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MG MGB Technical - idle problems

I've aquired a '72 that I intend to restore. It has been sitting outside and not ran for at least 5 years. Anyway I got the thing going but it will only run with the choke pulled fully out. Any ideas as to what the problem could be. I have an idea that it could be a problem with the fuel pump. it has an electric one fitted and it doesn't sound to healthy. Could this be the cause?
P I Barnes

PI. Might I direct you to my website, www.custompistols.com/ with a view towards the MG section? All of the MGBs used an SU electric fuel pump. Some have had this pump replaced with various aftermarket pumps. When the SU pump is working properly, it will normally click a number of times when the ignition switch is turned to the "run" position. After that, you turn the key to the "start" position and engage the starter. When running, the pump, if it is an SU, will click as needed, but, not all of the time. David DuBois, who has studied SU fuel pumps for something like 35 years and rebuilds them as a retirement job, was kind enough to write several tech articles which he has allowed me to post for everyone's use.

These tech articles will help you to identify what pump you have, determine if it is the correct pump, provide you with pressure flow and flow rate characteristics, and, if you have questions, contact Dave through a "hot link". He also has a tech article on trouble shooting the fuel system and on installing a duel pump system which may be of help to you.

There are other tech articles on how to troubleshoot the ignition system, perform a compression check and time the ignition system which may be of use to you.

Les
Les Bengtson

Hi.

I don't see how a faulty pump could give the symptoms you describe. If the pump is supplying enough fuel to keep it running with the choke out it is supplying enough to keep it running with the choke in.

The pump may still be sick, of course, and a visit to Les's site is well worthwhile.

The symptoms do suggest that the carb(s) are not supplying enough fuel OR there is a significant air leak in the vicinity of the inlet manifold.

What carb(s) do you have, what year is the car ?.

Have you drained any old fuel ?.

Was the car originally taken off the road because it wouldn't run properly, or was there some other reason ?.

Don

Don

Can I suggest that you do a full tune up and take the problem from there.
Iain MacKintosh

You really need to start from the beginning with a full ignition then fuel check and tune-up as after 5 years it's most likely so far out of kilter that you are about to start a daisy-chain of fault-finding. I know that when I got my Roadster everything, from the plug gap to the valve clearances to the points gap to the carb balance was a country mile out of spec, but oddly all so wrong that they somehow balanced and the car ran to drive it home. I started on my new hobby by carefully setting the points gap (about the only thing I knew how to do), after which it would barely start and then maybe run for a few seconds before dying. Pulling the choke, kicking the throttle and tapping random objects in the engine bay seemed to help, but wasn't until I bought the manual, serviced it and set every last item to spec that it ran properly.
Steve Postins

Absolutely agree with the previous two posts. There is no point poking and hoping at any time, and especially without doing a full set up of valve clearances, plug gaps, points dwell/gap, timing, and carb balance and mixture. You might also give it a birthday by buying it new plugs, points, condenser, leads, rotor and cap as a matter of course.
Paul Hunt 2

Yes, fit these first and then do the basic static settings before you start the motor and dynamically set the timing and balance and set mixtures. The whole thing will be done in a couple of hours and hopefully you will have a car that runs reliably which you can then further tweak as time goes on. Some of us are still tweaking or fiddling years down the line !!
Iain MacKintosh

Thanks for the advise. I think I'll leave the engine for a while until I've got the body sorted. Was just wondering if there could be anything obvious!

Thanks again
P I Barnes

P I, two things come immediately to mind: the first is old gasoline which has lost some of its "oomph", and the second is a stuck needle valve in one float chamber, so that there's no fuel in it, and you're running on only one carburettor. The quick fix is to pop the covers off the float chambers, and take a look, suck out the old gas, and refill with fresh fuel in the chambers. That will let you know really quickly if it's a fuel quality, or fuel quantity issue.

The tune up, is a must-do, also, but I can sense how curious you are about this one issue. Cheers - - Alec
Alec Darnall

By all means do the above, and:
A common problem with SU or other needle metered carbs (ZS & Amal & other motorcycle) which have been sitting is a buildup of varnish deposits on the needle and jet. It varies by fuel and other factors, but the net result is a jet that is too small, and a needle too large - especially near the at rest, ie idle, position. Since there is only .001-.002 difference in the jet and needle dimension at idle, it doesn't take much. Short of taking the carbs apart for cleaning, a heavy dose of "fuel injection system cleaner" and a fill up of new gas can work wonders. Put it in and run the car a bit to get it up into the carbs, then let it sit. Repeat every day or so if you can't actually drive it. A few days of this frequently straightens it out.
FRM
FR Millmore

This thread was discussed between 22/09/2005 and 23/09/2005

MG MGB Technical index

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