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MG MGA - mixture problem (?)


I just got my MGA back on the road recently. I was having carb problems and had a local mechanic tune the carbs. He seemed to do a very good job, and another mechanic who didn't know him even commented on how good a job the first mechanic did.

Yesterday I ran a rubber tube from the carb manifold to the distributor (pertronix), which with my crossflow head was only about 3 inches away. It seemed to be working even better. I estimate that I drove the car about 70 miles before I had any problems.

I was driving the car tonight and the engine was losing power and sputtering a little at high RPMs. I pulled the spark plugs out and all four were covered in black soot, so I'm assuming they were fouled.

I don't know what could have caused this, but I have an idea. They first few times I started it I flooded the carbs with gas because like an idiot I didn't know what I was doing. The engine started but would run rough for a while and sometimes I would have to wait a couple of minutes. Is this enough to foul the plugs. If yes, why did they only give me trouble tonight when I started it correctly?

If that doesn't make since, I have a couple of other theories:

1) The carbs weren't adjusted correctly. I doubt this, though, considering the fact that another mechanic commented on how well they were tuned.

2) Air is leaking into the manifold from the rubber tube I put on. (Wouldn't this make it too lean, though?)

3) I've been having fun and revving the engine to 5,000 RPM's when accelerating and that caused problems...

Any ideas would be welcome.


Darian Henderson

PS To clarify my poorly written sentence, I drove the car about 70 miles since the mechanic adjusted the carbs, and about 15 miles after I ran the rubber tube to the distributor, before I had any problems.
Darian Henderson

As your car has spent many years off the road and has only done a few miles recently, it might be picking up rust particles from the fuel tank and fuel lines. These particles cause the valve in the top of the SU bowls to leak.
Do you have an inline filter just before the SU's?

Mick Anderson

I have a fuel filter before the SUs. No air filters, though. (They're in the mail.) I have a glass fuel filter and have noticed particles in it.


Darian Henderson


What happens if you remove the vacuum tube - adding this will have advanced the timing even at idle, so you really need to wait for the air filters to arrive and have the car retuned with the vacuum advance so the mechanic has everything correct again - adding the air filters will cause it to run richer too, so he will have to adjust the mixture.
dominic clancy

Don't forget that most "carburetor" problems end up being Ignition problems. I have found that once properly set up carbs give very few problems. Other than dirty fuel and sticking or sinking floats not much goes wrong.
R J Brown


So advancing the timing makes the mixture richer? What if I just marked where the mechanic adjusted it with a marker and made it a little leaner?


Darian Henderson


That's not what I wrote. Let me try again.

1. Adding the tube brings the vacuum advance into operation, so the timing changes, which will affect running. The tube has nothing to do with mixture, just timing. It brings lots of advance into play at high revs.

2. Adding air filters will cause the car to run richer.

Adding tube and air filters will affect the tuning to the point that a readjustment wil be very beneficial to the smooth running of the engine.

If the car is spluttering under hard driving, and the plugs are black it could be:

a) it is running much too rich at these revs. Carb needles?
b) it is an electrical problem that shows when you are running hard. Unlikely to be fuel pump because of the black plugs, More likely to be ignition electrics related
dominic clancy

Hello Darian.
Did you check the jets to be sure that one or both are not stuck partially down after using the choke? If you don't know how to do it push up firmly on the choke linkage under each jet. If you feel any movement at all, the jet was likely stuck.
This can cause a very rich mixture, as I once discovered.
Good Luck,

It may mot be a mixture or ignition are your valve guide seals? The car sits, oil drains down from the valve area, down the valves and accumulates in the cylinders. When you start the car you burn the oil and deposit soot on the plugs.
Rob Carleen

Dominic I think you will find that adding the vac pipe will add advance at idle and will lose that advance at high RPM or with the throttle open. Advance at high revs is achieved by mechanical means
Bob (robert) Chairman north lincs and district pigeon fanciers society

While you have this thread open, I already found in the archive that GS is the factory needle for the 1500 MGA. This would account for why it is sooting its plugs on No 6s, but not for what they are doing in there in the first place. Has anyone on here fitted a Peter Burgess Econotune head to a 1500 A? If so would No 6 be good choice? It's probable that this head will go on the car to give it a bit nmore urge without spoiling its present unmolested condition (not my car, I have a B, it's owned by a regular at our natter who does not understand it to this level). I looked at them on Minty and the No 6 may be bit too rich at the higher stations.
Stan Best

Thanks everyone. I've marked the original settings with a black marker and have been playing with the mixture a little. Hopefully I'll get this sorted out.

Darian Henderson

A couple thoughts:

A lean mixture causes white plugs. A lean miss causes black plugs. Don't rule out a lean mixture. Preignition causes white, melty/broken plugs-probably not your issue.

If you take the advance hose off, does the problem go away? If so, skip the next two paragraphs.

From your symptoms, I'm with Ralph- something is stuck in the carbs and the vacuum advance is a red herring.

If not, I'd try a different coil. They are cheap and easy to replace. The Pertronix may have stressed your old one beyond its limits.

By "Pertronix", do you mean a pertronix unit inside the original Lucas dizzy? On points cars, there is a little wire that can break or short. Pertronix eliminates this wire but there may still be issues with binding, shorting or grounding.

If the carbs or coil don't fix it, I'd put my money on the Pertronix install causing a phasing problem-the rotor misaligned with the cap electrodes. This would cause a miss or cross fire at either minimum or maximum advance. Most likely causes are the unit itself or the rotating shutter positioned wrong.

Hope this helps,

Bill Eastman

This thread was discussed between 29/09/2007 and 04/10/2007

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This thread is from the archive. The Live MG MGA BBS is active now.