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?

 

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.

 

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).

 

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

 

Go to NAPA and buy 1 R45 and try it.

 

View 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.

 

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...

 

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.

 

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.

My '69Z is dead stock except for a revised advance curve, 72/76 jetting, and I've run AC R45 plugs in it for the last 14 years with zero fouling or plug problems of any kind (same plug I run in my '67 Corvette). 43's are "race" plugs - WAY too cold for normal street use. :thumbsup:


:beers:

 

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

 

View attachment 76754

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

 

In my opinion that is oil, you can see it accumulated between the threads on the plug as well. I think you have a valve guide/seal and/or ring problem. Excessive oil is getting in that combustion chamber one of those two ways or a little bit of both.

 

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

 

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.

 

Ash is light brown and can also be white. That baby is black.

 

No octane booster and it's Kendal 20W 50 with liquid titanium. As I said that plug had been in best part of a year which equated to maybe 1,500 miles of varied driving conditions.

 

Try the R45 yet?

 

I will get an R45 and try that and report back, hopefully it's something that simple.

 

My friend owns my 68 Z/28 which I bought in '74.

 

The other plugs do become a little oiled up but nothing as a major or as concerning as the passenger rear.

The plugs shouldn't be oily at all. If it's running rich they will have a black chalky appearance.

If it's oil, The R45 will help a lot. I have a Jeep w/ an old sbc 350 and the #5 plug would foul about once a year from oil. I put a hotter plug in just that cylinder and it hasn't fouled going on four years.

 

If it were me I would do a compression test on all cylinders.

 

Here is a list of plugs equivalent to AC R45. Accel 124, Autolite 306, Champion RJ8C, NGK B6S, NGK stk. 3510, Splitfire SM030C. Another thing check your timing try running the initial at 16*-18* and the total 36*- 38*. BTW put your foot into it every chance you get, it is what it designed for. Another thing look for blue smoke on start up. This is a sign of valve seals or guides needing repair.

 

Thanks for all the helpful suggestions guys especially about putting your foot into, love that one. I now have a R45 to try out so will see what happens.

 

Since you've got a correct carb I don't reckon you have an Edelbrock manifold with the PCV plumbed to its vac port do you?

 

Yep it runs the original Winter's intake manifold.

 

to Steiner: what are the issues you are hinting about with a PCV hooked to an Edelbrock intake vacuum port?

Keith

 

Since it sits on the #8 runner that's sometimes where the evidence will wind up if the PCV happens to have an issue with sucking oil.

 

So a quick update. The R45 plug made no difference and I changed both rear plugs to compare and the passenger rear #8 I believe is oily again. I've pulled the PCV pipe off where it enters the carb and there was a show of oil sitting in the pipe from the carb so have to assume it's the PCV valve that's at fault. I'm going to cap it and see if it oil's without the PCV connected and see what happens.

Should the PCV let oil through at all if it manages to draw it through the valve covers?

 

I've pulled the PCV pipe off where it enters the carb and there was a show of oil sitting in the pipe from the carb so have to assume it's the PCV valve that's at fault.

Should the PCV let oil through at all if it manages to draw it through the valve covers?

Manifold vacuum draws crankcase vapors through the PCV valve and into the PCV port on the carb base, so it's perfectly normal to see some oil in the hose from the PCV valve to the carb and in the pipe it connects to in the carb base. Baffles in the valve cover are only partially effective in keeping oil out of the system - when you're moving hot oil vapors around, you're going to see some oil.

:beers: