1968 Z28 header plug fouling issue - Team Camaro Tech
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post #1 of 37 (permalink) Old Jun 20th, 14, 08:57 AM Thread Starter
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1968 Z28 header plug fouling issue

Hi guys would appreciate some thoughts from people who may have suffered a similar issue.

My 68 Z28 302 currently runs Accel 437S shorty plugs which in themselves are okay as plug, I realise others have other preferences for plugs. Main trouble is that the passenger rear plug fouls so quickly that it's becoming annoying and this is I would imagine due in part to the oversized carb these little 302's run along with the inlet manifold and the high flow of fuel to this rear plug that doesn't get burnt effectively. The other plugs do become a little oiled up but nothing as a major or as concerning as the passenger rear.

Has anyone got any suggestions as to another standard plug I could run on this rear one as it doesn't need to be a shorty. I'm not sure on whether it needs to be hotter/colder etc. always gets confusing when I look at that aspect, any help would be great.

68 Z/28, Teal Blue
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post #2 of 37 (permalink) Old Jun 20th, 14, 09:16 AM
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Re: 1968 Z28 header plug fouling issue

That plug cross references to an AC Delco R44 and I think the 302 came with a colder R43 from the factory... Someone else chime in if I got that wrong...

If it's only the one cylinder thought I'd be looking for something else that just effects the one hole. Plug wire? Valve adjustment? Rings?

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post #3 of 37 (permalink) Old Jun 20th, 14, 09:19 AM
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Re: 1968 Z28 header plug fouling issue

My suggestion is run a hotter heat range.
R43's are nice and a cold plug, but, only if the engine is around 5K and above all the time.
At lower RPM's, the plugs don't get hot enough to burn off the electrodes.

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post #4 of 37 (permalink) Old Jun 20th, 14, 09:24 AM Thread Starter
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Re: 1968 Z28 header plug fouling issue

So other plugs do/can foul badly after time but not to this extent. It seems to be related directly to the distance the plug is from the channels on the inlet. The middle cylinder plugs burn fine as they seem to run hottest but front driver also suffers but not as badly as the rear. I believe this to be a carb/inlet issue due to the massive flow of fuel which may not be getting burn off correctly. At the end of the day if you were building a 302 you would never carb it to this extent, 650 max.

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post #5 of 37 (permalink) Old Jun 20th, 14, 09:27 AM Thread Starter
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Re: 1968 Z28 header plug fouling issue

Any suggestion on a hotter plug would be appreciated as yes I agree over 5k they would be great..

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post #6 of 37 (permalink) Old Jun 20th, 14, 10:12 AM
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Re: 1968 Z28 header plug fouling issue

Quote:
Originally Posted by Demon G View Post
So other plugs do/can foul badly after time but not to this extent. It seems to be related directly to the distance the plug is from the channels on the inlet. The middle cylinder plugs burn fine as they seem to run hottest but front driver also suffers but not as badly as the rear. I believe this to be a carb/inlet issue due to the massive flow of fuel which may not be getting burn off correctly. At the end of the day if you were building a 302 you would never carb it to this extent, 650 max.
Remember that with carburetor sizing, the CFM rating is how much volume of air, and fuel, mixed together by the carburetor can flow. You can and perhaps may need to go to smaller jets if the plugs are fouling from a rich mixture. If however you have worn valve guides, bad valve seals, rings letting oil by, that will cause oil fouling so carburetor size and fuel mixture/jetting leaner wouldn't help. Running a hotter plug as has been recommended will help either situation (rich jetting and/or more oil in the combustion chamber) to some extent but there is a limit to how much good a hotter plug will do. There are charts online for at least some manufacturers spark plugs that show the heat ranges, a good parts store counter person should be able to recommend the next hotter plug above what you have if there are hotter plugs by that manufacturer available.

A picture or two of your fouled plugs would make it easier for us to perhaps tell you what it looks like is the cause, rich fuel mixture or oil that is fouling your plug(s).

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post #7 of 37 (permalink) Old Jun 20th, 14, 11:06 AM
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Re: 1968 Z28 header plug fouling issue

Go to NAPA and buy 1 R45 and try it.

