Heat Riser Question
Thread Starter
Instructor
Joined: Aug 2009
Posts: 112
Likes: 0
From: Caledon Ont
Heat Riser Question
Does the heat riser close off exhaust with vacume applied? And if so vacume should be applied on start up then taapper off through the thermo switch as the engine warms? Anyone know?
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
Yes vacuum closes the riser. The thermo switch turns off the vacuum and dumps the vacuum from the actuator.
Thread Starter
Instructor
Joined: Aug 2009
Posts: 112
Likes: 0
From: Caledon Ont
Thank you that explains alot, it was directly hooked to vacume and the car ran awful, felt like the carb I played with that then noticed the heat riser, I unplugged it and the car ran better so now that I know this I can ra adjust the carb again.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
I can't say later years didn't have a 'vacuum operated' heat riser...but '71s and earlier didn't. They have a bi-metalic coil that unwinds as it warms up. At cold temps, the valve is closed which dumps exhaust gases into the intake manifold [under the carb]. That warms the carb and choke coil to help manage cold engine warm up. Once the heat riser warms up and the coil unwinds, it opens the valve so that almost all of the exhaust goes to the pipes.
