Spark plugs
Thread Starter
Advanced
Joined: Sep 2013
Posts: 53
Likes: 0
From: Plymouth NH
Spark plugs
I have a 67 L71 coupe. I have less then 6000 miles on the AC R44N plugs that are in the car. The engine wasn't running that well so I check them. I have never seen such nasty looking plugs in my life. I was wondering if you guys ran a special plug for that motor that I am not aware of? Thanks in advance. John
Burning Brakes
Joined: Nov 2006
Posts: 844
Likes: 5
From: Pasadena TX
I currently have AC 45XLS plugs in mine now and have run Autolite 403s in it before the ACs. Both were extremely clean when I checked them. I had the ACs out a couple of weeks ago trying to troubleshoot an issue and they were almost as clean as when they were put in last year.
Drifting
Joined: Jan 2013
Posts: 1,772
Likes: 196
From: Asheville NC
Hello Govoni,
On my L79 (that was also just rebuilt) I have gone through lots of plugs due to oil fouling with deposits so big on the plug I wonder how it got that bad before I noticed. As a result I replaced the valve stem seals on the two fouling cylinders and the problem has vanished.
Before that I tried AC 7 platinum plugs and it still fouled but me cost much more per plug to discover it. Today I am running Autolite 85 plugs which are copper core - and they perform very well.
Eventually the heads will need to come off and have the guides replaced.
You need to determine if it's oil fouled and then if its valve guides leaking, rings or intake leaking/sucking oil. Is it all cylinders or just one or two
Phil
On my L79 (that was also just rebuilt) I have gone through lots of plugs due to oil fouling with deposits so big on the plug I wonder how it got that bad before I noticed. As a result I replaced the valve stem seals on the two fouling cylinders and the problem has vanished.
Before that I tried AC 7 platinum plugs and it still fouled but me cost much more per plug to discover it. Today I am running Autolite 85 plugs which are copper core - and they perform very well.
Eventually the heads will need to come off and have the guides replaced.
You need to determine if it's oil fouled and then if its valve guides leaking, rings or intake leaking/sucking oil. Is it all cylinders or just one or two
Phil
Le Mans Master
Joined: Feb 2008
Posts: 6,507
Likes: 1,926
From: Folsom CA
Spark Plugs need a good environment or they will show you what is wrong in your engine. You can go to any major plug manufacturer Bosch, Champion, NGK or Autolite websites and look at pictures of fouled plugs to give you some idea if you are looking at with fueling or oil burning issues.
A new engine should burn clean. Either fuel or oil issues should be fixed right away.
If a plug fouls out and you keep driving the car you could ruin the rings in that cylinder.
A new engine should burn clean. Either fuel or oil issues should be fixed right away.
If a plug fouls out and you keep driving the car you could ruin the rings in that cylinder.
Melting Slicks
Joined: Feb 2011
Posts: 2,892
Likes: 168
From: Middletown Ohio
2016 C1 of Year Finalist
AC 44 is a sort of cold plug.
Try AC45 or 46 if you can find them.
Also there is a NGK which is between the 45 and 46 from a heat range standpoint. I'll get the number later.
I bought a set and they are great in my 340 HP 62 Corvette which has approx 11.25 to 1 compression (advertised).
NGK XR4, stock #5858
Also, use Shell 93 octane, mine contains ethanol and runs fine.
Try AC45 or 46 if you can find them.
Also there is a NGK which is between the 45 and 46 from a heat range standpoint. I'll get the number later.
I bought a set and they are great in my 340 HP 62 Corvette which has approx 11.25 to 1 compression (advertised).
NGK XR4, stock #5858
Also, use Shell 93 octane, mine contains ethanol and runs fine.
Last edited by ohiovet; Dec 1, 2013 at 02:42 PM. Reason: Added part number
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
1) No Corvette needs Avgas or any leaded gas.
2) Avgas is no 'drier' than pump gas- that's an old myth that won't die.
Switch to pump gas of sufficient octane rating, leave the additives on the shelf and I'll bet your problems go away.
Race Director
Joined: Apr 2005
Posts: 11,277
Likes: 2,139
From: Bowling Green, Kentucky
This is exactly why I asked the question. I think you've found the source of the contaminated plugs.
1) No Corvette needs Avgas or any leaded gas.
2) Avgas is no 'drier' than pump gas- that's an old myth that won't die.
Switch to pump gas of sufficient octane rating, leave the additives on the shelf and I'll bet your problems go away.
1) No Corvette needs Avgas or any leaded gas.
2) Avgas is no 'drier' than pump gas- that's an old myth that won't die.
Switch to pump gas of sufficient octane rating, leave the additives on the shelf and I'll bet your problems go away.
Some of the worst plug fouling I personally saw was on my new 1978 Z28 after about 5000 miles. I attributed it to engine break-in, combined with typical leaded gas fouling. Installed a new set of plugs (same heat range) and it never reoccurred.
I think the late 1970's were about the worst for GM factory quality control....be it sheet metal, interior, or engines.
Larry
Team Owner
Joined: Oct 2000
Posts: 38,897
Likes: 1,926
From: Washington Michigan
Team Owner
Joined: Oct 2000
Posts: 38,897
Likes: 1,926
From: Washington Michigan
Team Owner
Joined: Mar 2003
Posts: 26,118
Likes: 1,874
From: Greenville, Indiana
I don't know how that could be. There's way too many people here and there that use E 10 that don't have any problems with "messing up their carburetors". On the other hand, there's a few that insist that E 10 ruins not only the carburetors but the engine as well. Maybe you could tell us about your experience.
Do you have a carburetor problem that is causing your plugs to carbon up?
I don't know but I think a light crusty color of heavy carbon sounds like fuel additives in the combustion. Not oil burning.
Maybe you could tell us about your experience.Do you have a carburetor problem that is causing your plugs to carbon up?
I don't know but I think a light crusty color of heavy carbon sounds like fuel additives in the combustion. Not oil burning.
Race Director
Joined: Apr 2005
Posts: 11,277
Likes: 2,139
From: Bowling Green, Kentucky
Sounds like either the additives or the lead scavengers (EDB) from the AvGas.John needs to change plugs, cut (stop using) the additives, and try using 93 octane non-leaded gas......or a mix of unleaded with Av Gas.
Larry
Race Director
Joined: Apr 2005
Posts: 11,277
Likes: 2,139
From: Bowling Green, Kentucky
This ASSUMES that the engine was rebuilt to stock specs. We don't know if the heads were milled, the actual compression ratio, the cam that was installed, or the distributor revisions (if any). The engine just might need extra octane. If the engine were plain stock, it should be able to exist on current 93 octane unleaded gas.
Larry
Larry