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MG MGB Technical - Rear HIF Carb Leaking Fuel !!!

OK, I have had this "72 MGB-GT for about 5 months now.
This car had been sitting idle for several years when I
purchased it. It was not running very smooth when I
first drove it home. My other MGB is a "79 with ZS carb, so these dual carbs are a new experience for me.

Anyway, I borrowed a balancing tool from a club member and got the carbs running pretty good two months ago. Since I have had this car, I've noticed the number 3 & 4 (rear) spark plugs have always had carbon build up when I take them out to check them. The front two plugs always look just right. So I know the rear carb has been running rich.

The last few times I had the car out the idle rpms was dropping down very low, almost to stalling, and I had been noticing a strong fuel oder. So last night I took the air cleaners off when I got home and thought I would try to lean the rear carb out somemore.
I turned the adjusting screw about 1 full turn counter clockwise and then took the car out for a hard drive up an long incline, just to make sure I hadn't leaned it out too much. It seemed to run good.

But when I pulled into my garage and turned it off I could still smell strong fuel oder and could hear a hissing, sizzeling sound. I opened the bonnet and could see fuel leaking from the rear carb onto the hot
exhaust manifold. I could not tell exactly where the leak was coming from but could see that it was coming from the rear side of the carb in the area of where the mixture adjusting screw is.

Can anyone give me the most likely cause of this leak and what I need to do to correct it. I am assuming I may need to rebuild the carb?

Thanks,
John Fraioli
1979 Inca Yellow MGB
1972 Teal Blue MGB-GT
John F

More than likely a flooding float chamber. Either muck in the needle valve, a worn needle valve or a faulty float. More than likely one of the first two. If you don't want to dismantle the float chambers, try just diconnecting the fuel line, attach a piece of tube and blow through the needle valve. Then try sucking and see, by placing your tongue over the end of the tube, whether it holds a vacuum. If it does, fine and dandy, if not you will have to examine the valve and float.
Are you sure they are HIF's and not HS4's (float chamber on the side?) The latter are easy, 3 screws and you're in, HIF (Horizontal Integrated Float)you will need to take the offending Carb off.
Allan Reeling

@ Allan

the way Jhon describes the weakening of the rear carb, it must be a HIF version.

@ John

in addition to the information given by Allan, it also happens that the inlet pipe can start leaking when there is no clamp upon the line ore the brass pipe comes lose in the body. Another point to look for is the bottom seal of the float chamber. If the car had been stored for a while, it might become leaky, sometimes only due to unsufficiant tightened bolts, sometimes from dying in.

On one of my B's i had this, but it was the front carb where the gasket could be changed without pulling the carb from the manifold. not sure if there is enough space for the rear one to do this too.

Ralph
Ralph

Had the same problem after my car ( HIF, 1974) sat idle for over year.

The float chamber cover was full of muck and made the float stick open.

I was able to remove it without removing the carb, using a stubby straight screwdriver to remove the 4 screws. Jiggled the float, and gave it shot of card cleaner.

Have a bowl handy to catch the 2 - 3 oz of fuel.
Phil Collura

The float chamber vent ports are by the mixture adjuster screws on HIFs, both front and rear. If they aren't connected to a charcoal canister they *must* be piped down to clear the exhaust.

Sometimes you can clear a sticking float valve by electrically disconnecting the fuel pump and running the engine till the float chambers empty, then reconnecting the fuel pump. The resulting rush of fuel through the now wide-open float valves (while running they are only open a smidgen) can often shift any debris. If the problem happens again after a while then maybe you have dirty fuel. If it happens again immediatley then maybe the float valve is faulty, or maybe the float is partially sunk. It's better to get HIF carbs off, invert them before removing the cover, and inspect everything inside carefully. You will then need a new O-ring as they don't like being disturbed unless they are pretty new. Another symptom of a leaking float valve, if you have the OE SU pump anyway, is if it clicks with the ignition on but the engine stopped more than once per 30 secs. However this also happens if the non-return valves in the pump itself are leaking back.

When the overflow is fixed you will have to setup the carbs from scratch again. If you have to move the mixture very far from the starying point of two full turns down from being flush with the bridge to get the correct mixture, or move one noticeably more than the other, then that indicates there are still problems that will affect running.
Paul Hunt 2

This thread was discussed on 15/08/2007

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