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MG MGB Technical - sounds like pinking but is not pinking!

I had a long drive down to brighton on the bank holiday weekend (180miles) each way and on the way there experienced some strange sounds. I initially thought it was pinking but it isn't and i don't think it is coming from the engine.

Oil pressure is normally 40 idle 60 driving. This dropped to 55 driving and now fluctuates wildly at idle between 55 and 40 (a few blips a second)

The noise came and went then would come back sometimes on the tinniest load sometimes on normal driving, sometimes on hills sometimes not. The car also started to kangaroo (like learner drivers but not quite as bad) in 3rd gear.

I found an mg specialist in brighton. He reset the distributor, had it up on the ramps, checked the wheels, exhaust and prop shaft and could find nothing wrong other than slight play in the diff which he said was not bad at all. He drove the car but could not place the problem. He said it wasn't the engine. mayber the diff/propshaft coupling getting dry after all these years.

The car was given a rest for 2 days and then driven the 180 miles home (in alot cooler weather). No sign of the noise. Oil pressure still fluctuating wildly at idle and the kangarooing has changed to a slight juddering which i was away of but only in 3rd and 3rd overdrive.

Can you help me locate the problem?
MM

A few years ago my GT started pinking badly for no apparent reason. I tried everything I could think of but without success - the slighest incline brought it on. Eventually I was crawling round a bend in traffic with the roof back and I realised that I could hear the same sound - and it wasn't pinking, it was one of the UJs starting to break up. So despite the guy in Brighton having checked that, it could be worth you getting underneath and checking the UJs very carefully - any play would make them a suspect.
Miles Banister

Ok. lets look at the clues...

It is certainly worth checking the uj's, but they wouldn't explain the oil pressure or kangarooing (would they ?).. you may, of course, have two separate probs.

When the oil pressure is jumping about, are the revs going up and down at the same time ?.
If the revs are varying, the oil pressure change is maybe just a symptom, but the lower oil pressure at speed suggests otherwise.

Petrol 'goes off' with time, and effectively loses octane.. did you have old fuel in ?.

Don
Don

It sounds like you have a fuel problem. The knocking could be an intermittent lean condition, and the surging could be a very lean condition. Maybe the fuel pump has intermittent problems? How old is it? Check all the wiring to it, and replace the fuel filter - if you have one? What type of carb(s) do you have? Recently or never rebuilt? Is your vacuum advance good? Are the fuel lines in good condition?

Do you have a non-vented fuel cap with no emissions equipment? Try removing the rubber washer from the fuel cap if you hear a rush of pressure when you open it while running.
Jeff Schlemmer

Jeff, is that a problem, if you notice that there is a rush of pressure on removing your fuel cap? I don't believe mine is vented and may have experienced this problem.

Apologies for my simple-ness, my GT is my first classic car I've worked.
M J Kirkham

Hi again.

As fuel is sucked out of the tank air should be let in through a vent to replace it.
If the vent is blocked it can cause fuel starvation, or sometimes even cause the tank to collapse.
A slight rush of air is normal on some modern cars as the have spring-loaded vent valves for pollution control (to let air in, but not let petrol fumes out), but as far as I know this was never applied to UK B's.

Apologies are not necessary, we all had to learn, and most of us still are learning.

Don
Don

This thread was discussed between 31/05/2005 and 01/06/2005

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