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MG MGB Technical - starting problem

Over the weekend I got my B (74, HIFs) set up for the season, checked the timing, adjusted the carbs a tad, and dialed the idle down to about 900. Took it out for a run to the post office (about 20miles roundtrip) and it ran great. Parked it, fairly content.

Went out each of the last two mornings and can't get it to start and keep running. With full choke, it'll fire and rev up to 2000 or so for 5 or 10 seconds, then die. It'll do this a couple times, then not fire again on later attempts. Thought maybe I had it a little rich and was flooding it, but cranking with half or no choke doesn't make it fire either.

The only thing (other than sitting) that I can think might have changed something is that it was about 65 over the weekend, and the last two mornings have been about 40 and 50, respectively.

Any ideas appreciated.

Steve
Steve Aichele

Have you checked your fuel pump and filter? Can't see how the drop in temp would keep it from running as you say, especially if the car is in decent tune. Sounds like it's fuel starved.
Lewis

Fits the symptoms. I can hear the pump pumping, but maybe the filter's full. I just can't work out why it would fail parked rather than when I was winding it up over the weekend.

I'll check that when I get home.
Thanks
Steve
Steve Aichele

Don't look for logic, sometimes things just happen. Sounds like fuel starvation to me as well. You say you can hear the pump pumping, with an original SU type it should only click when it is supplying fuel to a running engine. If it clicks all the time either a float valve is stuck open and it should be dumping fuel on the ground either from the carb breathers or charcoal canister, or a non-return valve has failed such that it can't develop any pressure to the carbs, or there is an air-leak or blockage on the suction side. Remove the fuel feed pipe from the carb(s) and direct it into a container and switch the ignition on. The pump should click away and deliver *at least* one pint per minute with no air bubbles in the fuel stream.
Paul Hunt

Steve,
Try turning the key on and off 5 or 6 times and then starting it. If it starts and then quits after about 5-10 seconds, repeat the process and see if it starts again. It kind of sounds like the fuel pump may not be running. Sometimes they fail where it cycles once every time you turn the key on, but doesn't continue to cycle. After several starting attempts the pump will have cycled a few times and the car will sometimes start for a few seconds and then quit. By turning the key off and on quickly while it is running you can sometimes keep it running, and that definitely confirms the fuel pump.

Good luck,
Ralph
Ralph

I went back through it - plenty of gas to the carbs, so I figured it must be something in the mixture adjustment. Redid that, got it running, got it balanced, and went to adjust the mixture a tad and the engine accelerated noticeably when I touched the adjustment screw. No turning, just pressure on the screw.

Both jets seemed to move similarly relative to the bridge, and there's no excess gas dumping out by the mixture screw (suggetsing the gasket is OK). Is there a part that would fail internally with these symptoms?

Thanks
Steve
Steve Aichele

Has this just suddenly happened on previously working carbs? Or has something been done to them since it was working properly last? Is this on just one carb or two? If only one could be something misaligned or broken inside the float chamber. If you remove the piston cover and piston, and look down at the jet in the bridge while turning the screw, it should only move up and down slowly and gradually as you turn the screw whole revolutions. Much more diagnosis will probably need you to remove the carbs and turn them over to remove the float 'cover' and look inside generally and whilst you are turning the mixture screw. A pip on the mixture screw engages with a slot in the end of a bi-metal strip, the other end of that is what controls the height of the jet. You may well need new O-rings for the cover afterwards, as they harden being in petrol all the time and once disturbed can seep.
Paul Hunt

I had them [professionally] rebuilt last year and they worked fine. I managed to get them further out of adjustment than I would like last weekend by forgetting the "right is rich" rule.

I did remove the piston and chamber, and both jets seem to move smoothly and at equal rates relative to the bridge.

I guess there's probably not else except to take them off and have a look inside.

Thanks
Steve
Steve Aichele

If it speeds up when you put pressure on the screw - Isn't that a sign of an air leak on the throttle shafts? You could hold pressure on the screw where it is speeding up and spray carb cleaner around the shaft areas. The rpm will change if there is an air leak since the carb cleaner will temorarily block the leak.
One thing - on a car that has been running well, the carbs shoulnd not be adjusted until you have adjusted the valve clearance and replaced the ignition points, plugs, condensor, and rotor and checked the timing.
If you think about it, out of all those things the carbs are the only thing that will show almost no change from wear. You will find quite often that after all the other things are right the carbs don't need any adjusting at all FWIW.
Ralph
Ralph

Ralph - probably true if it were the idle speed screw, but as I read Steve's post it is the mixture screw he touched which changed the revs.
Paul Hunt

Steve,
You mentioned you checked the timing. The distributor is secured in position?
Regards
Roger
Roger T

The acceleration seemed to occur when I touched the mixture screw (not the idle screw), the distributor was secured. I'll probably take a couple hours this weekend and have a look inthe carbs.

Odds are good I'll be back with more questions, monday if not before.

Steve
Steve Aichele

This is a long shot with a 74 since you don't have a CAT but the starting and running for 10 to 15 seconds or so and then die could be symptomatic of a plugged exhaust system.

In my case, the CAT overheated and lost it's inner parts down the pipe plugging things up. Replaced the entire system to solve that one but it runs wonderfully now.

Greg

This thread was discussed between 05/04/2005 and 08/04/2005

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