Carburetor phenolic spacer

[Leif A.]

I'm wanting to isolate as much heat as possible between intake and carb. I have a stock L79 with original 3890490 intake and 3810 Holley carb. I have been told I should install a gasket at the intake, followed by a phenolic spacer, then a stainless steel gasket and finally the carb. Also, told to get a gasket that eliminates the "hot slot" on the intake due to today's gas blends. Has anyone done something similar and, if so, where did you order the parts. I would need to know how thick of a spacer would fit under a stock hood. Thanks in advance.

[Edward J.]

Re: Carburetor phenolic spacer

Leif, the problem with spacer plates is just how much extra room do you have under the hood( clearance)? I think if you shut of the heat slots under the Carb, that will help. another thing is a stainless steel shim under the Carb. base would also help, But not sure if they make one for a Holley carb. maybe some else can comment. Today more than ever with ethanol fuels heat is really a problem under the Carb. with higher outside temps.

[Jerry B.]

Re: Carburetor phenolic spacer

I believe the 1970 LT1 used a stainless plate under a Holley.

[Duke W.]

Re: Carburetor phenolic spacer

I'm wanting to isolate as much heat as possible between intake and carb. I have a stock L79 with original 3890490 intake and 3810 Holley carb. I have been told I should install a gasket at the intake, followed by a phenolic spacer, then a stainless steel gasket and finally the carb. Also, told to get a gasket that eliminates the "hot slot" on the intake due to today's gas blends. Has anyone done something similar and, if so, where did you order the parts. I would need to know how thick of a spacer would fit under a stock hood. Thanks in advance.

L-79 just has a gasket between the aluminum manifold and carb. There is no heat slot in the L-79 manifold. The phenolic spacer, stainless heat shield are unique to the 300 HP engines with the AFB only.

First thing to do is verify that the heat riser valve is free, then wire it open. Also, check the spark advance map including verifying that the VAC is functional. Set initial timing at 8-10 degrees, which should put the total idle advance in the mid-twenties. After this go through the idle speed mixture adjustment procedure in the service manual to ensure it's correct.

Then measure exhaust manifold temps with an IR gun. They should be no more than 500F with the engine fully warmed up from driving and after a few minutes at idle.

Percolation is a big problem with modern gasoline, but the degree of the problem varies widely. The subject has been discussed many times including ways to mitigate it, so do some archive searching.

Duke

[Joe L.]

Re: Carburetor phenolic spacer

I'm wanting to isolate as much heat as possible between intake and carb. I have a stock L79 with original 3890490 intake and 3810 Holley carb. I have been told I should install a gasket at the intake, followed by a phenolic spacer, then a stainless steel gasket and finally the carb. Also, told to get a gasket that eliminates the "hot slot" on the intake due to today's gas blends. Has anyone done something similar and, if so, where did you order the parts. I would need to know how thick of a spacer would fit under a stock hood. Thanks in advance.

Leif-------


As Duke mentions, your manifold does not have a "heat slot". That's a good thing. The "heat slot" design was used mostly on cast iron manifolds before 1970. As far as aluminum manifolds go, the only ones that featured it were 1968-69 L-36.

You can install a thick insulating gasket as was used on some 1970-72 Holley applications. These are available in the aftermarket. Neither the 70-72 originals or the current aftermarket are "phenolic" but will perform just as well. However, I don't think that installing such a gasket is going to do you a lot of good. If you install it you'll have to fabricate a new and longer choke rod.

[Leif A.]

Re: Carburetor phenolic spacer

Duke, Joe and Ed,Thank you all for your responses. Duke, I will check all the items on your list. Get's awfully hot here in Texas

[Stuart F.]

Re: Carburetor phenolic spacer

I use the 3/8" thick unit from a 63 L-75 (300 hp) on my L-76 w/ aluminum manifold. I also need to use the regular L-76 base gaskets on top and bottom in order to get the proper seal and most insulation. Mine has an AFB Carter, of course, but I have no hood clearance issues with this total thickness of about 15/32" compressed. I have an electric choke and use a fuel hose from my filter to the carb to make up the difference in spacing.

It helped a lot here in Florida.

Stu Fox

[Leif A.]

