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MG MGB Technical - Interesting Spark Plug Colors

'73 BGT back on the road after a lot of "issues" this year, but it is running much better.
However, I checked the plugs this morning, and the color variation seems odd. #1 and #3 look like the text book tan color, and seem to be burning nice and clean. #2 shows more carbon - not real bad, but definitely more than #1 and #3. #4 is gray rather than tan.
I know that modern fuels can make color analysis difficult, but I can't figure out the differences. I could understand it better if #1 and #2 were the same, and #3 and #4 were the same.
Has anyone seen this before?
Thanks for any info/insight,

Pat
Patrick Callan

I presume "sooty" rather than oily.. But if rich were the case you would expect at least No 1 to show similar carbon deposits. Sooty can also suggests incomplete combustion. That could be caused by low compression (valves/rings), weak spark (plugs, lead, dis cap) exhaust system not helping to clear the combustion chamber (holes, cracks, poor manifold/downpipe joints). But since 2 and 3 are siamesed the latter would seem unlikely.As for the grey plug No 4, of course it could be running weak but as above you would expect No 3 to show the same signs. Could be being "steamed".
Allan Reeling

2 and 3 run slightly richer than 1 and 4 due to the head and manifold design. 1 and 2 may be about right for colours, but having 3 show up ok and 4 be greyer could simply mean the rear carb is set too lean. Maybe richen it up a couple of flats and go for a drive to see how that changes things. If it's still wrong, well then you'll have to look deeper. Pictures would help too.
Roadwarrior

Come on Patrick, let's have some feed back!
Allan Reeling

Allan,
Sorry about that.
Weather and projects have prevented me from getting back on the road. I adjusted the rear carb to see if a little richer would get #4 to improve, even if it added some carbon to #3.
Just went out this AM for a ride. Waiting for it to cool (very hot and muggy here today), and then I will try to get a picture of all four plugs.
Hope to post more later today.

Pat
Patrick Callan

Ok, if I uploaded it correctly, the figure shows the plugs after my run today (left to right, 4,3,2,1). It is hard to see the color well, but it looks to me like #1 is not a clean as it might be, #2 is showing to rich, #3 is pretty good, and #4 is still a little lean.
So, I will try to lean the forward carb just a little, and richen the rear car a little more, and try again hopefully tomorrow if weather permits.
I should note that today's temp and humidity were both quite a bit higher than the previous day run. Not sure if that impacted anything, but #1, for example, looked cleaner to me the last time.
Hopefully, I am not too far off.

Pat

Patrick Callan

Hi Pat have you considered using Colortune to get the mixture correct and equal in all cylinders?

This is a "see though" spark plug that enables you to see the combustion in each pot as you move the plug to each cylinder. Set the carbs to give a Bunsen Blue (within a range of Blue)in the cylinders.

I moved from sea level to 1100 mtrs AMSL in the Natal midlands and reset the carbs 3 years and 12000kms ago using 2 colortunes in no1 and 4 simultaneously and have not touched the mixture since, running beautifully. Total mixture setting time was under 20 minutes, well worth the cost and improved economy.

Cheers


Ray
Ray Bester

Ray,
I used the Colortune for quite a while, but have had difficulty with it since they started putting additives like MTBE and ethanol in the gas here. I understand that these types of additives change the color of the burn - achieving the correct blue seems hard to do.
I have had pretty good luck with John Twists method, but did the spark plug check recently and noticed that I wasn't "spot-on."
Can you still get no-additive fuel?

Pat
Patrick Callan

Assuming everything else is OK, i'd still say the rear carb is too lean and the front carb maybe one flat too rich. Unless you need some kind of emissions to pass a law, running a little richer is how they like to run.
Roadwarrior

Patrick,
I think it's time you checked a few things; see my previous post, those plugs suggest to me something other than just mixture/balance issues.
1. Do a compression test,
2 Check out the health of the 4 sparks. Either by removing the plugs and observing the spark while earthing the plug, or using a spark tester on the ends of the leads.
3. Check the soundness of the manifold gasket.
Allan Reeling

if you google 'reading spark plugs' you will find a couple of very useful articles. no more to add apart from general agreement with above.
G
Graham Moore

I'm going to have to shelve this one for a while - pressing projects re-directing my priorities.
Condition seems to have improved - one "flat" adjustment at a time.
I will re-post at a later date when I can spend more concentrated effort. For now, it seems to be running well, so I am enjoying my rides through the countryside whenever I can get out.

Thanks to all for the info and suggestions.

Pat
Patrick Callan

This thread was discussed between 05/07/2015 and 13/07/2015

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