take it out to double check that it doesn't have a partial leak. If not, I guess that it must be the diaphragm and I'll order one from Burlens, although I did replace it 1 1/2 years ago. This would be the third diaphragm for the carburetor with extremely limited mileage if so . I'll report back after I find out more.

[Phil Nottingham]

The new diaphragms last years - mine has been on for 10!

[stantondavies]

I've had one on for nearly twenty years. So far, so good!

[Ken Nelson]

As a further followup to the leaking carb: I just removed the diaphragm from the carburetor and do not find any hardening or obvious splits. One edge near a screw hole may have been slightly crimped during earlier assembly, so I am ordering a new one to replace it with. But even if it was split, would that cause fuel to drip from the intake throat of the carb? This fuel was dripping out of the air intake where the air cleaner/silencer housing would bolt on, and not from the bottom joint of the float bowl attachment. Wouldn't the float bowl shut off valve still keep the level below where it could run out the air intake? Yet I just put another brand new SU shut off valve in the top of the float chamber and set the float lever to shut off a little before the 7/16" setting the book gives. Seems unlikely 2 in a row would be bad?

[Phil Nottingham]

It is strange - petrol will flow by cappiliary action though - it could be cracked carb body casting?

[Ken Nelson]

Final (I hope) followup on my leaking carburetor. Since the diaphragm didn't appear torn or hardened I went to a local MG restoration shop to buy a "Grose Jet" assembly. The owner (John Twist-"University Motors" in Grand Rapids, MI) told me he no longer uses Grose Jets because they are now made in China with poor quality control. But he did have a rubber tipped fuel shut off valve that I purchased. I came home and installed it and reset the float bowl height to 7/16 " and reassembled it. Voila, it seems to work OK now! So the intake throat fuel leakage seems to have been because the 2 brass tipped valves didn't seal perfectly. Maybe this is only a problem because the fuel tank is high and forces fuel by gravity even when the engine is shut off? Perhaps if the tank were under the car as in MG's, etc a minor leak wouldn't be noticed or affect starting? I have my fingers crossed. Ken

[Phil Nottingham]

Its annoying when so called "new" parts are faulty rota arms are terrible - Viaton tipped valves were introduced years ago and were good but unleaded fuel dissolved the Viaton!