reading spark plugs
 

wondering which plugs to pull to check jetting, mark4 502,, stock heads,stock intake with 1in spacer holley 800,496 take off exhaust, roller cam 230,236 @.050 with 547 lift are there traditionally lean cylinders best to check them? thanks Randy

check em all but # 6 is always are 1st choice

#2 and #7 are generally the leanest since they are the farthest away from the carb.

i am a bit confused about this whole plug reading deal...

back in the before times, before egt recorders and the like when it was essential to bea able to read plugs to prevent detonation and get the mixture correct, i had some pretty authoratative instruction from the engineers at champion about how to do this. they were very clear...

1) it is almost impossible to set the mixture by reading the color on old plugs. once they are colored dark and contaminated by residue, the mixture would have to be wildly lean for a long time to make them change back.

2) assuming you are starting with near new plugs, a " clean cut" was essential. that meant that you cut the ignition at max rpm and power and coasted into the pits to read the plugs. if you idled in all you did was artificially darken them and lead yourself to believe you were richer than you thought.

3) once you approached critical mixture, the plugs looked exactly the same on either side of the line except you had to examine them with a magnifying glass to look for teltale signs of detonation. i can tell you that i could change the main jets a full 10 % which , on the dyno made a HUGE difference, yet the plug color looked absolutely identical.

4) the heat range of the plug has everything to do with how clean the plug remains... so... if you are running plugs a heat range colder than what your motor might want as optimum, there is a chance that the color is lieing to you anyway... and the other way around if they are hotter...

but the bottom line for me is that , unlike the cars where you could "coast in" , i don't see any point at all at going out and blasting around to check your mixture if you are then going to idle back in for 30 minutes loading them up etc and then checking them. all you are going to find out ( if anything at all) is what the mixture of your idle circuit looks like at 800 rpm...

you are correct cant idle after ,have to make a 5000 or 6000 rpm pass and keep her under control and shut her down and pop the hatch and start reading ..has worked great for us but with the O2s and pyros makes life easier :drink:

Exhaust temp guages help with this problem,