Spark plug heat ranges

[Steve Bakken]

SO I DECIDED TO CHANGE CAP, ROTOR, AND PLUGS ON MY OLDS 455. THE ENGINE TENDS TO DIESEL A BIT WHEN I SHUT IT OFF, SO I DECIDED TO GO A HEAT RANGE COLDER ON THE PLUGS. THEY ONLY HAD A SET THAT WAS TWO RANGES COLDER, SO I DECIDED TO TRY THEM. I FIGURED IF THEY'RE TOO COLD, THEY'LL JUST FOUL OUT SOONER. THE FIRST TRIP OUT, THE CAR RAN VERY SLUGGISH. UNDER HEAVY THROTTLE IT SNAPS OUT OF IT AROUND 2500 RPM'S. IS IT POSSIBLE THAT THE PLUGS ARE CAUSING THIS, OR SHOULD I BE LOOKING FOR SOMETHING ELSE THAT MAYBE WENT AWRY WITH THE CAP AND ROTOR CHANGE?

[Jeremy airjer W]

Why not put the old plugs back in and see if the problem persists. If it does not that you know your on the right track with the plugs.

[Steve Bakken]

I sure wish I could. After changing plugs, I saw no reason to keep the old ones so I threw them away. And on an old car that only gets driven once every couple of weeks, by the time I realized I had a problem, they were long gone.

[RebelSS]

What year is it? Obviously, prior to '76. HEI ignition or points?

[Macgyver55]

Most often on carbureted cars "dieseling" is caused by the throttle being open too much at idle. You"ll need to first adjust the mixture screws to get the idle as high and smooth as you can, then back off the idle speed screw to lower the idle to the suggested RPM, or as low as you can reasonably set it so it will idle in gear without dying. It can be a bit tricky on modified motors with bigger cams and such, but more times than not it will cure the "run on" just by lowering the idle.

[Steve Bakken]

I should have thrown out a bit more info at the beginning. It's a '76 block with HEI. the carb is a Quadrajet from Year One for a 1970 442. I have adjusted the idle mixture as rich as they will go, and the idle speed is at 400 RPM's but with a fairly healthy cam and exhaust, its still a bit on the lean side. It has always ran great for the last ten years but I thought I would try a bit cooler of a plug to see if I could band-aid the lean idle. I just don't see how two steps colder would affect the actual way it runs. The spark should be the same, just the actual temp of the plug itself should be a bit cooler, right?

[Macgyver55]

Quote:

The spark should be the same, just the actual temp of the plug itself should be a bit cooler, right?

Correct. Changing a heat range hotter or cooler should not noticeably change the "seat of the pants" feel on an average street car. The use of hotter and cooler spark plugs in the past was more of an effort to control pre-detonation (pinging)on hard acceleration and carbon build up on the plugs than to stop "run on". Carbon build up itself can cause run-on, but you still need fuel entering the cylinder for it to occur. Timing that is too far advanced can cause detonation and higher cylinder temps but when that is occurring your throttle is open. Does the car "ping" when accelerating? Do you use premium fuel?

Pre-detonation and run-on although similar are slightly different things. One of them happens when there is spark and one happens when there is no spark. In order for a motor to run-on there must be fuel entering the cylinders after it is shut off. If you are too lean at idle, then your throttle plates need to be held open more than necessary to allow enough fuel in to attain the proper idle speed. Getting the idle mixture correct so you can adjust the throttle plates to be closed more at idle will prevent the air/fuel mixture from being able to enter the cylinder. That will frequently cure the problem even with the recommended spark plug unless carbon build-up is an issue.

Quadrajets can be tricky carbs (I've rebuilt literally hundreds) but when properly adjusted can be a great performing and dependable carb for street machines. I personally have on on my own hot rod!

As far as the sluggishness up to 2500 RPM, have to checked the mechanical and vacuum advance mechanisms in the distributor?

[RebelSS]

I'll agree with what Macgyver55 has said so far. Q-jets (aka "Pukejets") are great performing carbs when properly jetted and dialed in. My last 3 'rods had them. Anywho, as stated, I'd check that mechanical advance for goo/crud, and just clean it up and lube it anyway. The big things I'd be looking for right now: a vacuum leak, and plug wires. Check all those hose/lines for any loss of vacuum, pretty common theme here. Vacuum advance occurring properly? Have you tested those plug wires/coil wires? Get 'em tested or throw a new set of good wires like Accel,etc, on. Check you timing, I don't know if you have factory HEI or aftermarket, smog heads, etc, so I can't give you timing degs. But, initial timing should be in by 3500-4000 RPM, with vacuum advance pulled and plugged. Hopefully you do have those plugs gapped to .080, and personally I always preferred the AC plugs in them. Have found some of the platinum plugs not well liked by these engines. Not sure I would have dropped down 2 heat ranges, in my 60's and 70's building days, that was considered a big drop. You might want to re-address other issues first and then go with recommended plug. One last thing, are you running (carp) ethanol in it? You should have 91 non-oxy running through it.....IMHO. Good luck, keep us posted!

[Steve Bakken]

There is no pinging under any condition. Compression is 9.7-1 so I run at least 92 octane. I checked the timing and the distributor is tight and right at 8 degrees btdc where I had it before. The mechanical Advance is moving freely. I only have the plugs tapped at .045 which I believe is factory spec and where I had the old ones at too. I did put the old cap and rotor back on to eliminate the possibility of thos parts being bad. Spark seems strong. It jumps 3/4" when I pull the wire off. Gonna pull out the ignition module and have it tested. If that and all else checks out good, I guess I'll have to pick up another set of plugs and give that a whirl