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MG TD TF 1500 - Carb Tuning Using Spark Plugs



I have been running rich. A situation exacerbated by lots of short runs to work. Gives a thick sooty coating on the plugs and eventually starts to run rough and misfire.

Timing is right + running a pertronix.

Read all the stuff about carb balancing and tuning and listening through a tube etc and thoght I could cheat.

All the plugs were covered the same so I concluded that the carbs were balanced - both running equally rich.

I tweaked the mixture down a quarter of a turn on each. Engine runs fine ( perhaps better).

Pulled the plugs again after two days and here is what I found.

Plug 1 is a nice light brown ( if anything it is a little lighterh than it appears in the photo)

Plug 4 is better but still a little rich looking

Teaked the rear carb down a further smidgen tonight and will pull the plugs again in a day or two.

Question - am I bonkers as this seems a lot easier than all the rest of the faffing around suggested in the manual ?

Anything I need to be wary of ?
Dave Moore

The way you are setting your carbs for mixture is the way I usually do it. I think is the most reliable one because it really shows you what is going inside the combustion chamber...but...
You should do it in an nice country road, drive the car for 3 or 4 miles, do your settings, drive the car, do your settings until you get the rigth color.
Of course just driving the car to work will show a rich mixture because o the traffic, maybe overheating, maybe you pulled the choke for a long time, etc..
it will never be an accurate reading, while if you do it in a country road you will not those problems....
Besides if you have a nice brownish color in your traffic settings once you take your car to open road it will be too lean and have overheating problems and probably even perforate your pistons.... you will not have enough fuel in open road conditions....
Jose Vicente Vargas

dave, my experience has been that once i became familiar with the book procedure it takes far less time than the trial and error method you described here...and i'm not the ace of the base.
how are you synchronizing the carbs? perhaps i am mistaken, but i believe the maintenance manual faffing assures the same mixture through each carb and the same VOLUME of the same mixture through each carb.
if the carbs don't have any leakage problems the whole book procedure is 15 minutes. best of luck. regards, tom
tom peterson

Dave ,
One thing I learned from this BBS:
If your not doing this.
Run the car "at speed" ...shut it down and coast it in...then check the plugs.
You get abetter idea what is going on it you shut her down this way.
David Sheward

I think I'll be ok on the volume since all the plugs were evenly covered. I'll double check the air flow using 'the book' and a peice of hose pipe :-)


Good advice on a longer run and swithch off and coast.

I tried before to attach a pic taken off a shortish run and then some meidum density traffic back to my house..I think you will see that the front is probably too lean...

I think I'll tweak up the mixture a tad from where it is now on the front carbie to start with and then take to the road for a continuous run and tune session.

Thanks as always guys.



Dave Moore

This thread was discussed on 06/07/2010

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