I have a 76 350 standard bore with a Holley 600 cfm cab and Weiand X-celerator intake. The cam specs are as follows:
Cam lift in .260 exh .273
Valve lift in .390 exh .410
Lobe cntrs in 108 exh 116
SAE duration in 258 exh 269
.050 duration in 194 exh 202
SAE timing 14 btc
.050 timing -12 btc
I currently have the timing set at 14 btc and at 1000 rpm it runs fine HOWEVER any rpms over that the engine begins to RAPIDLY overheat and backfire through the carb. Water pump and radiator are ok. Any suggestions on how to fix this?

do you have springs inside your radiator hoses?

What is your fuel mix at that RPM? Backfiring might indicate lean, and that would cause high temps.

Fan clutch?

No springs inside radiator hoses although I have been trying to find them. As for the air fuel mixture the carb is set as it came from out of the box. There is a fan clutch and it appears to be working. I have been told that having the pcv valve vacume hose connected to the manifold instead of the carb will cause this problem. Has anyone else heard this?

You can use some coat hanger wire spiraled around a pipe to make the spring till you get a real spring. Just a temp fix thought because hanger wire will rust.

Or,maybe coat it with plasti-dip?

I have tried all the places I could find in Hemmings motor news that specialized in Chevelle parts. No one sells these things?

Change the fan clutch. Put in a "thermostatic" (expensive) not a "centrifugal" (cheap, probably what's on there now). Look for a little coiled bi-metal on the front. Expect to pay ALOT, probably near $40.

The el cheeeepo "centrifugal" design assumes that any time the engine RPMs are above 1800 or 2000, the car will be travelling fast enough to "ram" air through the rad, so it decouples. Stupid. The "thermostatic" kind decouples when the air passing through the rad is below 210° or so, and hooks up when it's hot.... the discipline that's actually correct for the situation at hand.

Any idea where to get one? Will any of the local part stores (Pep Boys, Auto Zone,etc.) carry one or will I have to find a speed shop?

I had a simular problem with a 327 in a 67 chevelle, sounds like the basics are covered make sure you check for exhaust restrictions or lean condition you might be under carb'd. I could not find any faults with my 327 I installed a 4 core radiator with a fan shroud and a flex fan and it took are of my problem good luck.

Yeah, pretty much any parts store will have it. They'll usually have a range of tehm, from $16-18 up to $40 or so. Go for the high end.

A clutch decouples when not needed, which saves MASSIVE power (as much 15-20 HP, if your car has high-ratio gears); yet gives you huge cooling when you need it.

I would not recommend a flex fan at all no matter what. They work on the same principle as the cheeeep "centrifugal" clutch, which is to reduce cooling at high RPMs, which is exactly the problem you have now. An alternate method of accomplishing the same misguided result won't solve it.

Seems to me that once you are over 2000 rpms you are past the effective range of the fan, the fan is supposed to cool at low or idle speeds. The airflow is what cools over 2000rpms.
I would look at the radiator or hoses.
Ron

don't overlook the simple things, I bought a car awhile back that had the fan blade on backwards. Also make sure the timing is right and that a thermostat is installed.

Have you pulled a plug to see if it indicates a lean condition?

The plugs should be a light tan to a tanish gray if you are properly jetted. With the backfire issue I would be looking at timing and air/fule mis before a radiator/cooling issue........

The radiator and water pump check out good. The radiator is a 3 core (row) and I have tried both a a 165 degree and 185 degree thermostats. I dont know if this means anything but I am runing dual exhuast (2.5 inch pipe) connected through a reducer to the stock exhuast manifolds. The drivers side exhuast is normal while the passenger side exhuast is black and sooty. Does this mean anything to anyone? Holley reps tell me the carb size I'm using is ok in this configuration. Also can some one tell me what the difference is between SAE Timing and .050 Timing. I ask because the cam that came with the engine rebuild kit has two different setting listed on the box. SAE Timing of 14 BTC and .050 Timing of -12 BTC. I have assumed that this means the timing should be set at 14 degrees when setting the timing at idle. I cant get a response from the manufacturer so if someone could tell me if I am right or wrong about the timing issue please let me know.

Did you check to see if your heat riser is stuck shut. That would cause it to overheat and run poorly, it also would give you different color exhaust pipes from side to side as you mentioned.

I think your confusing cam timing with ignition timing. Your initial ignition timing should probably be somewhere between 10 – 16 degrees (BTDC) before top dead center. Also check that your mechanical advance is functioning properly. Try for total timing somewhere around 36 degrees. Remember to check both initial and total with your vacuum advance disconnected and plugged. Then reconnect it when you are all done. Retarded timing will cause both overheating and popping out the carburetor.

Hopefully it is not this, but a poorly sealing head gasket on one side could be causing the overheating, popping out the carburetor, and the sooty pipe on one bank as well.

Joe