What are my plugs saying? Comp. results

[Timothy Bond (41147)]

I wanted to try out a new compression tester, and maybe find out why my 67 327/300hp car seems to be idling more roughly and uneven lately than it has been. I noticed that some of the plugs are not as "normal" looking as they usually do, so maybe something is up?

My compression results are:

Left bank (front to rear): 171, 173, 168, 175 psi

Right bank (front to rear): 170, 172, 175, 165 psi

Left bank plug photos (left to right is front to rear):


Right bank photos (left to right is front to rear):


As you can see, cylinders 7 and 2 seem to have noticeable different looking plugs.

Also, since I thought the compression in cylinder 8 (rear of right bank) seemed lower than the rest, I squirted oil through the plug opening and ran the check again at got 196 psi! I did the same for cylinder 7 (rear of left bank) and got 191 psi. How much increase in pressure would be considered normal? Also, what would be a normal "expected" compression reading for my engine? My engine has seem very few miles (maybe 1000) since I (as a youngster) replaced rings about 15 years ago. History of the rest of the engine is mostly unknown. Heads have been recently rebuilt with new valves, etc.

Plugs are AC Delco R45S, as recommended in this and other forums.

Advice? Comments?

[Duke Williams (22045)]

Re: What are my plugs saying? Comp. results

Nominal cranking compression pressure for the '63 300 HP engine is 160 psi with 20 psi variation allowed. You're within that range. You should check your CSM to see if there is any difference in spec for '67. As long as the engine idles smooth and has good SOTP power and throttle response, I don't see any major issues.

The plug that has a dark grey insulator appears to be a bit gas fouled. It could be a mixture distribution issue, and you should go through the idle speed and mixture adjustment procedure in your CSM.

Adding oil will usually raise compression pressure. You should have done all to look at the comparative data on all cylinders, since it's comparative data rather than absolute numbers that tell the story.

I've come to believe that leakdown testing is a better way to check cylinder health.

The brown stains are likely deposits from fuel detergent to other additives, which are normal. The generally clean appearance of the insulators without signs of glazing means the plugs are the correct heat range for your driving conditions.

Duke

[Dale Schafer (48165)]

Re: What are my plugs saying? Comp. results

You might check the spark plug wires, especially two I see. Dale

[Mike McCormick (1498)]

Re: What are my plugs saying? Comp. results

I agree with Dale. Cylinders 7 and 2 fire sequentially (1-8-4-3-6-5-7-2), but are located on opposite sides of the engine and are fed from different carb venturis. I would look for something in the ignition system, bad wires (leakage, cross-firing) or a bad distributor cap.
Good luck,
Mike

[Dale Schafer (48165)]

Re: What are my plugs saying? Comp. results

I think that after checking the ignition like Mike says, I would then put umbrella seals on the exhaust valves, since compression is good. We use to use Studebaker seals but now Fel Pro has them listed for Chevrolet. This way you don't go into the motor until you have exhausted (Pardon the pun)every thing outside the internals of the motor. Carb.,ignition, valve seals. Dale

[Timothy Bond (41147)]

Re: What are my plugs saying? Comp. results

Well, I found my problem. It appears to have been the short wire in between the coil and the distributor, probably combined with mis-adjusting the carb to try to compensate for uneven idle.

I checked all plug wires and they were all reasonable, ranging from about 6K ohms up to just over 10 K ohms for the longest runs. I did find some corrosion in three of the plug boots, but not necessarily on the plugs that shows black carbon buildup. When I checked the coil wire, it read 14K for only an 8 inch piece, and it would change as I wiggled it. Turns out the center conductor was broken where it bends back under the crimped connector. I cut the wire, opened the connector and crimped it again shorter and now the resistance is about 2K ohms.

After readjusting the carb idle screws and rechecking dwell, it appears now that the idle is smooth and even. Thanks for all the helpful posts!!

[Dale Schafer (48165)]

Re: What are my plugs saying? Comp. results

Tim, First I apolgize for saying put umbrella seals on exhaust valves I meant to say intake. I am not sure you have found your whole problem. I don't think a coil wire would make one or two plugs look like that,(coil wires don't pick cylinders). I still suspect the intake guides on those two cylinders. Lets hear from some others on this, as there is a wealth of knowledge on this site.. Dale

[Timothy Bond (41147)]

Re: What are my plugs saying? Comp. results

You could be right. I will check the plugs again after some time and see what they are looking like. I think I had the carb adjusted out of whack to try to compensate for the coil wire problem.

[Duke Williams (22045)]

Re: What are my plugs saying? Comp. results

What's the rate of oil consumption?

Duke

[Timothy Bond (41147)]

Re: What are my plugs saying? Comp. results

I am not aware that it uses any oil to speak of. Rings were changed about 3000 miles ago (but many years). Machine shop worked the heads about 800 miles ago, with new guides, valves, seats, seals, etc.

It is possible that my bad coil wire isn't really associated with the plug fouling problem, as pointed out. At first I thought that it couldn't be associated, but then I realized that the coil wire has probably been bad (creating intermittent connection) for some time, and I have been probably trying to run the carb idle screws too rich to get it to idle. Maybe that could be the association. I know that now the engine idles smoothly, and all cylinders are firing as I pulled on wire at a time and each one results in the same rough idle change. I also checked and the cylinder that had the worst carbon fouled plug now has a bright white/blue spark that will jump a good 3/8" gap to ground.

This set of plugs is relatively new (AC-45S), changed probably only 6 months ago. Prior to that I had a set of Champions in there (can't recall the number), and when I pulled them out all looked good and the same. Sometime recently is when I noticed that my idle had deteriorated, so that is probably when the coil wire went bad.

[Duke Williams (22045)]

Re: What are my plugs saying? Comp. results

Clean up the dirty plugs and reinstall them. If the engine operates as a good 300 HP engine should - smooth idle and lots of low end torque, don't go looking for problems.

There's a good chance that your carb adjustment to compensate for the bad coil wire caused some mixture distribution issues. With good plug wires and a properly adjusted carb. the plug appearance should be more consistent the next time you pull them out.

Enjoy driving the car and don't worry.

Duke

[Timothy Bond (41147)]

Re: What are my plugs saying? Comp. results

Now THATS the kind of advice I like to hear! :-) Thanks Duke!