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MG MGA - Rich Running

Hi All,

I have a problem with a '57 MGA that I'm baffled by.

The car apparently started "sputtering", stumbling and "mis-firing" last summer. Prior to that I'm told the car performed beautifully. The owner, for whatever reason, decided that it needed carb kits to correct the problem. He had an employee that does all his small engine work, install genuine SU kits. After that the car wouldn't start, so I was contacted.

I found that the jet "cup washers" were up-side down, and that the jets weren't centred. Correcting these items would solve the problem thought I. They did allow the car to start, but it still had a serious mis-fire, and was running extremely rich. The mis-fire was due to a combination of badly fouled plugs and an intermittently operating Pertronics ignition system.

I changed the car back to a normal distributor and installed a new distributor cap, points, rotor and plug wires, along with new Champion RN5C spark plugs.

The car ran great for about ten miles, then started mis-firing again. This was due to badly fouled spark plugs. The engine was running far to rich.

I have "leaned" out the mixture to the point where the jet adjusting nuts are all the way up, and yet it still runs very rich. The jets are all the way up, and the choke is disconnected. The pistons in the carbs slide up and down very nicely, the carbs are "balanced", as far as equal draw and mixture, and yet it still runs rich enough to foul the plugs in short order.

Any ideas as to my next steps would be greatly appreciated.
JR Jim) Ross

My favourite for this type of thing is the rear air cleaner box being fitted upside Down. See here on my website: first item.

Steve Gyles

Jim, if it isn't the rear air cleaner fitted upside down as Steve suggests, then it may be that the main jets (which both slide downwards when you pull the choke) are not sliding fully back upwards when you push the choke in.
This would cause the super rich running you describe.

If you see that the jets are still pulled out, you should be able to reach under the carbs and easily push them back in.

If this is the cause, then either the jets are still not centred or the choke mechanism needs adjustment.

Hope you sort it soon

Colyn Firth

Thanks Colyn and Steve. Appreciate the suggestions, but since my original post was so lengthy, I didn't include all of the things discovered and done.

This problem exists with no air cleaners, and with the choke levers disconnected and the jets held up with a rubber band to ensure their inactivity.

Please keep the suggestions coming, since I have no other ideas.
JR Jim) Ross


Check the float levels and reset to 7/16"
Make sure that the float valves are in good shape.
Remove the dash pots and reset both jets to 0.06" below the bridge.
Check that the needles are flush with bottom of air piston.
Do the standard tune up from the above settings.
If the car is still running rich, I would suspect that the upper cork seals around the jets are leaking.

L. Christiansen

Just encountered the same problem with an MG TF. In a past life the jets were not properly centered. Mixture needles rubbing on jets wore the needles and juts to the point where excess fuel was allowed in at all speeds, and it ran very rich no matter what was done to lean it out. Solution was to install new needles and jets.

Do not assume that anything you were told is true. Check the numbers on the needles, and condition of the needles and jets. How accurately can you measure the jet hole that should be 0.090-inch diameter?
Barney Gaylord

Hi All,

I found the problem. Both needles had been mounted recessed from the "bridge" of the dashpots, which of course allowed ext5ra fuel, and the distributor drive had been re-installed almost 180 out of position

After correcting this the car runs nicely.

The reason for the wrong position of the distributor drive was that someone had changed the distributor driving dog, and had reversed it's orientation.

Thanks for all the help.
JR Jim) Ross

Glad you have it fixed Jim, it always feels good when you manage to solve a problem.

I had a similar problem when I switched a failed electronic distributor for a brand new standard Lucas type.
The engine wouldn't fire and when I investigated it turned out that there was about 40 degrees difference between the positions of the drive dogs on the 2 distributors.

I wasn't expecting that!

Colyn Firth


Always good to find the cause and report back for the benefit of us all.

The distributor aspect should only be a red herring. It can be fitted in various positions depending on how the distributor drive spindle is inserted and engaged in the camshaft drive, but provided the plug leads are swapped round to suit it is not an issue. Most of us go for the orientation that puts the vacuum housing in the conventional position.

Steve Gyles

This thread was discussed between 25/05/2016 and 06/06/2016

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