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MG MGB Technical - MGB starting / firing problems

I was wondering if anyone could help me on a technical question. I'm having trouble with a 1975 Jubliee model that's been parked up for a while.

Its got fuel supply problems - just had K & N filters fitted and when I ordered them they said not to bother with fitting new needles but the car will run for 5 minutes or so and then pack up. It will re-start after a lot of trying and then does the same again. To me it seems like its choking itself. If I need / should order new needles what are recommended for K & N fitted vehicles? Second question: I'm also not discounting a fuel pump problem either as its been parked up for a while and the pump could be sticking so I've ordered a newer style elec pump to replace the SU unit but I have to admit can anyone tell me where the damn pump is situated as I can't find it!
M Lightfoot

It is not unusual to change to a richer needle when fitting the less restrictive K&N filters. I and several other posters to this board went from the standard ABD to AAA on the HIF carbs. Prior to this change, my 73 B gave symptons of being too lean to the extent that the choke was still reqd when hot to ensure good accel from traffic lights.

Barry
BJ Quartermaine

The standard needles are too lean, but the problem shows up at speed, not idle. Both the ABD and AAA have the same diameter at idle, you will feel it leaning out at speed- it plain won't pull past about 50. Richening the mixture to get it to run right at speed will then make it far too rich at idle, and it won't tick over.

If it runs for 5 minutes then stops, I would change the condenser.

The pump should be on the right hand side of the boot half inside at the bottom of the front bulkhead, under a tin cover that looks homemade, half outside behind the axle. Usually a clean of the points is all that is required, that can be done from inside the boot.
Martin Layton

First check fuel delivery. With a delivery pipe removed from a carb and directed into a container it should deliver *at least* 1 Imperial pint per minute (and in practice significantly more than that) with negligible bubbles.

But the easy test for a dodgy pump is if it chatters away like mad just before it restarts. If it does, that indicates the float chambers were empty which does imply a pump problem.

I've just resurrected a barn-find doing much the same thing. Initially it was the points but after tickling them up a bit it started running. Delivered enough fuel to start and run the engine for a few seconds then it stopped. Eventually it wouldn't do even that. In that case the air chamber bolt was loose so it was probably sucking in air or otherwise failing to develop a vacuum to suck fuel up from the tank (the pump is higher than the tank in rubber bumper cars).

Another possiblity is a blocked fuel tank vent, which should be in the filler cap on UK cars. Modern filler caps are unvented so you could have the wrong one. In this case simply removing the filler cap will allow the engine to start and run normally.

Otherwise, clip on an inductive pickup type timing light to the coil lead and each plug lead in turn and watch the flashes while cranking. They should be regular and consistent, with those on the coil lead at four times the rate of any plug lead of course. Whilst on plugs 1 and 4 point it at the crank pulley and check the timing is about right at 8 to 10 degrees BTDC.

Back to fuel, after cranking for a while with no start, remove the plugs and look at/sniff them. If they are wet it means the engine is flooded, but you should be able to smell fuel anyway in that case. If there is no fuel smell no fuel is getting through for some other reason, there should be a strong fuel smell but not be wet.
Paul Hunt 2

M Lightfoot - The AUF 300/AZX 1300 series pumps should deliver very close to 2 pints per minute of fuel withthe lne disconnected from the carburetrs nd directed into a container. For a complete troubleshooting guide for fuel delivery problems, see my article on th subject at: http://www.custompistols.com/cars/dave/ddFuelDeliveryTroubleshooting.htm
Paul's suggestion of a blocked fuel tank vent is the first thig to check for (mainly because it is a very easy check), but unles the fuel tank is completely full to the top, the engine should run longer than 5 minutes before sufficient vacuum is developed in the tank to stop fuel flow.

The pump is located on the off side (have I got that right?), just forward of the rear axle. You will need to remove the wheel on that side to get at it unless thecar is up on a lift. Cheers - Dave
David DuBois

Thanks for your feedback. I've now found the pump in the boot. I'd retraced everything and got back to the tank to find it in the end!!!

Will take the needle comments on board I think on investigation this afternoon that there is an earth fault on the pump circuit. The problem is very intermittent. It runs great for ages and then ... nothing. Anyway. Thanks for all the comments.



M Lightfoot

This earth fault is very easy. The fuel pump ground is shared with the license plate lights; it's right there in the boot - follow the number plate light wires and you'll find that the black wires go to a grounding connector that bolts to the body. That's the connection you need to clean up.
Sam

On an RB I don't think removing the wheel is any help as it is too high up and not level with the bottom of the battery box like on a CB, whereas removing the wheel is probably essential. I've just swapped the pump on an RB and one you have the car safely supported on axle stands you just crawl under and deal with it from below, with suitable eye protection.

If it is the ground that is bad the pump *body* will show 12v when the fault is evident so fairly easy to determine, but in order to eliminate that (i.e. there isn't 12v on the body when the ignition is on but the pump isn't pumping) you do need to confirm you have 12v on the spade in the boot as well, you can get bad connections back through the white wire and its connectors (by the fusebox) as well as in the ground.

With 12v on the spade then check there is 12v on both halves of the points, and that eliminates those as a possibility, but they are the most usual cause of non-operating pumps.

Always remember there may be more than one fault.
Paul Hunt 2

I've just fitted a new fully elec fast delivery pump and re done all the earth connections in the pump and license plate circuit. Ran like a dream for an hour or more. Probably won't start tomorrow but feeling hopeful at this stage :-)
M Lightfoot

This thread was discussed between 16/06/2007 and 21/06/2007

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