Re: Carburetor phenolic spacer

I use the 3/8" thick unit from a 63 L-75 (300 hp) on my L-76 w/ aluminum manifold. I also need to use the regular L-76 base gaskets on top and bottom in order to get the proper seal and most insulation. Mine has an AFB Carter, of course, but I have no hood clearance issues with this total thickness of about 15/32" compressed. I have an electric choke and use a fuel hose from my filter to the carb to make up the difference in spacing.

It helped a lot here in Florida.

Stu Fox

Thanks, Stu.

[Leif A.]

Re: Carburetor phenolic spacer

L-79 just has a gasket between the aluminum manifold and carb. There is no heat slot in the L-79 manifold. The phenolic spacer, stainless heat shield are unique to the 300 HP engines with the AFB only.First thing to do is verify that the heat riser valve is free, then wire it open. Also, check the spark advance map including verifying that the VAC is functional. Set initial timing at 8-10 degrees, which should put the total idle advance in the mid-twenties. After this go through the idle speed mixture adjustment procedure in the service manual to ensure it's correct. Then measure exhaust manifold temps with an IR gun. They should be no more than 500F with the engine fully warmed up from driving and after a few minutes at idle. Percolation is a big problem with modern gasoline, but the degree of the problem varies widely. The subject has been discussed many times including ways to mitigate it, so do some archive searching.Duke

Duke,You state to wire open the heat riser valve. Not being a mechanic by trade, just a DYI kinda guy, looking at the Corvette Shop Manual I'm assuming you're speaking of the Manifold heat control valve. Mine is new and moves easily. Am I to wire it open permanently or just during the process of adjusting timing, fuel and air screws and idle speed screw. Kindly excuse my ignorance but I ask to learn...what is the purpose of wiring the heat riser valve open? Thank you for sharing your vast knowledge.

[Duke W.]

Re: Carburetor phenolic spacer

I recommend wiring it open (weight pointing down) and keeping it wired open all the time other than for judging.

You may notice that the valve is not full open at idle when the engine is fully warmed up, but it opens fully if you blip the throttle. This behavior pushes more exhaust gas through the inlet manifold passage at idle and that added heat exacerbates the percolation problem.

You might notice slightly poorer warm up driveability, but most don't.

It was designed to improve driveability during warmup in cold weather. The heat promoted rapid fuel vaporization, but since most of us only drive our vintage Corvettes in mild to hot weather, it's an albatross hanging on the car. Combine the heat riser valve with today's gasoline's lower midrange distillation curve and you have a perfect recipe for percolation.

Some owners have removed the butterfly, which effectively eliminates the valve without any external evidence, and some (like me back in the seventies) have installed the FI spacer, which is just the heat riser valve body less the machining necessary to install the valve shaft.

Those things can be a real PiA. On cars that have sat for a long time it's not uncommon for them to be seized closed. Now that will really make the gasoline in the carb bowls boil since all the exhaust gas from the right bank is forced through the inlet manifold passage to the left bank exhaust.

Get some stainless steel safety wire and give it about three to four wraps. It's easy to undo if you go through Flight judging and redo after the event.

Duke

[Leif A.]

Re: Carburetor phenolic spacer

Duke,Thanks so much for the clarification. A little stainless steel safety wire it is.

[Jerry B.]

Re: Carburetor phenolic spacer

Hello Joe , what is a "heat slot". thanks

[Joe L.]

Re: Carburetor phenolic spacer

Hello Joe , what is a "heat slot". thanks

Jerry------


The "heat slot" is found on many pre-1970 Chevrolet cast iron manifolds. It's found on the forward part of the carburetor flange. It's seen as a wide "slot" with holes on either end. The holes go through to the exhaust crossover passage in the intake manifold. The purpose of the slot is to provide "instant" exhaust heat to the carburetor so as to, theoretically, improve cold temperature fuel atomization and warm-up.

In actuality, the slot does very little of benefit and very much of detriment. It's a destroyer of carburetors. I recommend that the holes at either end of the slot be plugged with drive plugs or drilled and tapped for internal hex drive pipe plugs, thus disabling this crazy function. GM wisely abandoned this system after 1969.

[Tom R.]

Re: Carburetor phenolic spacer

Hello Joe , what is a "heat slot". thanks

image.jpg
This is the heat slot on a 67 300hp base engine with the the two riser holes that are plugged. Worked pretty well along with wiring open the heat riser valve on the passenger side exhaust manifold as had been suggested earlier. Easy fix.
Tom