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MG MGB Technical - Starting/Idling Problems

I'm having trouble with my car...it has been idling at about 500 rpm, unless I pull the choke out a bit while at a light or stop sign. Now it does not want to start at all, and when it does, idles at about 400 rpm and then dies.

So far, I've changed the spark plugs, wires, coil, battery, and alternator trying to fix the problem...all to no avail. The car has a lumenition ignition system and twin carbs.

Any ideas? Perhaps the carbs or the distributor?

Chris

Did you try turning up the idle speed?
.

I did, but it didn't help.
Chris

Is the timing correct?
.

It was last time I checked it (in October). Since I can't get the car to start and idle I check it now.
Chris

Chris-
Here's a few things I'd check...I'm assuming that the starter turns over with the usual amount of vigor; it's simply a matter of the engine not firing.

Remove the dizzy cap and make sure the inside is dry. Wipe it out as needed and/or use a moisture displacing aerosol spray (silicone-type).

Are the plug leads connected in the right order?

Pull all the plugs from the motor. Are they still new-looking and correctly gapped? If the enging has been causing you problems and you've been relying on the choke to keep it running there's an excellent chance that the plugs are covered with soot and not able to produce an acceptable spark.

Pull the leads from the motor and connect one plug to one of the wires so you can monitor for a spark whilst a mate cranks the motor. If there's no spark you can work backwards to the dizzy and coil.

If the electrical bits are indeed dry and sparking, then look to the air/fuel.

Is there actually clean/fresh petrol in the tank? Is the fuel filter allowing petrol to pass thru? Is the fuel pump working?

Are the air filters allowing air to pass thru or are they dirty and obstructed (over-oiled if using K/Ns)? Still unsure of the filters? Take them off and try to start the motor.

Re connect all the plugs and leads. Spray a shot of engine starting fluid and crank the motor. No choke and maybe half-throttle.

I'd be curious to see how you are getting on with this.

If I may make a recommendation..."Your expert guide to MGB and MGB GT problems and how to fix them" by Roger Williams is one of the more brilliant books I've found on the MGB.

Good luck

Matt

M Ruch

Chris. It might be good to know what model year your car is, what carbs you have on it and where you live. There were three, very different carbs used on the MGB over the years. The starting procedures vary, depending on the model year and the carbs currently installed.

I do not have any experience with the mid-range models having the HIS-4 carbs. I do have extensive experience with the HS-4 carbs installed on the early versions and the Zenith-Stomberg carb installed on the later models.

Thus, it is important to know the year of the car and, if you can recognize it, the type of carburetors on the car.

My experience with the MGA (H series SU carbs) and the 68 MGB GT (HS series carbs) has been that they would start and run well when my American cars would not. My experience with my original 79 MGB, with a Z-S carb was that it did not like to start in cold weather, even when the factory procedure of allowing the pre-heater to warm for 30 seconds was followed. Add to this that most of us have replaced the original Z-S carbs with a Weber DGV or DCOE and you can see our problem.

We cannot advise you unless we know what your model year is, what carb you have and where you live.

Les
Les Bengtson

My $.02

Vacuum. Vacuum Vacuum

Actually $.03
tom

The first port of call should always be 'what did I do last'. But if this started on its own, and seems to be developing gradually, it looks more like something drifting out of tune. Changing stuff without diagnosis (if that is indeed what happened) is expensive, and frustrating when it doesn't help. The first thing that should be done is to check valves, plug gaps, points dwell (if appropriate) and timing adjustment and centrifugal and vacuum advance mechanism operation. When all of those have been eliminated start on the carbs, checking choke operation and perhaps more importantly release, then doing a full air and mixture balance and setup from scratch. If the mixture has to be turned far from the starting position of two full turns down froim being flush with the bridge, or you have to move one carb more than the other, then a carb or inlet problem like a vacuum leak is indicated.

If turning the idle screws did 'nothing' then either you turned the wrong screw, turned it the wrong way, or there *is* some major defect with the carbs or linkages.

Electronic ignition can be a swine when it starts playing up as pretty-much the only thing you can do is substitute it. But generally they either work or they don't.
Paul Hunt 2

Chris-Are you around? I'd like to hear what you found.
M Ruch

Haven't had a chance to look at the carbs, but I've eliminated everything else.

Paul, I agree with you about changing stuff without diagnosis...I did a bit of that trying to fix this problem and it was frustrating. It's certainly like shooting in the dark.

I'll post this weekend once I figure it out.

Chris

To back up what Paul has said, the usual cause for poor running that gradually gets worse until it won't idle is the points gap has healed up due to wear on the heel of the points.

I used to drive my mini until it wouldn't idle, then regap the points. An MGB is so easy to work on in comparison.
Martin Layton

Chris
Check for air leaks between carbs and the inlet manifold, (there are two nuts that secure each carburettor).
Dont assume that new components are not faulty. I had a new spark plug that was faulty and caused the same symptoms.
Robert
R J Collier

Turns out that their is no fuel in the bowl of one of the carbs.
Chris

Ah, float valve stuck closed then.
Paul Hunt 2

This thread was discussed between 22/01/2007 and 26/01/2007

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