Im experiencing a very odd issue with a friends 69 z28. Its got a 302 in it still that we just rebuilt. It has high compression forged pistons from sealed power and moly rings. We used a new set of AFR aluminum heads, Victor Junior intake and a used Holley 750 double. After starting the engine for the first time, and getting into about 15 minutes of the cam break in process we noticed that the headers on 6 and 8 were beginning to get red hot. Engine coolant temp was about 230 so we shut it down. We richened up the carb the next day, we put a large fan in front of the car to help cool the radiator. Coolant temps while running are still right around 190-200 so that seemed to help but the headers are still getting HOT. The passenger side bank is almost 100-200 degrees hotter than the drivers side. (With 6 and 8 glowing).

So far we've checked:

Plugs appear to be normal. They are not white, they have a nice dry tan color. After we richened up the carb, they are actually starting to foul.

Weve checked the intake manifold for air leaks and none are apparent.

Coolant seems to be flowing, so I dont _THINK_ theres an air pocket in the head.

What are we missing ?? It seems like its running pretty good. But that header worries me. Its already killed the ceramic coating on that side.

 

The only 2 things I have seen cause this are severely retarded ignition timing, or incorrect cam timing. What are you using to set the ignition timing (dial light, pointer?)

 

ignition timing is really late....bust out the timing light..


I love the little DZ 302s...
I still have my old one in the garage got to love 8,500roms with a small block

 

Could the retarded ignition cause only 2 cylinders to heat up? We are using a dial style timing light. We have the ignition set at 0 right now.

 

try 4

 

How much mech advance is in the dist?
If just two cyls are doing it I would look for an intake leak at the gasket or a vacuum leak or the valves not adjusted right.
The overheating is probably the timing to far retarded. Ya should start around 36*total and work from there to find what it likes.
It could be an intake flow problem or in the carb,but that is the last place to look.
Most of the carb problems that I have seen were caused by the distributor.

 

Are you sure that it’s not that those 2 are just slightly hotter then the rest so you’re seeing them glow first?

I’d bet on ignition timing or valve timing also, possibly a mismatched timing mark/balancer, spun ring on the balancer or the timing chain installed one tooth off.

If it’s really only 6 and 8 then I’d bet on a vacuum leak at the intake to head flange, but that should be obvious with a vacuum gauge connected and it wouldn’t cause the engine to overheat unless the cooling system was marginal to start with.

 

I would look for exactly the opposite of what you might think is SHOULD be. Not lean, but RICH. From all the times I seen it happen, an overly RICH condition has caused headers to glow due to the raw fuel being burned in the pipes instead of in the holes where it should be. Combine a too rich condition with too much timing retard, and it could lead to exactly what you've described. The ceramic coatings we're all used to nowdays are good to about 1200* F., so I would definatly want to find the trouble before you burn it down any further..

Dave C.

 

I had the same thing happen to me right after a cam install... what happened was that the timing chain was off one tooth and it bent all of my intake valves. =( New ported heads and correctly installed timing chain and I was ready to go!

-B

 

Could this not just be due to a lack of cool airflow to the back cylinders when running the motor at 2000rpm for cam break-in? This happened to me too and I assumed this was the problem. Motor has run great for 5 years and withstood 2 yrs of racing it. Not saying anyone is wrong, hell I might be. Just thinking outloud.

 

holes? you mean combustion chambers?

 

i agree with ccccya, if not an vaccumn leak, you will find the fuel is puddling in the two headers and burning there.

 

Yes. Sorry for the slang. I tend to do that when answering things here...old habit..

Dave C.