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SU Carb Flooding

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A

Adelaide

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I have a BN1 fitted with twin 1 3/4 inch SU carburetors on a 100M manifold. Recently the rear carby has started to flood with petrol flowing out of the float bowl overflow tube whenever I try to start the car after it has been left undriven for a period of time. If I tap the float bowl the flooding stops and the car starts and drives normally.
I have straightened and reset the float fork, replaced the needle and seat, checked the float for leaks and even tried a spare float bowl cover but the problem persists. It only seems to happen when the car has been left for a few weeks.
Can anyone offer any suggestions?
 
A symptom: you straightened and reset the float fork. Just wondering - why you had to do that.

Also, is the float set to the proper height?

Tom M.
 
Check the pin securing the float to make sure it is straight, not binding, etc.
 
My SWAG is that it's still and issue with the float valve/seat. I've futzed with these for years, trying to understand what is, in principle, a simple idea. For instance, do you adjust the lever so that it's (mostly) level with the cover with the little spring-loaded tip compressed, or relaxed? I finally came to the conclusion that the spring is there just to damp float movements, so the valve should close with it completely compressed. This changes the adjustment slightly but, if set otherwise the needle can be cocked at an angle to the seat, which could cause binding. That approach has worked well for me. Also, the hinges have to be adjusted 'just so,' else binding could be an issue as well.

Years ago, I installed Grose jets in my BJ8's HD8 carbs, which eliminated some of the guesswork and they've worked flawlessly for over 100K miles. However, I tried them in my BN2's H6es, and they stuck. They have a small ball for closing the inlet, and a larger one that contacts the lever; the small ball would sometimes lodge in its seat. I'd heard quality had suffered after the manufacturer was sold, and I think my experience confirms it. Modern needle-type valves should have a 'viton' tip, but I suspect it's possible that modern fuels can degrade them anyway. If a ridge forms on the conical tip it could cause binding.
 
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Check that the float (or float lever in the case of HDs) isn't dropping too far. When the bowl dries out after sitting a few days/weeks, the float may drop too far. If that happens the angle of the float metal piece that the float needle rides on and the float needle can be extreme enough that it can bind when it tries to rise as fuel comes in. Once there is normal fuel level in the bowl, the float never drops far enough to cause the bind, so works normally.
 
I would have this problem after the car sat for a long while. I figured the fuel had turned to varnish and the float would stick to the bottom of the bowl. I would let the fuel pump run for ~10 seconds, then turn it off, clean the windshield, and after a few minutes the float would unstick and all would be well.
 
Thank you to all who responded to my query. I have now replaced the conventional needles and seats with Grose jets and that seems to have fixed the problem. Only time will tell!!
Cheers Jock
 
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