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MG MGB Technical - petrol vacuum issue?

Dear Experts,
this is pretty minor, but also a bit annoying, and i wonder if its common and if so is there a standard fix......my 1973 MGB GT cuts out. Changed the coil, condensor, points etc, to no avail. the car runs well, then sporadically dies. resembles lack of fuel. checked fuel pump, all fine and good. after cut-out, wont restart. count to 30 secs, then restarts fine, good idle, no problems (till the next time). Last couple of times this has occurred i have skipped the 30 second wait by jumping out and easing the fuel cap back - i hear a hiss, then all is well. so figured that the cap wasnt breathing correctly, and replaced with a brand new breather petrol cap. guess what - no change, and still a hiss when i pull it back after cut-out. My car is a uk version, so doesnt appear to have any evaporative loss system/charcoal canister devices. examined tank, cant locate any breather valve which may be blocked. any advice or tips much appreciated, as the only next step i can think of is to remove the washer in the pertol cap/drill a very small hole in the cap to prevent vacuum build, if indeed you think that i am correct in diagnosing this as the root of the problem. apologies if this is a laughably simple thing to be asking about - i am all rather new to this you see!
many thanks for your time
mick
m rae

It's not laughable Mick as I have the same problem on my 72 roadster. I think that the 72 car should have a vent on the tank but I have a rubber bumper tank complete with filler neck on my car and also a vented cap and have the same problem as you. I did consider drilling the top of the filler neck and brazing a small pipe into it, fitting a plastic tube and leading this into the rear chassis leg. Drilling a very small hole in the cap is another method I have considered but not done yet. Anyone got any other ideas.
Iain MacKintosh

That's not the first time I heard that happening to a car. The hose on the fuel filler neck sounds better than drilling the cap.
Mike MaGee

Well, occasionally in my part of the world you'll see an old pickup truck with a red shop rag stuffed into the fuel filler pipe, instead of a proper "gas cap".

Same problem, do ya' reckon?

<Silly giggle> - - Alec


.
Alec Darnall

I had the same problem. I drilled 2 1.5mm holes in the (locking) cap and punched 2 holes in the rubber sealing ring. No more hiss....Could you test it by leaving the cap off putting a piece of thick polythene over the filler neck (secure with string / cable tie) put a pair of small holes in it and see how it goes. Perhaps not fill tank to brim..!!
Michael
Michael Beswick

You *do* get a hiss after driving for a while as the breather valve is spring-loaded to stop fuel running out after a roll-over, or fumes to escape, you should not drill open holes in the cap. Whilst it is always possible to have bought a faulty new cap to replace a faulty old one, maybe the pump is at fault in that it hasn't got enough oomph to suck against even the weak spring pressure of the cap. If the UK cars ever got a vented tank it certainly wasn't as early as 72, and I'm not aware they ever did, but North American (and eventually all LHD cars) used a vented tank and a non-vented fuel filler cap as part of their emissions changes from the early 70s.
Paul Hunt 2

Paul - i reckon you may be spot on. I too had a concern about the fuel pump - got a new pump on yesterday and problem solved - still a wee hiss as you rightly describe, but no issues with fuel not making it through any more. Good job my drill is knackered!
thanks again
m rae

My pump is new as well (SU electronic) but when a wee hiss turns to a clang from the bottom of the tank as it pops back out it's getting a bit past a joke. I wasn't aware of vent valves in the cap so maybe that is the problem.
Iain MacKintosh

Like Mick, I seem to have the same problem with my 1971 MGB American version. My car runs like a train (not a dutch one)but suddenly stops. No shaking or whatever. It is suddenly completely dead. Most of the time, sometimes after opening the hood(?)it starts again. Because it stops so suddenly I was told that it was an electrical problem. So I changed the coil, the condensor the points etc. This was not the solution, the problem is still there.
When I turn the contact on I hear what I think is the fuel pump. It is rather noisy.
I'm a beginner as well so any tip is welcome.
Thank you for the help so far.
Fred
Fred Gielens

Fred, next time your car stops turn off the ignition and then remove the carb float chamber lids. If both are full it's unlikely to be a fuel delivery problem although it could still be debris between the float chamber and the needle. In any event a dead stop is usually electrical but not conclusively so.
Iain MacKintosh

Fred - you say completely dead. Is this no lights? No ignition? If you have ignition does it crank but not fire? When it dies and before you have done anything i.e. the ignition is still switched on, the car is still moving and its momentum turning over the engine, is the tachometer still registering? Is the ignition warning light on?

If the tach is still registering then you have power to the ignition, so the problem could be condenser, HT or fuel, any can make it die suddenly and completely *or* stumble before it dies depending on what the problem may be.

If the tach isn't registering and the ignition warning light is on you have lost power from the ignition switch (white). If the latter is off then the ignition LT circuit has failed i.e. coil, points, distributor ground wire, and connections.
Paul Hunt 2

Lain and Paul
Tank you for your reaction.
I'm not sure about the tach or the lights but I know the ignition warning light goes on after a few seconds. Any way I now have an idea where to look for if/when it happens again.
I will let you know
Thanks again
Fred Gielens

This thread was discussed between 05/10/2005 and 13/10/2005

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