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MG TD TF 1500 - Spark Plug Color and mixture

I removed the plugs for cleaning and found the rear two very black. I assumed the mixture was too rich on the rear carb and weakened it one flat. I checked the rear two plugs again later and they were still black. I weakened the mixture one more flat.

A friend suggested that the float in the rear carb may have a hole in it, so I removed it and it was fine.

The air filters were not too dirty.

I checked the centering of the jet by lifting the piston and letting it drop. It clicked, so the jet is centered and not sticking.

I checked the plugs again and the rear one was better though blacker then I had remembered it being before this episode, the third from the front still quite black and the front two seemed blacker than I remember, but I am not sure.

Does anyone have any other suggestions before I replace everything and weaken the rear carb another flat? It has been several years since I have tuned the carbs, but the car has run fine and I don't know whether it is normal for one carb to go out of tune. Thanks for any suggestions you can give me. Ron

Ron Coates

Hi Ron,

Carbs don't often go out of tune, and if anything, the throttle shafts wear, and instead go lean instead of rich.

You may have an oil problem not a richness problem.

I would warm up the engine and set the valves, and then do a compression check. If that is even amd reasonably high, I would swap some plug wires. If not, I would do a wet test and see if the rings seal with oil injected in the cylinders. If not, probably burnt valves.

Most of the time poor running or combustion is compression or spark, not fuel.

warm regards,
dave
Dave Braun

HI Ron,

Dave B makes some good points re checking compression etc before altering carbs, but you also need to tell the difference between oily plugs (oil past rings, valve guides) and rich mixture plugs, normaly carb related. Lot of info in Archives, but if carb related likely to be internal seals, or jet too low. Float chamber position can be swivelled and also affects fuel level, again all this is in Archives.

Suggest you do one thing at a time, having firstly ruled out engine problem as per Dave B.

Good luck, John.
J.C Mitchell

I would still suspect the carb, or air filter on that carb. Maybe the jet sticking down a bit? I'd make sure the jets are up, run it pretty hard with fresh plugs, and check again. If still a problem, switch the air cleaners, if you have separate ones. You could do a carb tune between all of these steps, but try to do one thing at a time to isolate the problem.
Tom

I think Tom was right. The jet lever spring was not pulling the jet head up against the adjusting nut very quickly and not all the way. I pulled the jet lever down so the jet head was as low as it would go and smeared some petroleum jelly on the jet. After moving up and down a couple of times it was very free. I did the same on the front carb. I retuned the carbs and the car runs fine. I hope the next time I look at plugs they are clean. Thanks for your help. Ron
Ron Coates

This thread was discussed between 11/06/2008 and 12/06/2008

MG TD TF 1500 index

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