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MG MGB Technical - SU OVERFLOW PIPE

I have been having trouble with a high and inconsistant idle on my 70 B. Today I blocked off the overflow pipes and notice a big improvement. Now I know the overflow pipes that connect to the carbon canister must be there for good reason so can someone tell me what will happen with this temporary solution.....
Bob R.

Hi Bob.

If there wasn't some sort of vent at the top of the float chamber it wouldn't be able to fill properly.

Old carbs just had a simple hole in the casting, but a pipe was added on later designs to (I guess) deposit any overflowing fuel onto the road rather than onto the hot exhaust manifold.

It seems fairly obvious that the vent was a route for escaping petrol fumes. I am not familiar with US emission control mods, but I suspect that blocking the tubes is masking some other fault, I'm sure someone else will come along with the definitive answer.

Don
Don

The overflow pipes are there to allow air flow in and out of the float bowls. This permits free flow of fuel in and out of the bowls.
I'm just guessing, but by blocking the pipes you may be causing a higher pressure in the bowls as the fuel pump forces fuel against the unvented air. This will cause more fuel to flow past the needles and result in a richer mixture. If that's really what is happening you probably have a vacuum leak that's fouling up your idle.
Conversely (see I told I was guessing), blocking the pipes may cause a lower pressure in the unvented bowls as the fuel is drawn out of the bowl by manifold vacuum. This will result in a leaner mixture.

Bill
Bill Boorse

There is one way the vents can cause problems on non-emissions cars, and two ways on emissions cars.

The vent is what has been said - it allows air to be vented from the float chamber as it fills with fuel. If it were no there air pressure would stop the level rising enough for the float to shut off the supply, and the pump pumps fuel straight up the main jet and into the inlet. The same thing can happen if the vent get blocked, and this affects all cars. It is only incidental that if the float valves fail fuel is pumped out of the vent.

Emissions cars can have another failure mode and that is if the fresh-air vent into the canister gets blocked, or the carbon granules get cruddy and block the flow of air through the canister. The canister is connected to the inlet manifold via the rocker cover and the crankcase, so air is continually being pulled through the granules to purge them of vapours from the tank and float chamber, and then through the crankcase to ventilate that. If the fresh air source gets blocked the suction from the inlet manifold lowers the pressure in the vent lines, which tends to suck the fuel out of the carb jets, which *will* affect running. This is precisely how the anti-runon valve works, although it needs a secondary vacuum source direct from the inlet manifold as the existing source isn't quite enough to positively pull the fuel out of the jets and stop the engine.

So, check your vent pipes and canister for blockages. The way to check them *is* to block the fresh air inlet with your hand, whereupon the engine should run roughly or stall.

Paul Hunt 2

This thread was discussed between 17/01/2006 and 18/01/2006

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