Manifold Hot Spot

[Ralph E.]

Let me start by saying I have owned my 1967 for over 12 years. The engine bay has been detail numerous times. This year I have notice the intake manifold paint is burning off below the carburetor near the choke. This has never happened before. The cars running temp is usually 180*-190*. Occasionally on very hoy days it will approach 210* in traffic.
What could be causing this? Is this something to be concerned with? Could it be the paint? If so, why at this spot?

[John H.]

Re: Manifold Hot Spot

Let me start by saying I have owned my 1967 for over 12 years. The engine bay has been detail numerous times. This year I have notice the intake manifold paint is burning off below the carburetor near the choke. This has never happened before. The cars running temp is usually 180*-190*. Occasionally on very hoy days it will approach 210* in traffic.
What could be causing this? Is this something to be concerned with? Could it be the paint? If so, why at this spot?

Ralph -

That's the location of the crossover passage for the exhaust gases directed under the intake manifold plenum floor by the heat riser valve on the outlet of the passenger side exhaust manifold; it's perfectly normal for the paint to be burned over that passage on 300hp and 390hp engines. You can reduce the effect somewhat by wiring the heat riser valve open. On '65 and earlier, you can block the passage without affecting the choke, but on '66-'67 that passage provides heat for the choke's remote thermostat.

[Stuart F.]

Re: Manifold Hot Spot

Due to the "change" you noticed, it would probably be wise to at least check the operation of your heat riser valve. More often than not you would expect it to fail "open" (spring relaxed over time), but I saw one on a 69 L-89 the other day that would stick tight at half open. One could force it by hand past the sticking point in either direction, but it is doubtful that it would over come that point during normal warm up/cool down by itself.

Just a thought.

Stu Fox

[Jim T.]

Re: Manifold Hot Spot

Wiring your hear riser fully open will contribute to longer muffler life.

[Ralph E.]

Re: Manifold Hot Spot

Thanks John, Stu, and Jim, appreciate your input.
My bad, forgot to mention that I also had to change the heat riser. When I removed the exhaust pipe I dropped the heat riser and it broke.
Dumb question. Which way should the heat riser be installed. Mine is installed with the counter weight on the passenger side facing front.
Another observation is the car is a little harder to start at times. For example if I take it for a cruise and stop and park for 45 minutes or so it is usually doesn't start right up. The starter has to work a little. Could this be that the fuel in the accelerator pump is getting too hot.

[Timothy B.]

Re: Manifold Hot Spot

Ralph,

The heat riser weight should face the rear firewall, if it moves freely wire it open and see how the engine runs. Make sure the heat riser is not installed upside down, the weight should move down to open the butterfly.

There are more and more problems with these cars and ethonal gasoline, where is your initial timing set and does the vacuum advance work OK.

[Ralph E.]

Re: Manifold Hot Spot

Ralph,

The heat riser weight should face the rear firewall, if it moves freely wire it open and see how the engine runs. Make sure the heat riser is not installed upside down, the weight should move down to open the butterfly.

There are more and more problems with these cars and ethonal gasoline, where is your initial timing set and does the vacuum advance work OK.

Tim,
Thanks now I am sure the heat riser is installed correctly. As suggested I wired it open. Car runs cooler. I can actually sit in traffic and not see the temp increase. Only difference now seems to be the idle as not steady.

[Al R.]

Re: Manifold Hot Spot

What is it with the heat riser valves not opening correctly on the 66-67s. The new one on my 67 327-300 does not open either. The weight turns freely when pushed down and released. I could not see any reason for it not to work correctly when I removed it to look for a solution--not easy when an A/C compressor is also present.

[Donald O.]

Re: Manifold Hot Spot

I had to replace my heat valve about 12 years ago. When I did, I took a dremel tool and cut the inner butterfly flapper off the new unit before installation. It looks correct, and sounds even better.

DonO

[Timothy B.]

Re: Manifold Hot Spot

Al,

I would not say the heat riser valves don't operate correctly, it's the lower boiling point of the ethonal fuel combined with the exhaust heated plenum.

To a large extent the heated manifold helps with fuel vaporization, I can tell a big improvement in my car after the plenum is at operating temperature but in my case I had a problem with the accellerator pump fuel boiling because on the Holley it's at the bottom just above the manifold.