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MG MGB Technical - Front Carb overflowing.

G'day.

Some time ago I noticed that the front carb was spitting fuel out of the overflow on my '67 roadster. I tried adjusting the float level (plastic floats with metal hinge arm) but it kept happening so I got new needle/seats for the carbs. Unfortunately that has not cured the problem.

I have tried opening up the float to body clearance from 3mm to 6mm on the front carb with no luck. I have changed fuel pumps to eliminate potential over pressure problems, but the rear carb doesn't have this fault. What next? BTW, there is no fuel in the float itself.
Tony Oliver

G'day too,

Typical problem of a worn/dirty float chamber needle.
You might consider swap it to fix thi issue.
good luck.
R.G
Renou

If you are still using sold metal needles change to the viton tipped needles as they seal much better.
John H

Are you sure the float is empty? I had a similar problem on my V8 and it was only when scratching my head after changing the valve and seat that I looked *very* carefully at the float and found it did have some fuel in it. Is the hinge pin bent or damaged in any way? The floats seem to be common to both sides, I'd swap them over to see if it is some interaction between float and valve that is causing the problem. Then I'd swap the hinge pins over, and then the jets over. That should tell you what is wrong, but if it is still in the rear carb it is difficult to think what could be causing it.
Paul Hunt 2

Thanks Gents.

I will follow Paul's advice and start to swap from one to the other. I had a spare float that I fitted so I am fairly sure the float is OK, but I will check 'to be sure to be sure' as the Irish say.

John, I am using new needles with what looks like a black rubber compound on the tips. I'm not sure what the material is.

Cheers

Tony
Tony Oliver

The black tip is Viton.
Darrell

Tony,

in most cases a fibre washer under the needle valves and a new filter close to the carb cures this problem, but you should also try to check the floats on a lettre scale for equal wights, as Paul allreafy recommeded.

Ralph
Ralph

All inputs about Viton welcome gents.
What is it? I never heard speaking about it.
Thanks in advance.
R.G
Renou

R.G Viton is a high quality rubber that is petro resistant. It is used for the tip of the needle valve in the float as it seals against the seat much better than a metal needle.
John H

I don't know about the cars "down under" but
in the U.S. some cars had the SU carbs that
had a two hoses leading to the float top covers.

One hose was for fuel feed to the float needle
valves.

The other hose was for fuel evaporative venting
to the charcoal canister.

The hoses and barbs look the same and they
can be easily crossed up. If so, then unstoppable
fuel overflow is the result.

Maybe this is the case?

Just a thought.
Daniel Wong



The earlier cars do have the overflow pipe vent to the ground. My 64 B had the same problem as Tony's. The problem was fixed after I thoroughly cleaned the floats bowels and added several shims. I used replacement floats which don't have the metal hinge. These are available from Moss or directly from Burlen



Regards

Les
Les Jacobs

*All* SU carbs, and probably all carbs with a float chamber, have to have two ports - one to let the fuel in and the other open to atmosphere to equalise the pressures inside the float chamber. Without this second vent port the float will never rise to shut off the fuel, and fuel will be forced up out of the jet by the continually pumping pump. Because this port can also vent fuel if the float valve sticks open or is overwhelmed by a non-standard pump, it has to be plumbed, either down past the exhaust to vent onto the ground on non-emissions cars as Les says, or in the case of emissions cars via the charcoal canister (where fumes are trapped) to atmosphere. If you get the fuel supply and vent pipes the wrong way round, the float chamber fills, the float rises, shuts off what is now the vent, but the pump keeps pumping and fuel comes up out of the jet. The same can happen if the pipes are on the *correct* ports but the vent port hose is blocked.
Paul Hunt 2

This thread was discussed between 20/09/2007 and 23/09/2007

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