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MG MGB Technical - Twin carb overflows

Car - 1965 MGB CB Roadster
Has anyone had any experience with the effects of playing with the bowl overflows. I fitted some hoses to them incase of a sticky needle & seat excess fuel will be routed safely to the ground instead of on and around the exhaust manifold. Doing so caused no end of grief. First the open ends were hanging just below the chassis rail and caused fuel starving problems while cruising only. Second re-routed the hoses to under the guard behind the headlight - same problem still remained at 40mph or 60mph. Ripped the lot off and the car behaved and performed like it should.
My friend has identical car with hoses that end in the space around the engine mount and all works fine. My question is how little amount of vacuum is needed to cause a drop in fuel level in the bowl? Is my fuel pump doing enough? It is a standard aftermarket one. I have no problem with fuel while under load at any speed.

. . . . . .. . . . henry
henryo

Henry,

Your '65 had aluminum overflow pipes fitted at the factory. As on your friends car they should terminate in a bracket mounted to the side of the block but above the level of the sump.

Cliff
Cliff Maddox

Henry,

The vacuum at the end of your overflows does not lower the level of the fuel in the bowl, it raises it by pulling the fuel back from the jet.
This drop in fuel level at the jet is your problem.
Applying vacuum to the fuel bowls is the way Triumph cars stall engines, to prevent "running on".
Finish the overflow tubes in the same place the factory did.

Mick
(Member, but using my work computer)
M F Anderson

I am so pleased with this thread! Ever since I restored my 71B, ready in 1997, I experience the same. As soon as I hit around 70 mph for more than 10 seconds or so, the engine starts to loose its power, accelerating has no effect at all, and the car slows down. All you can do is back off, and after 10 seconds the engine comes back alive again and all is normal. I was convinced this is a fuel starvation problem, measured the fuel flow (3 ltr per minute, free flowing), still replaced the fuel pump, test drove the car with a strobe light connected to the ignition, and put the gun inside the car so I could observe it etc. The car was originally a US version (hence the "mph" ....), with the charcoal cannister filter in the engine compartment, to which the carb overflow vapor hoses were connected. During the restoration, I removed all the anti-pollution gear and did exactly what Henry had done, making up pipes that end underneath the car, for obvious reasons.

Never ever has it occurred to me that there could be a relation with the apparent fuel starvation at high speed! I don't think it would ever had occurred to me either, had it not been for this thread!
Therefore, thanks Henry for discovering and reporting it, and the other posters for their good comments.

Since the exact position of the overflow pipes lower ends seems to be a little critical, does someone perhaps have a picture of the original version?
Kees Boerop

Thanks all for the comments.
Kees : Did your problems stop with the pipes removed?
I will look in to the correct system.
I only decided to put overflow hoses on as I have started doing regularity trials ie round the houses driving events and don't want fuel leaking on to hot exhaust. My fire extinguisher lasts for 13 seconds and I would like to save that for me if needed.
I will repost if I find any startling revelations.
Has anyone ever pulled out the choke while driving full throttle under load? I did and I was surprised with the result.

. . . . . henry
Henry

"Did your problems stop with the pipes removed?"
This will be the first thing I'll try when the weather permits me to pull the B out of its wintersleep, and drive with the top down, of course! Will post the result.
Kees
Kees Boerop

Whilst researching a final cure for my running-on problem I was amazed to find how little vacuum is needed to pull the fuel out of the jets to stop the engine, and this is the method MG used on North American cars from 1973. Originally the front carb pipe dropped more-or-less straight down, the rear pipe coming forward to meet it. They pass immediately behind the engine mount being secured with a P-clip to one of the bolts. I've run both the V8 and the roadster with and without pipes and never had the starvation problem you mention.
Paul Hunt

This thread was discussed between 23/03/2003 and 27/03/2003

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