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MG MGA - Sooty plugs - poor starting

I have only had my car for a week, but something is not right. It struggles starting from cold, idles at only about 500rpm and even when hot, can stall at traffic lights unless I rev the engine.
I have had a look at the plugs and they are very sooty. The points have been replaced by electronic system so I guess that eliminates that. The choke cable is incredibly stiff (two hands needed!) so I dont think it can be too much choke either.
I have standard twin SU's but tried to balance twin SU's many years back and converted a sluggish engine to an unstartable one!
Is there a simple way to go about sorting this, or is it something best left to the experts? I would rather do it myself.


It's not rocket science. I learned to do it myself as a kid many years ago. Get yourself a good manual that details the procedure. I acquired a book in the mid '60s on tuning the MGA and Magnette that spells it out clearly. I can't recall the exact name, but I'm sure there are others. You have to be methodical in checking that everything is working well, and not sticking, including the throttle and choke linkages. And the jets must be centered. There is a simple SU tool kit available - from many sources - that has a centering tool and some tubes and wires that insert in the top of the dashpot pistons to balance the throttles after getting the mixture set on each carb. There was a thread recently here about making these balanceing devises, but you'll still probably need the centering tool, plus a jet wrench for setting the mixture. Get a good manual, and go for it.

k v morton

It's all decibed very logically in this nice booklet, available for download (free) here
dominic clancy

Others may disagree, but when starting from scratch or from an unknown mixture strength on someone else's car, I like to use the gunson color tune. It's very good at getting both carbs even and in the ballpark of proper mixture. I then take the car out on the highway and test drive, examine the spark plugs, and ajust the mixture as needed. Stick your hands under the carbs and feel the jets. Make sure they're not stuck out when the choke is off. You may not have the proper needles installed, but cross that bridge later, you should at least be able to set the idle mixture so it's not fouling plugs. I'd also recommend Bosch Platinum spark plugs, they're a good self-cleaning plug that will still fire even if you get them sooty black. Last year I had my mixture set well for highway cruising, but it was a bit rich at idle. I had also switched to a cooler NGK plug. I found that when I sat in traffic, the plugs would tend to foul. Granted I was probably running a little bit too rich, but I switched back to my old Platinums and they didn't foul.
Mark J Michalak

Use Barney's method

Mike Ellsmore

Thanks for all the encouragement. I will have a go then. Just one small question. Do I need to clean the plugs first to get all the soot off, or will they just burn themselves clean? If I need to clean them should I use sand paper or s there a better way?

Thanks for the help. I better check my bike is still working before I kill the car!


A point on the very stiff choke. You may not be aware that to operate, you pull the choke out and twist about a quarter of a turn clockwise to lock in the desired position. Once in this position it is still possible to push in and out, but it is quite stiff. It is just possible that your choke is in the locked position. Try rotating the knob slightly anti clockwise, but don't force it.

Steve Gyles

Do you know if the carbs have been rebuilt? The metering needles and jets may be worn and need replacing. No amount of adjustment will remedy this. Also, on centering the jets: if you slacken the large nut directly under the carb (where the jet goes through),you can remove the piston rod oil filler cap on the top of the suction chamber and push gently on the top of the piston rod. The jet will centre itself. Then you can tighten the nut. You don't really need a centering tool. I have done this myself many times on my MGA, my MGB and my 1927 Morris Empire Oxford. Don't be disheartened by this hiccup. It's not a big problem.
Good luck
M S Randell

Thanks everyone for your feedback. Started to play with it today. I think I need smaller fingers as I found it difficult to remove air filters and even more difficult to replace. To be honest, I got scared to mess about with carbs too much (I am learning so one step at time!) but thanks to Paul's help I just turned down the mixture very slightly, cleaned plugs and adjusted fast idle. It started first time and idles better so I am delighted. Thanks for your help everyone

This thread was discussed between 20/04/2009 and 21/04/2009

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