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MG MGA - Carbon and exhaust

Hello, It has been several years since I have been on this site. I own a beautifully resotred 57 MGA. I have owned it for 9 years. It runs great. Starts easily, idles smooth, good pickup and no pinging etc.

The only concern is black soot comming out of exhaust. The pipe is always black and will spit the soot out onto the floor on startup. I have adjusted the carbs and they appear right. Is it running too rich? Any suggestions appreicated.

Dave in Kentucky
D Gates

Dave, check the plugs colour. If they are black as well then you are too rich.
Art Pearse

The carbon is caused by an incorrect burn of the fuel air mixture. The most likely cause is a too rich condition. It could be as simple as leaving the choke on too long upon start up. This richens the fuel mixture wich causes the carbon buildup. If you look at your spark plugs, you will probably find that the plugs are either fouled or full of carbon, eventually this will lead to fouled spark plugs, carbon buildup on the valves, cylider head pistons etc. Excess carbon build up can cause dieseling (engine run on) after the engine is shut off due to hot spots on the carbon.
JP Jim


Black exhaust and soot on the floor at start-up is a typical response in properly tuned H2 and H4 carburetors. If you are getting good mileage, and nice power, don't worry. Basically, the enrichment cycle is pretty strong, and the fast idle circuit to deal with the richer mixture isn't adequate when a low normal idle (650-800 RPM is my choice) is sought. You will probably find that you can go full rich on start and immediatly after start go to the notch just before no choke, and get smooth running, even smoother than you have on say, half choke. You may have to blip the throttle a bit to keep the engine running until it idles for a few minutes.

If you want to check your plugs as Art suggests, run along at 3500 RPM (or so) and shut off the engine. The idea is to have the spark plugs at normal running condtion when you shut them down and check them, as some needle sets tend to run a bit rich at idle.

Dave Braun

This thread was discussed on 12/11/2009

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