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MG MGA - car running too rich

My Mark II runs very rich. The plugs are always carboned up. The car seems to run ok but it blows black smoke when you step on it. How do I get the cars to run leaner. Do I turn the nut up on down. It seems to me that up would be the way to go but it is all the way up tight against the spring and it does not seem to make any difference.
GK George

GK, have you checked that when not in use the choke cable is slack and the lever arm not inadvertently operating the jet?
J H Cole

If its running that rich, and its not the choke, then you will probably need to change the needles.

I seem to recall FX are standard

J Bray

check that the jets return completely after pushing in the choke cable.If the leather washers inside the jet bearings are dry the jets can stick and you are running to rich.Solution is to dismantle the jet bearings and to put the leather washers in oil to soften them.Afterwards you assemble everything and center the jets.
Thats what I had with my newly rebuild carbs.

Good luck,
H. Jorens

To lean off the mixture you turn the nut upwards, if you are doing this with the engine running then always push the jet upwards to ensure it is actually moving to the new set point.

I would recommend you first take a look at the float height on both carbs. If the level in the bowl is too high you will always run rich and adjusting the mixture will not work.

The way I set the float height is to take off the dashpots and pull the choke out to lower the jets by 1/8". Then turn on the ignition and look for the level of fuel above the jet top. If it is clearly over the jet then lower the float height a bit and try again. You may need to take a little fuel out of the bowl each time (to give a good starting point).

The ideal is for the fuel to sit just inside the top of the jet when the pump stops ticking.

To check out your needles then one way is to carry out a set of "Plug Chops" basically you run the car under several different conditions then shut down the engine and coast to a halt. Then you can see the mixture on the plugs at that operating area. I use wide open throttle up a long hill, steady fast cruise (65-70 mph), then slow cruise (35-40 mph).The last one is normal idle. This will give you 4 readings of mixture which you can put into a needle selection chart to find a better setup.

Hope this is some help


P Reardon

The jets can wear, especially if the needles are "rubbing" on one side or the other. I have said this before, SU carbies are one of the simplest known to man but if they are not 100% your car will not run properly. They are not cheap to recondition (properly) but it is probably money well spent.
Barry Bahnisch

I had the exact same problem several years back. If you have the same fault as I had, it is as others have suggested above, the choke was sticking on. I found that after I used the choke, although I pressed the button back in, the choke levers did not return.
Try pulling the choke out, then pushing it back in and then see if the levers have returned back by applying pressure to the levers by hand from under the bonnet.
Good luck
Graham M V

Best of all buy a Gunston and look into the inferno itself ! They are very easy to use, you see the actual burn colour in the cylinder.... great bits of kit.
Neil Ferguson

When I put the carbs back on I noticed that the choke cable not only had no slack, I had to pull up on the choke leavers to get the cable in. So I adjusted the cable and it seems to be running better now. Thanks for the help.
GK George

This thread was discussed between 27/03/2012 and 28/03/2012

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