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MG MGA - MGA Spark Plugs

I am struggling to get a 57 MGA to idle smoothly and run leaner. I've rebuilt the carbs and checked for air leaks and all seems to be OK. Does anyone have photos of what the plugs should look like after a short run with new plugs?

Mine are turning black with dried carbon after a short 15-20 minute run with new plugs, is this correct? Attached is a photo of the progression - the plugs to the right are after the run.

Bob Abrams


For several year, like you, I could not lean out my plugs. I eventually sorted it by putting better quality filters in the canisters. Those that came with the canisters (made in China I suspect) were of very dubious quality. I forget the name of the filters (problem with my age), but others on this site will know because I followed their advice.

Steve Gyles

You could try running with the airfilters removed in case these are causing the richness. The plugs should be a light brown colour when the mixture is correct. Try driving hard on a road for 5 minutes or so, then pull over and take the plugs out to look at them, i.e., don't idle at all.
P. Tilbury

Are you sure your carbs are too rich. Is it possible you are pulling oil in from worn valve guides? That will make your plugs ugly in a very short time

Thats my next big fix

R Egge

I can smell richness in the exhaust. I also get an excessive amount of black smoke out of the exhaust when accelerating especially after idling for a bit. I'll try the suggestion of air filters off and run for 5 minutes with no idle and see what I get. I assume I should start this process with clean plugs versus trying to burn off the carboned up plugs - correct? Thanks to all for your suggestions.
Bob Abrams

A few years ago I changed the filter elements oil wetted in my Vokes & Beck cleaners with K&N elements as per Barnely's details. The air fuel mixture immediately became much leaner and the plugs went to almost white. I rapidly had to richen up the jets. I suspect there are a lot of asthmatic MGA's out there that could benefit from decent breathing.
J H Cole

Like you I struggled a bit for a year or so with black plugs, black/blue smoke, excessive oil consumption and rough idle. My fix was K&N filters, new dizzy and Sealed Power ST2001 valve stem seals(as per Barney's suggestion). Now she idles smooth, plugs are light brown, no smoke and no oil consumption problems.

Kris Sorensen


Thanks for the suggestion. It appears to be black smoke but Im set to check the valve seals. Compression is 135 140 on all cylinders but not sure how to check for valve seal leakage any suggestions?

Can the seals be changed from the top side if I use a compressor in the cylinder to keep the valves closed?

Thanks again,

Bob Abrams

Looking at your plugs i would not suspect the valve seals as they do not apper to be oiled at all, only rich. You can use compressed air or fill the cylinder with rope and turn the engine so the piston holds the rope against the valves. You appear to be just running rich from idle to crusing speed. Just take the jet adjustment nuts up a flat at a time and then recheck the plugs.
J Heisenfeldt

No I don't know other than to remove the head and have a machine shop check it. Others may have a suggestion. My head had been done a year before so I knew the clearances were OK.
Yes I believe you can change the seals using compressed air. We did the rope down the spark plug hole routine and it worked fine. Still not an easy job to remove the valve springs and all the bits and get them back in place. We used 2 screw drivers to compress the spring while the other person removed the other parts and installed the seals then compressed the spring again and put it back together. It was a bit more involved than that but that's the general idea. Thank goodness we only had to do the 4 intake valves.
Kris Sorensen

Stock valve seals on an A are just "O" rings under the top. A seal that attaches to the valve stem can be adapted. NO need to pull the head to check for seals or to check play between valve guide and stem. The head only needs to be off if guides are worn and need replacing. With the proper tools the seals could be replaced and the guides checked in 2 or 3 hours.
R J Brown

When you rebuilt the carbs, did you just replace all the gaskets, or did you also replace the needles and jets? I've seen jets worn oval that will never lean down enough.

If you took the needles out of the pistons, did you replace them (or new ones) with the flange flush with the bottom of the piston? Recessing them would cause an over rich situation.

Did you get the right needles? Memory says GS, but you should check that.

Have you adjusted the nuts right up to the point where they stop against the jet holder, and is it still too rich?

One trick. The jet will be recessed into the bridge a little. You can 'cheat' and lean the whole range out a tad by resetting the needle so that the flange above where the taper ends is actually a little proud of the piston face (only a tad, please, maybe 1/8"). Don't do to much or the flange may hit the top of the jet. See if that helps.

That's about all the reasons I can think of offhand for rich running. Good luck.
Bill Spohn

Thanks to all for your input. I only replaced seals and am running with the needle and jet that were in there when I bought the car. The needls have 2 different numbers on them (226 and 231) but am not sure what this means. Should I try replacing with GS? I am located in Florida so at sea level.
Bob Abrams

I would replace the needles for sure - and make sure the holes in the jets are dead round (check using the shank of a suitably sized drill bit if you can find one)
Bill Spohn

The pictures look like a mixture problem rather than an oil-fouling problem. I couldn't cross reference those numbers with any SU needles. You can check these with a vernier caliper against specs or replace with the correct ones for your engne, GS is standard for your engine if it has not had severe modifications.
Inspect (with a magnifying lens) the sides of the needles near the widest part to see if it looks like the needle was hiting the inside of the jet. If so, it might be best to replace the jet as well, otherwise, check the jet for roundness with the drill as mentioned previously. These parts are to ten thousanths tolerance so a little damage is significant.
Just for the heck of it and while you have your vernier out, take the suction chamber and piston off and measure the jet drop (distance below the top of the jet bearing)for each carb with the jet pushed up against the jet adjusting nut. You should have the same reading for the front and rear carbs, about 0.053" would be about right if you have the correct needles. This would be a starting point. Further tweaking can be done by checking the color of the spark plugs after driving. The plug to the far left is about right to a bit rich in terms of color. if all of your plugs looked like this or a bit lighter you would be good to go.
All of this is predicated on other things being correct, pistons not binding in the suction chambers, throttle shafts not worn, float chamber rubber grommets not deteriorated, floats not too high and fuel pump pressure not excessive i.e. not an aftermarket fuel pump (2-3 psi, max).
TJP Pollak

Ok .... first and foremost I am not too adept at mechanical issues with my 1600 and usually defer to others far more knowledgable than I, but I will relate a recent experience similar to the one described here. If my fix is the answer then it's more my luck versus my expertise. I had exactly the same problem with fouling plugs and "rich" smelling black exhaust. I had installed brand new (not rebuilt) carbs purchased from Burlen a couple of years back and other than setting the mixture, choke, throttle and balance did nothing else. I assumed that being from the factory and all they would be set up right internally. My plugs were always coated with a thick black carbon film. The one thing I had never done was check the float bowl settings. When I did just this past week, I found that the adjustment was so far out that the floats were not making contact with the needles to shut the fuel off and hence it was passing directly thur to the carbs continuously. This would seem to finally explain my hard starting and flooding situtaion as well as my having to stop twice as often as other cars I was travelling with to re-fuel. I followed the instructions in the shop manual and set the float levels using a 7/16" rod as a gauge. I then took the car out for a spin. The difference was immediate. The car ran like it had never run before and when I pulled the plugs after the initial run they were a nice toasted tan color. It had not only fixed the problem but cleaned the fouled plugs as well. With the current price of gas I would think I've just saved myself a pile of money. Hope this may help !!

Mike 1960 MGA
Michael Hosier

This thread was discussed between 08/05/2008 and 31/05/2008

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