What is a "heat riser"?
Thread Starter
Melting Slicks
Joined: Jan 2003
Posts: 2,714
Likes: 32
From: Shreveport LA
LA Events Coordinator
What is a "heat riser"?
The other morning it was REAL cold and I hadn't started the Vette in a few days. I had to run an errand, the morning was clear and I thought "Why not drive the Vette" I started it (it was kinda hard to start), let it warm up some, started to drive off and noticed the RPM's were at 400 (instead of the 600 like usual). I pulled over, and after about 2 minutes, the RPM jumped to the normal 700 (in park). I called a friend of mine and he said the heat riser wasn't working properly. 1) is that true and 2)what is the heat riser? My friend is out of town for the next 2 weeks.....
Safety Car
Joined: Jul 2002
Posts: 4,269
Likes: 3
From: St. Charles IL
Cruise-In 5-6-7-8-9-11-12 Veteran
A CI-6 Car Show Winner
Original smog equipment on your 75 had a heat riser valve on the passenger side exhaust manifold outlet. The valve is operated by a vacuum actuator that is controlled by a temperature sensor in or near the thermostat housing. When the engine is cold the valve would close and cause back pressure on the passenger side. The hot exhaust gasses would then be forced through a passage up through the head and then through a passage in the intake manifold to the driver's side exhaust manifold. The heat traveling under the base of the carburator would help the car warm up faster, and also would provide a heat source for the adjustable choke coil. Sounds like maybe your choke opens a little too soon and can be adjusted by loosening the 3 screws that secure the black cover and rotating a mark or 2 towards "rich"
Tech Contributor
Joined: Aug 1999
Posts: 15,193
Likes: 4,018
From: Connecticut, USA
Look at the pass side ex manifold between the end of the manifold and the pipe and there about an 2" spacer there with an external spring. That's your heat riser. It works like a choke on the exhaust to help heat up the carb through the choke tube. There is a paddle valve,like a choke on the top of the carb primaries, inside it. They rust up and stay closed a lot of times. I used to remove the pin and valve and use it as a spacer when running stock exhsut. You can wire it open to see if that's your problem. Look in a vette catalog like Zip and they should show one.
Gary
Gary
Drifting
Joined: Nov 2002
Posts: 1,692
Likes: 208
From: Loveland, CO
It's hard to say if the heat riser was the cause of your problem. The heat riser is a flapper valve on the passanger side exhaust manifold. When the engine is cold, a bi-metallic spring holds the flapper closed and forces exhaust gasses under your intake manifold to more quickly heat the intake gas charge. As the spring gets hot, it relaxes and allows the exhaust to flow out its regular exhaust pipe. If you have headers, the heat riser has probably been removed. In any event the heat riser is designed to warm up the engine quicker and run better upon startup. They can stick open with age and not function properly. 
