Hi Joe,

It's not necessarily something you have to deal with! You may want to try a heat shield between the manifold and the carb. Here's one that Summit sells:

https://www.summitracing.com/parts/oer-3969835

Here's the one on my car:



Tom

 

Hi Tom:

The heat shield you show is helpful for blocking radiant heat that comes off the top of the manifold runners, bit it does not help much with the fundamental problem created by the hot slot in the 300 HP cast iron manifold.

To help pre-heat the carb while the heat riser valve was closed, GM intentionally routed hot exhaust gas through the hot slot, which directly heated the manifold and the carb under the primary side. To keep the hot gases from corroding the carb base, GM used a thin stainless shield between the hot slot and the carb. So, at least for the hot slot area, heat entered the carb directly by conduction through the carb base.

The radiant-heat shield that you show would pretty much conduct the same heat right into the base of the carb. To reduce the heat that is directly conducted into the carb base, there are four things you can do:

1) Block off the hot slot.

2) Remove the heat riser valve entirely and replace it with the spacer used on fuel injection cars (although the OEM spacer fits 2.5" exhausts, so different spacer might be needed for the '67 327/300 manifolds that have a 2" outlet).

3) Block off the heat riser passage that sits right under the carb. The heat riser passage was commonly used by GM and is not the same thing as the hot slot (which was used only briefly). For example, the OEM aluminum intake has the heat riser passage but not the hot slot. The 327/300 manifold has both. Keep in mind that the heat riser passage also heats the little "choke stove" that actuates the choke. So, blocking the heat riser passage would likely require converting to an electric choke.

4) Add a thermal spacer under the carb to help block conducted heat. These are usually made out of a phenolic material that is a very good heat insulator. On the 327/300, there is at least .500" of available height for a carb spacer. These insulating spacers are highly effective at reducing the heat that gets conducted directly into the base of the carb.

It occurs to me that the OP could simply install a .500" carb spacer and block most of the conducted heat that comes from the heat riser passage and the hot slot. The manifold under the carb would still be quite hot, but the direct conduction path into the base of the carb would be pretty well blocked. Adding a radiant heat shield like the one you show would help block any remaining radiant heat that comes off the exposed surface of the intake.

 

The main purpose for the hot slot channel in the cast iron intake manifold is to prevent ice build up around the primary throttle blades during warm up. Air travels at sonic speed past the blades and moisture can freeze around the blades, this usually shows up as a lean roll in the idle until the carburetor base comes up to temperature. Heat can be your friend here and you can control the heat in the plenum during hot months by wiring the heat riser open. In cooler months once again heat can be a good thing.

My feeling is that shield works pretty good even with the heat riser operational, the heat soak after shut down seems to be the issue and that shield is in just the right location.

 

I agree that "heat can be your friend" when the ambient temperature is very low and the engine is stone cold. That's why GM included things like the heat riser passage and the hot slot.

When GM designed their fuel systems, they had to address the possibility that the owner might be trying to start the car during winter in Minnesota when the temperature was -20 degrees F. Under those conditions, all those systems that helped to heat the carb were essential, especially if the owner expected to simply start the car and drive away.

Even though I live in Boston, I rarely have to start my Corvette when the ambient temperature is below 30 degrees F. During the worst of winter, there is salt on the roads and I avoid driving the Corvette.

I run my car with no heat riser valve and a plugged hot slot, but I never have trouble starting the car when the temperature is 30 degrees F.

However, under such conditions I do have to let the engine warm up for about a minute before I put the car in gear, or I risk having it stall. I'm willing to put up with this in exchange for better heat soak behavior during the summer, but GM probably could not expect an ordinary customer to put up with it, which is why they went to such trouble to make cold starts work smoothly. Fortunately for GM, the gas they had to work with in the 1960s was much more tolerant of heat soak than the more volatile stuff we have today.

If the OP's screen name is an indication that he lives in Nevada, he can probably afford to disable some of the cold-start systems in favor of improved heat soak behavior.

 

I agree with you about the gasoline that's available for our cars today and your approach regarding the heat riser although I prefer the operation of the valve most months.

The hot slot and carburetor icing problem I mentioned above is present when temps are in the 30-50* range when humidity is in the air. At temps below 30* the air is dryer because of the freeze point and moisture drops out. With proper primary throttle blade position tighter gaps can present the problem, I have experienced this on my 67 300hp engine and stock Holley. Idle speed of 600-650RPM with the stock PCV only requires approx 3/4 turn on the idle speed screw and the secondary blades completely shut. The hot slot problem is more a issue of the stainless baffle burning through and letting the heat directly on the carburetor.

The plenum heating from the H/R valve is a aid to warm up and is designed to keep the plenum hot for fuel vaporization. Fuel injection cars did not use this because the atomized spray from the injector is in a near vapor state. I am no engineer but today's fuel and it's lower boiling point is designed for modern F/I cars that operate with the fuel rail at high pressure so the boiling point is actually raised higher, our carburetors fuel is at atmospheric pressure.

 

I had fuel boiling problem on a new 350/290HP crate with edelbrock 1406 in my C2. Engine even did not want to hold decent low idle for too long when warmed up. Blocking the heat cross-over and installing a heat spacer made all the difference. Don't forget to lock the heater valve in the open position when going this road (as I did while wondering why exhaust gasses leaked from the valve during warm up)

 

Those are great photos! It looks like you used threaded pipe plugs like I did.

I have read about the carburetor icing problem at ambient temperatures around 40 degrees. Fortunately, that has never happened to me (at least, not yet!). I guess the OP just has to decide what sort of tradeoffs he is willing to make.

I think that a lot of great information has been posted on this thread, so he has a good base for deciding how to proceed.

 

I threaded and blocked my hot slot in my '63 with the intake on the car...put a bit of paper towel down in the hole full of wheel bearing grease and tap the holes, vacuum the holes out when done and carefully extract the towels with the remaining bits of metal. It worked fine. However that stops the heat from getting in the hot slot, it doesn't stop the heat from sitting under the carb as it rests on the intake. I used fuel injection gaskets to block off the heat passages on the intake manifold. No more heat soak/perc after shutdown.

If you don't think that intake doesn't get hot from those heat passages just note the scorched paint in the pic.