Spark plug color differences

[DaveAhrendt]

Hi all,

Is it common for plugs 2 and 4 to be different in color from 1 and 3 on an XPAG? I've just gone through the carbs and have been tweaking the mixture. It looks to me that 1 & 3 seems pretty good while #2 looks rich and #4 looks lean.
Besides the difference in the insulator color you can differences in the ground electrode.
I'm not sure what you can do externally to change this behavior.

What do you guys think?

Dave TC0524

[Steve Simmons]

It isn't entirely uncommon, and could be caused by slightly leaky valve guides or rings rather than combustion mixture. Maybe try a leak-down test and see if 2 and 3 come up with higher numbers.

[DaveAhrendt]

Thanks for the idea Steve.

I just checked the compression and the numbers are pretty consistent but a bit low but that may be my gauge. We used this gauge on my friends TC last week and got the same basic numbers as mine. We tried his old tester that needs two people or three hands- one to push in the plug hole and one to run the starter. His gauge consistently gave about 20 psi more than mine. The car appears to drive fine and has as much power as it used to have.

This is with the throttle closed and all plugs removed.
#1 130
#2 127
#3 125
#4 130

I'm going to be away for a couple of days starting tomorrow. I'll borrow his leak down tester and test when I get back.

[Duncan M]

Keep in mind that gradually over time the head on these engines tends to warp upwards towards the middle. Or maybe it is that the head nuts tend to loosen more towards the middle? Any coolant disappearing?

[Tom Lange, MGT Repair]

And another hand to hold the throttle wide open as you do the compression test...

After you have done the compression test, .squirt a tablespoon of oil in each cylinder and do another test. If you get a significantly higher reading, it will indicate you have worn rings.

A leak-down test is far better, as it tells you where the sealing is diminished.

Tom Lange
MGT Repair

[DaveAhrendt]

Tom,

I didn't bother to keep the throttle open because when I tried this last week or so I had the entire intake manifold sitting on the bench. I wanted to test my gauge as it read about 120'ish on my friends TC which we thought was low. The last time I recorded my compression was a couple of years ago and it was about 145. So in this case it didn't seem to matter whether the throttle was open or closed as I got the same reading +/- as I got without the intake installed. This was also on a cold engine if that matters. I forget the engine heat state on my friends TC.

I'll try the compression test with oil but I'll be really disappointed if that helps as I probably have less than 5K miles on this rebuild.

I doubt this should have any effect but I have a roller cam and roller rockers in this engine.

[Steve Simmons]

The cam and rockers won't matter, but hot versus cold can make a difference sometimes.

[stephen stierman]

Keep in mind that if the head has not been milled the cranking pressure is not going to be all that high.

[DaveAhrendt]

Thought I'd report on the results of doing the leak down test. This was done on a hot engine.

Leak Down 100 psi input

#1 97 psi - 3 %
#2 97 psi - 3 %
#3 98 psi - 2 %
#4 98 psi - 2 %

According to everything I've read this indicates an engine in very good condition. I couldn't
hear anything at the carbs or tailpipe.
I guess I'll just drive it and ignore the plug color for now.

[Steve Simmons]

Those are very good numbers. Basically new engine numbers.

[Duncan M]

Could be some sort of weak spark condition.

[DaveAhrendt]

That's an interesting possibility. Prior to the carb exchange I had dropped back to standard points from a Pertronix system.
I never did get around to checking the plugs after that. I may try to revert back to the Pertronix for a while and see if anything changes.