Heat riser question
Thread Starter
Racer
Joined: Oct 2010
Posts: 457
Likes: 0
From: Pewaukee WI
Heat riser question
My car has been on the road for about 150 mi this spring after the exhaust system (among many other things) were removed and worked on this winter. I noticed the other day that the heat riser was jammed shut. It took quite a bit of force, one pound or so for it to start to move, and then it was free as a bird. After checking it a few more time I found it to be jammed shut again.
I loosened the nuts on the exhaust manifold studs and as soon as I did that that the heat riser was free. I tightened the 3 nuts up evenly and the heat riser remained free. Seems that the flange may have been a bit cocked.
I took it out for a drive today and was surprised to see that the driver side exhaust tip was hot to touch, but the passenger side was somewhat cooler. Is there a way to determine that the heat riser is operating fully open at temperature? Once the heat riser is at temperature, should it stay open like an automatic choke, or does it open and close as the flow of exhaust increases or decreases.
Thanks for your help
Myron
I loosened the nuts on the exhaust manifold studs and as soon as I did that that the heat riser was free. I tightened the 3 nuts up evenly and the heat riser remained free. Seems that the flange may have been a bit cocked.
I took it out for a drive today and was surprised to see that the driver side exhaust tip was hot to touch, but the passenger side was somewhat cooler. Is there a way to determine that the heat riser is operating fully open at temperature? Once the heat riser is at temperature, should it stay open like an automatic choke, or does it open and close as the flow of exhaust increases or decreases.
Thanks for your help
Myron
Team Owner
Joined: Jul 2006
Posts: 22,201
Likes: 4,187
From: Kannapolis NC
Once the spring gets hot, the weight of the square chunk that sticks out pulls it down and opens the heat riser. As the spring cools, the tension overrides the weight of the metal and it slowly closes, just like a choke. When it's hot the lever should be down and up at 90 degrees when it's cool.
Le Mans Master
Joined: Sep 2003
Posts: 8,812
Likes: 2,029
From: eastern NC
It might be easier to borrow someone's IR Temp Sensor tool (if you don't have one already) and check the temps on the header pipes below the manifold when the car is thoroughly warmed up.
Good luck... GUSTO
Last edited by GUSTO14; May 5, 2015 at 12:13 AM.
Burning Brakes
Joined: Mar 2014
Posts: 904
Likes: 46
From: Crestline CA
Mine was sticking closed also(brand new one too). Makes a huge difference in power output when it sticks(even more than the difference when I got rid of the headers and went back to manifolds) got tired of screwing with it so used some safety wire to wire it open, no more problems and can be reversed for if/when I finally get around to getting my car judged.
Burning Brakes
Joined: Apr 2009
Posts: 1,167
Likes: 173
From: Somewhere North of The 49th Parallel Ontario
Took mine off and wired it open also. Made a huge difference on drive ability. I also installed the felpro intake gaskets that block the crossover runner under the carb. Previously had a minor problem with gas percolation on a hot summer day.
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Racer
Joined: Feb 2011
Posts: 299
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From: Troy New York
Safety Car
Joined: May 2014
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Keep in mind the heat riser even on a hot day will usually stay mostly closed at idle. Double check you have the exhaust extension on the pipes because it centers the pipe inside the heat riser and also keeps the donut centered. Without it the valve can be sticky like you report.
As a test just wire the heat riser valve open and drive the car, your problems will probably go away. One more thing to think about is to make sure the vacuum advance is working as this provides 15*+- additional timing at idle and helps keep the engine temps down.
As a test just wire the heat riser valve open and drive the car, your problems will probably go away. One more thing to think about is to make sure the vacuum advance is working as this provides 15*+- additional timing at idle and helps keep the engine temps down.
Team Owner
Joined: Mar 2003
Posts: 26,118
Likes: 1,874
From: Greenville, Indiana
Good point. If that sleeve is missing it can also allow the exhaust to blow out the donut and cause a leak and/or the doughnut can get crushed when you tighten the pipe to manifold.
Race Director
Joined: Apr 2005
Posts: 11,286
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From: Bowling Green, Kentucky
Team Owner
Joined: Jul 2006
Posts: 22,201
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From: Kannapolis NC
Both my cars are fully open (down) when hot at idle. As the cool, they slowly rise up. The engine rpm doesn't change the fully open position (on any I've owned) That would mean the spring tension at temperature would have to be perfectly calibrated so that the exhaust pressure could push it open and it would still partially close at idle. I can't see the spring being calibrated that perfectly, specially over time. If that were true, at what rpm is the exhaust strong enough to push it open and then allow some closure if slower? Hot=open, cold=closed. Warm, who knows?
Race Director
Joined: Apr 2005
Posts: 11,286
Likes: 2,146
From: Bowling Green, Kentucky
Both my cars are fully open (down) when hot at idle. As the cool, they slowly rise up. The engine rpm doesn't change the fully open position (on any I've owned) That would mean the spring tension at temperature would have to be perfectly calibrated so that the exhaust pressure could push it open and it would still partially close at idle. I can't see the spring being calibrated that perfectly, specially over time. If that were true, at what rpm is the exhaust strong enough to push it open and then allow some closure if slower? Hot=open, cold=closed. Warm, who knows?
That's why for many of us that just drive these cars occasionally and during warmer months, just replace it with a spacer or wire it fully open.
Larry
Team Owner
Joined: Mar 2003
Posts: 26,118
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From: Greenville, Indiana
In a perfect world, I agree that the heat riser should be fully open when the engine is hot and at idle. I just haven't seen it on my cars over the years. Maybe it's that the spring is old and sticking a bit………….but there always seen to be a bit of additional travel to the riser when engine RPMs go up.
That's why for many of us that just drive these cars occasionally and during warmer months, just replace it with a spacer or wire it fully open.
Larry
That's why for many of us that just drive these cars occasionally and during warmer months, just replace it with a spacer or wire it fully open.
Larry
Check me on this one as it has been awhile since I even looked at one.
Melting Slicks
Joined: Oct 2011
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From: Sacramento California
C2 of Year Finalist (performance mods) 2019
2018 C2 of Year Finalist
Does the heat riser have a gasket between it and the manifold?
Ill be swapping my riser for the FI spacer real soon.....
Ill be swapping my riser for the FI spacer real soon.....
Team Owner
Joined: Mar 2003
Posts: 26,118
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From: Greenville, Indiana