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post #8 of 37 (permalink) Old Jun 20th, 14, 11:40 AM
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Re: 1968 Z28 header plug fouling issue

Quote:
Originally Posted by Demon G View Post
So other plugs do/can foul badly after time but not to this extent. It seems to be related directly to the distance the plug is from the channels on the inlet. The middle cylinder plugs burn fine as they seem to run hottest but front driver also suffers but not as badly as the rear. I believe this to be a carb/inlet issue due to the massive flow of fuel which may not be getting burn off correctly. At the end of the day if you were building a 302 you would never carb it to this extent, 650 max.
You might want to rephrase that -If you were building a 302 to only rev to 5000rpm before it poops out you would never carb it with more than 650cfm. Reality a 302 is just waking up at 4000rpm and capable of running at 7000 + rpm! Chevy chose a Holley vacuum secondary carb because they understood Holley vacuum secondary carbs are what is referred to as "on demand" meaning the secondary's only open enough to meet the demand of the engine. The secondary is operated by vacuum created in a venturi (small hole/path) as air/fuel is sucked through the primary throttle bores.

Your loading up is a low rpm/idle rich condition when the secondary is not open unless you are running a mechanical secondary carb for some reason. That should be effecting all 8 cyls to some degree. A hotter heat range may help but if you're running rich that's a tuning issue. This can be made worse by periods of long idling and slow speed driving...

From there you need to be looking into the one cyl that is effected the most. Worn piston rings or valve seals may allow too much oil to leak into the combustion chamber. And valves not adjusted properly can change the dynamics in a cyl too. And yah runner length may be making it easier to see the symptoms on some cyls but keep in mind there are millions of SBC's out there running just fine!

...Dennis

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post #9 of 37 (permalink) Old Jun 20th, 14, 02:18 PM Thread Starter
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Re: 1968 Z28 header plug fouling issue

Attachment 76746

Sure an engine built to thrive on revs for circuit racing which the Z28's were designed for can get away with such a big carb to ensure fuel is always available on demand. These cars had a habit of fouling plugs back in the day probably because of a mix of large carb and long runners. The race cars would have just ditched the plugs after each race so wasn't a concern. The trouble is this car is used for pleasure and although driven hard occasionally it has to cope with traffic and idling at low revs which it doesn't like hence the issue. Most big blocks won't run a carb much larger than this so for everyday driving the plugs suffer.

The engine only has maybe 4'500/5,000 miles and has only been rebuilt once by a tenured engine guy who builds lots of top end motors far out of my league so I'm fairly confident unless something's failed it's the carb size, plug heat range and driving conditions doing the damage here.

The picture attached was taken after a period of off and on driving and I only checked the plugs because of a slight mis fire. To be fair I hadn't checked them for a while but since replacing this plug it has almost formed a black glaze over the end of the plug like an enamel so swapped it out again.

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Last edited by Demon G; Jun 27th, 14 at 12:40 PM.
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post #10 of 37 (permalink) Old Jun 20th, 14, 02:54 PM
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Re: 1968 Z28 header plug fouling issue

Guys drive their 302 Z/28's all the time with the factory 780 carbs and don't have the fouling problems you are blaming on the big carb...

...Dennis

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post #11 of 37 (permalink) Old Jun 20th, 14, 03:03 PM Thread Starter
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Re: 1968 Z28 header plug fouling issue

We'll I'm open to suggestions and will try a different heat range on the back plug so thanks for suggestion. I'm not entirely convinced re- jetting will help all that much but I will get it checked out as I'm willing to explore all avenues with this.

As I said historically it's well known that these cars had a habit of fouling plugs because of the carb size on a small block car so was my first thought when I encountered the issue.

The carb was re-built by Bob Kunz when the car was totally restored so hopefully all is okay that end but I will check.

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post #12 of 37 (permalink) Old Jun 20th, 14, 03:05 PM
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Re: 1968 Z28 header plug fouling issue

A closer up photo would help but from what I can see of that plug it looks oil fouled to me.

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post #13 of 37 (permalink) Old Jun 20th, 14, 03:18 PM Thread Starter
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Re: 1968 Z28 header plug fouling issue

Attachment 76754

This may be a bit better to see the plug a little closer.

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Last edited by Demon G; Jun 27th, 14 at 12:41 PM.
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post #14 of 37 (permalink) Old Jun 20th, 14, 03:33 PM
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Re: 1968 Z28 header plug fouling issue

That gap looks larger than .035. Make sure your PCV valve is clean and has suction.

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post #15 of 37 (permalink) Old Jun 20th, 14, 03:33 PM
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Re: 1968 Z28 header plug fouling issue

Are you using an octane booster?
If not, what oil are you running. Looks like either an octane booster deposit or ash mixed with oil.
Guessing just from that pic (and on line pics can be deceiving) I would say you have oil either getting by the rings or the valve seals.

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