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MG MGB Technical - pump blocked?

HI

My 71 Gt has just developed a problem which i think is a bit strange!
Driving along the other day and when i changed upto fourth gear the car lost power and stalled as i was gaining speed.
It restarted ok and drove home only using 1st,2nd and 3rd gears not picking up to much speed.
Next morning it started fine idles ok so i take it out for a drive same problem it seems to happen when i reach 35mph and above it is not to much of a problem when i am driving slow. The engine always starts again no problem.

The carbs were rebuilt in the garage and have been fine for 2 months or so, it has also had a new fuel tank and pump 3 months ago.

Why is this happening?? Please any ideas would be great. I thought it could be fuel pump but not really sure.

Thanks
NJ Rawlings

Sounds like a blocked fuel filter. A very dirty filter will allow just enough fuel through to run at low speeds, but when more volume is needed at higher speed it is not available. In line fuel filters were not fitted as standard in 71 but perhaps one was installed at sometime. There are also screens in the pump fittings and on the tank pick-up which can become blocked.
Also check that your petrol tank is vented. The cap should be vented on a 71RHD.
Andrew Blackley

I had a similar problem, turned out the PO had installed a no name fuel pump that wasn't strong enough. Fine while running at fairly low speeds, but once I asked for more power, all the fuel was taken by the first carb and the second was starved. Soon as I stopped, the float chambers would refill and everything was fine. So if you had the pump installed by others, you might want to check it out. Alternatively check the fuel cross feed between the carbs, plus everything that Andrew has said.
Good Luck
Ken
B.J. Quartermaine

NJ

The previous posting was from Ken Martin in Ontario. I knew he had had a similar experience so flagged him to this thread - he must have replied via my link so it appeared to be from me.

Good luck

Barry
73B

B.J. Quartermaine

Hi should have mentioned that the fuel pumptank etc were replaced a few months back and have been fine. But just had it's MOT and the garage fitted a new fuel cap, i have checked it and it says vented but the rubber backing has a complete inprint of the collar so i am thinking may it is sealing it's self. But again i don't know but it seems a good place to start.

Thanks for all your help
NJ Rawlings

Easy enough to check. Disconnect the fuel line behind the rear carb, and see how fast the gas pumps into a mason jar or similar. I think Les Bengston has the rate specs on his website. A gas cap failing to vent would cause the symptoms, eventually. But it would run normally for a while (inversely proportional to the amount of fuel int he tank) before sufficient vacuum built up to slow fuel flow.

Shutting down wouldn't necessarily fix it either. There should be check valves in the pump that would maintain the vacuum in the tank, so the problem would persist, at least for a while.

Steve
Steve Aichele

The vented fuel filler cap has a spring-loaded valve to prevent leakage in a roll-over so it isn't immediately obvious if it is vented or not. These develop a small amount of vacuum in the tank so you even get a small 'whoosh' when you remove the cap immediately after a run. Remove a fuel feed pipe from a carb (you will get a spurt if the ignition has been on recently) and direct it into a container and turn the ignition on. You should get at least 1 pt per minute, and in practice considerably more than that (or a pint in less time). Do it with and with out the fuel filler cap fitted to check both cap and pump.
Paul Hunt

This thread was discussed between 24/05/2004 and 26/05/2004

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