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MG MGB Technical - Float Bowl

Hello everyone,
My 71 MGB has had a gasoline odour to it for a long time. Last week when idling I noticed a puddle under the engine. I popped the hod and saw gas leaking from the top of the rear float bowl. It looked like not just the overflow tube, but the entire top of the bowl. I removed the float bowl lid. The gasket came off in 3 pieces and looked to be in very poor condition.I replaced the needle and seat, and gasket, and the leak disappeared. The car no longer has the gassy smell and seems to run a lot better, both idling, and stronger at speed. My question - Could just replacing the needle, seat, and gasket make such a noticeable improvement in engine performance?
G Nicholas

This is probably not responsive to your inquiry but, just coincidentally, last week when I started my '72 B with HS6 carbs and looked under the bonnet, I noticed the entire top of the rear float bowl covered with gas. I first thought it was from the overflow but on further inspection determined that it was from a loose fuel hose connection. No banjos on these carbs, just rubber hoses with hose clamps. The hose clamp either hadn't been tightened or it had worked its way loose. Several turns and it tightened up and the leak stopped. I thought I had some horrible problem and it was quite pleasant to find such an easy fix.

Jud
J. K. Chapin

Nice fix Jud
In answer to your question G
It's possible the gasket being broken allowed fuel to splash out while driving ,but if there was a puddle under the car it usually indicates that the float bowl is flooding, usually caused by a leaking needle and seat-
This could just be some rubbish in there or a worn needle or seat
Having replaced n&s and gasket you have fixed it
When it was leaking the fuel level in the bowl would have been above the top of the jet tube making that carb. run very rich resulting in rough running and I guess black smoke at idle---now with the fuel level back where it should be all is well again
willy
William Revit

Concur with Willy. A useful check on the float valves is to turn on the ignition but don't start the engine. After the first few clicks where it is filling the float bowls an SU pump shouldn't click more than once every 30 secs. If it clicks more than that it's likely a float valve is leaking, and the carb will eventually overflow. With the North American charcoal cannister this could take very many minutes to be visible at the canister air intake (disconnect the coil if leaving the ignition on for any length of time or it will overheat). It could also be the pump inlet one-way valve leaking back but that is less common.
paulh4

The bowl gasket is not intended to block fuel if it comes up to that level.

It is probable that there was a bit of dirt or rust between the needle and seat. If the needle was the chrome tip kind, it may have been worn out. Floats will get soggy with fuel also, some kind of pin hole thing.

I suggest running an overflow tube between the bowl lid overflows and the top of the shocks, into the opening to the wheel well to avoid fuel spilling onto the exhaust manifold.
Glenn Mallory

The gasket IS intended to prevent fuel leaking from the join between lid and body, as fuel level has to rise higher than that before it can escape from the overflow.

It's certainly true that in normal use it won't rise that high, but we all know that it does from time to time when there is a problem with the float or valve. Pipes are supposed to be (and were originally) fitted to take any overflow down past the engine, clipped to the side of the block or the engine side of the engine mount, so they move with the engine and don't stress the very small hoses that connect the lids to the pipes.

If the gasket were not intended to prevent fuel escaping there would be no point in providing a safe route down past the exhaust.
paulh4

This thread was discussed between 17/08/2019 and 18/08/2019

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