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MG MGB Technical - Carb flooding with petrol

Hi

I have a problem with the front carb.

With the ignition on, the pump ticking over nicely, the front carb now has petrol flowing up and out from the jet to the bridge area on the air intake.

I have the jet at two full turns on both ready for synchronisation.


Is the problem the jet, the needle or both?


The needle piston is a bit slow coming down too, have cleaned it, could this also be part of the problem.

Thanks in advance.

Iwan
Iwan Jones

Sounds like the float chamber vent is blocked, or the fuel pipe is connected to the vent instead of the inlet!

It has nothing to do with fuel being pumped up out of the jet but the piston should drop rapidly with a clunk, it is only damped on its rise. You don't say what carbs or needles you have, but if fixed needles it could be jet alignment that is incorrect. Take the needle out of the piston, reassemble and try again. If its still dropping slowly then it is the piston, try cleaning and polishing the inside of the cover and the outside of the piston. If that doesn't help you could try numbering each carb, cover and piston and swapping them round to see if you can get a combination that is good for both carbs. They are supped to be kept as matched sets but they could have been mixed up in the past. If it drops fine with the needle out then try recentering the jet (only applicable to HS carbs and fixed needles). The needle could also be bent, take it out and roll it across a clean surface on its shank and look for wobble at the tip, alhough again that is for fixed needles, it is more tricky with sprung.
PaulH Solihull

Hi Paul.

Sorry did not mention they are HS's.

Fixed the piston not dropping with a click. Bit of a burr of metal in the machined chamber area. Got rid of it. Piston dropping fine now.

However, checked float chamber, yes they both are on the right way round. Able to blow through both pipes/vents. Still flooding.


Ta,

Iwan
Iwan Jones

Iwan-
The overflow valve in the float bowl is probably sticking shut. Remove the top of the float bowl, turn it upside down, and clean both the needle and its seat with some carburetor cleaner.
Stephen Strange

Can't be sticking shut or no petrol would get through it into the float chamber and from there up through the jet.

It must be open and failing to close. The normal causes of this are debris in the valve itself, or a sunk float, but this usually results in fuel coming out of the overflow.

It's when the overflow is blocked that fuel usually comes up out of the jet, as air pressure inside the float chamber prevents a good float rising and a good valve from closing. But if you are sure the vent is clear then a) there must be some other cause of the flooding i.e. valve or float as described, plus there must be some other reason why the excess is not coming out of the vent.
PaulH Solihull

I had the same problem, every time I switched on I had a pool of petrol on the garage floor, I found it was dirt getting through and stoping the float needle closing. I put an inline filter between petrol pump and carbs. No more petrol on garage floor. Johnny Cook.
JG Cook

Hi all, Should have sucked instead of blowing.

The valve in the float chamber was brand new, cleaned up everything, blew through the vent pipe etc. etc.. However, decided to look at the vent again, and out came this gunk - it must have been very directional blockage. Anyway, seems sorted now, thanks Paul just as you said right at the start of your initial reply.

Ta,

Iwan
Iwan Jones

My pleasure :o)
PaulH Solihull

This thread was discussed between 30/12/2010 and 03/01/2011

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