Heat Risor Valve
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Heat Risor Valve
Can anyone tell me about my heat risor valve on my 1980 L82? I found that the arm that swings the valve open and closed has been welded. I have heard that this was done because they would start making noise. I went on line and found a new valve with a picture of it. I have determined that my valve has been welded in the 3/4 closed position. This can't be good! Can this cause the problem I am having with starting the car. It starts just great first thing, no Black smoke. Drive it all I want but if I park it for more than 15 minutes its hard to start and then it blows Black smoke. After that its fine again. I find myself watching the clock if I am stoped somewhere and making sure I get it started before it has sat longer than 15 minutes. Can anyone shed some light on my problem? I have a new Edlebrock and the carb that was on it did the very same thing! Help!
Melting Slicks
Joined: May 1999
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From: Moon Township PA
I have read where folks have fixed them open but not closed. As the spring in the riser gets old, the valves tend to fail in the partially open position. The fix is to 1. replace the spring, 2. wire the valve open (I did this), 3. replace the heat riser with a fixed length spacer and eliminate it completely.
Le Mans Master
Joined: Oct 2006
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From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
Just a thought, but the black smoke could be due to underhood heat. After you set for 15 minutes, the heat could be boiling the fuel in the carb, and causing flooding. To find out, drive your normal distance, let it set as normal, then open the hood and pull the lid on the air cleaner. Look down the carb and see if there's any fog. That may give you a clue as to what's gong on.
The heat riser needs to be either fully open or gone, unless you drive in an area that gets real cold. It just forces hot exhaust to pass under the carb (could be part of your flooding problem too) to make sure the fuel gets vaporized when the engine is cold.
The heat riser needs to be either fully open or gone, unless you drive in an area that gets real cold. It just forces hot exhaust to pass under the carb (could be part of your flooding problem too) to make sure the fuel gets vaporized when the engine is cold.
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Thanks for the advise! I will check for a fog. I have ran the car and then as soon as I parked it I opened the hood and removed the air cleaner to allow the heat to escape. This made a big differance, not 100% but at least 75% differance, but I think you are right, the trapped heat is boiling the gas in the carb thats why I was thinking maybe the heat riser is the problem. I have spent allot of money latley on the car and I have been trying to learn and do these things myself, however I am not set up to well to change the heat risor and yet that is the last thing I can think of that might be causing my problem. Thanks, Mark.
Safety Car
Joined: Oct 2000
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From: Carbondale Pa.
The heat riser on my 80 L82 vette was vacuum operated in conjunction with one of the temp. sensors on the water outlet (for smog purposes)and was noisy too, G.M. used to put a one way valve in line that stopped the noise, we usta call them black and white valves cause thats what color they were. Can anybody elaborate ?Peace,,,Moosie 
Drifting
Joined: May 2005
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From: Somers CT
All the heat riser does is restrict the flow of exhaust gasses when the engine is cold to help the car run better until it warms up. When it's cold the bimetalic spring allows the butterfly valve to close off the exhaust slightly, this causes some back pressure which forces some warm exhaust up thru the passage in the intake manifold to the other side thus warming the intake area and carb. When the exhaust heat gets the spring hot enough it changes state and opens the butterfly allowing full unrestricted exhaust. It cannot cause carb flooding. If it's operating properly even when closed during warm up, full throttle (or some portion) will overcome the spring tension and still allow exhaust gasses to pass averting too much back pressure.
You can eliminate the heat riser with a spacer and might only notice that the car takes a little longer to warm up in a cold climate. I've seen spacers on Paragon's site I believe.
You can eliminate the heat riser with a spacer and might only notice that the car takes a little longer to warm up in a cold climate. I've seen spacers on Paragon's site I believe.
Le Mans Master
Joined: Apr 1999
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From: Raleigh North Carolina
The newer ones do not have a bi-metallic spring.... they are controlled by vacuum. A temp/vac switch closes the valve when the engine is cold. When it warms up, the vac ends, and the valve opens via the return spring inside the vac control can.
Drifting
Joined: May 2005
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From: Somers CT
Originally Posted by Tom454
The newer ones do not have a bi-metallic spring.... they are controlled by vacuum. A temp/vac switch closes the valve when the engine is cold. When it warms up, the vac ends, and the valve opens via the return spring inside the vac control can.
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Race Director
Joined: May 2006
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From: Dayton, Ohio
Originally Posted by moosie982
The heat riser on my 80 L82 vette was vacuum operated in conjunction with one of the temp. sensors on the water outlet (for smog purposes)and was noisy too, G.M. used to put a one way valve in line that stopped the noise, we usta call them black and white valves cause thats what color they were. Can anybody elaborate ?Peace,,,Moosie
One other thing I do to help with this rattling noise it to put a couple washers under the actuator can--between the actuator and bracket.this shims it up and holds the heat riser valve closed tighter until its time to open.This and the 1-way valve usually takes care of the rattle.
Oh and Mrowpolitron your 15 min. black smoke problem is probably the choke pulloff.Could be the carb leaking down but you said it started good in the morning so I doubt that is it.
Last edited by ...Roger...; Dec 6, 2006 at 04:01 PM.
Le Mans Master
Joined: Apr 1999
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From: Raleigh North Carolina
Originally Posted by TopGunn
Good point, he didn't mention that he had a spring type.
moosie982 said..."The heat riser on my 80 L82 vette was vacuum operated in conjunction with one of the temp. sensors on the water outlet (for smog purposes)"
The Vette in question is an 80, so.....
Melting Slicks
Joined: Aug 2002
Posts: 3,334
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From: Midland TX
Don't know about the 1980 but my 72 has a thick gasket between the intake and the carb. If you don't have a good gasket between them, you will transfer a lot of engine heat to the carb. May be another source of your heat problem.
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Thanks guys for all of your input, but keep in mind that someone has welded my heat risor in the 3/4 closed position. Vacuum is still hooked up but nothing is going to move! It does sound like I am getting excess heat going back to the intake if the flapper is still in the heat risor valve. One tech said he didn't think the flapper could still be there because he thought my car should be running crapy if it was indeed in the almost closed position. I do think the thicker gasket does sound like a good idea. Does everyone one else agree? Mark.
Race Director
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From: Dayton, Ohio
Originally Posted by mrowpolitron
Thanks guys for all of your input, but keep in mind that someone has welded my heat risor in the 3/4 closed position. Vacuum is still hooked up but nothing is going to move! It does sound like I am getting excess heat going back to the intake if the flapper is still in the heat risor valve. One tech said he didn't think the flapper could still be there because he thought my car should be running crapy if it was indeed in the almost closed position. I do think the thicker gasket does sound like a good idea. Does everyone one else agree? Mark.