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MG MGA - Float chamber lid banjos

Hi all,
The diameters of the fuel inlet and outlet on the inside of the rear float chamber lid banjo are noticeably larger than the diameter of the fuel inlet on the front float chamber banjo. Does anyone know if this is the standard set up? I would expect them to be the same. I have spoken to Burlen about this and drew a blank - the front lid banjo is not listed on the parts diagram (although their website does show a single inlet banjo). The car is a 1600MkII with H4s.

The reason I am interested is that I have recently replaced the float valves/needles in both front and rear chambers (the front repeatedly overflowed). When I lifted the lid from the rear float chamber, the float lever was set way above 7/16" and there was a fibre washer between the valve and the lid body. I also replaced the levers (dimples had worn into the under surface where the needle had made contact).

I set both levers at the correct 7/16" but the fuel level in the rear chamber is visibly higher (~1/4-3/8") than in the front chamber and with the jets set the same the rear spark plug is fouled. Dropping the jets with the choke shows lots of fuel in rear but not the front. My suspicion is that the smaller diameter for fuel entry in the front banjo is creating a higher fuel pressure in the rear banjo, sufficient to push some extra fuel into the rear chamber leading to the higher level.

I've scoured the archives but not found any info. Many thanks for any info/ideas and apologies for the length of the post. Cheers
Richard
Richard Atkinson

FWIW it seems replacement banjos also have different diameter holes front to rear. Would suggest I am barking up the wrong tree regarding my theory about fuel pressure!
Richard
Richard Atkinson

Don't think the pressure theory is right. You would only develop differential pressure with a fuel flow and the holes are big enough that you would need a very high flow.
Art Pearse

There were two different designs for the float cover and valve arm. Difference is in the height of the arm pivot pin, with the later cover having a lower pivot point. The cover and arm must be matched to the correct type. Using the early type arm in the later type cover would result it excessively high fuel level or no shut-off and fuel overflow. See pictures and part numbers here:
http://mgaguru.com/mgtech/carbs/cb116b.htm

When setting lever height, turn the cover upside down and let the lever drop toward the cover by gravity. That is where you need to set the 7/16-inch lever height.
Barney Gaylord

When both levers are set to the same height, the floats must stop at the same height. If fuel height is different in the two chambers, then the floats may not have equal boyancy. Weigh the floats, or shake them to see if one might slosh due to a leak and fuel inside. Heavy float sinks lower into the fuel resulting in higher final fuel level. A leaky float might be repairable. See here: http://mgaguru.com/mgtech/carbs/cb116.htm
Barney Gaylord

Thanks guys. I have the later lids and the correct levers. The floats themselves are within 1g (20 and 21g) with no leaks as far as I can tell (I have a new genuine SU float and it weighs 25g). So it must be how I have set the levers. Can I ask, how close do you think the fuel levels in the chambers need to be? The same by eye, within 1/4", 1/2"? I don't want to get too obsessive about it. Many thanks,
Cheers
Richard
Richard Atkinson

Have you ensured the part of the float lever between the pivot and fork is not bent. It should describe a horizontal line across the top of the needle valve, the start of the curve to the fork should be only the last 3/16".

I would suggest the levels in the float chambers should be as similar as possible. The level transfers to the height of the pool of fuel in the main jet, This is drawn up to the top of the jet by the depression across the venturi. Any significant difference must affect the mixture strength a bit.

I usually set the float valves by pulling the choke out fully and repeatedly adjusting the float fork til I have the same level in both jet guides (just touching the top of the jet at full choke)After each adjustment I syringe some of the fuel out of the bowl and use the fuel pump to re-establish the level.

Probably more work than it's worth, but at least I know fuel level isn't a problem.

Paddy
Paddy Reardon

Thank you all for your advice. I have finally got all of the parts I needed to put everything back together. Have made myself useful in the mean time by cleaning and polishing the various bits so the carbs will look better at least! I like your suggestion to use the fuel level in the jet guides. Looking forward to getting driving again.

Cheers
Richard Atkinson

Pleased to report that I managed to get the fuel levels in the float chamber equal - the key I think was to get the straight part of the levers horizontal by adjusting the two outer tabs of the levers. Thanks very much for your help, much appreciated.
Richard Atkinson

This thread was discussed between 25/06/2013 and 07/07/2013

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