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I am replacing my exhaust and going from the dual to single to dual with a full dual exhaust without cats. I decided to replace the heat riser while I have it all apart. I went to connect the heat riser valve to the rod on the diaphragm and noticed when I connected it that it was partially closed. When the engine is off shouldn't it be fully closed? It looks like the rod has a spot you can put a wrench on it, is that for length adjustment?
Ya- and the oil wouldn't reach 212*F to boil off the moisture.
Oh wait- it won't anyway and there is no moisture if the car has been operated correctly.
Having a heat riser valve has nothing to do with cat converters. It's normal for the valve not shut completely on it's own. Exhaust pressure will force it closed.
The heat riser valve was there to direct hot exhaust gasses below the carb to help warm it in cold weather. It does not really help the engine warm up quickly; that is the purpose of the thermostat in the cooling system.
IMO, more problems result from having a heat riser and having it stick closed (or partially closed) than any advantage resulting from early warming of the carb. Unless you plan on driving the car much in winter, it would be better to remove the valve plate from the heat riser AND put blocker plates in with the intake manifold gaskets (to prevent heat flow under the carb passage).
For anything but cold weather, you won't notice much difference...and you eliminate the risk of a permanently closed heat riser valve.
Heat riser was intended to help warm the motor more quickly but that's it. They are always sticking, breaking or leaking. I usually take them off, knock the flapper out and weld the holes from the flapper pin closed. If you can install your exhaust without the riser, I would do it that way but many times it's part of the system and needs to be there for the pipe to bolt up properly to the manifold.
Heat riser sends exhaust heat up and under the carb. None of it heats the rest of the engine. It's a 'carb warmer' and sometimes used to also warm the incoming air to the carb inlet.....nothing more.
From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
Originally Posted by Mike Ward
Having a heat riser valve has nothing to do with cat converters.
True but it does have to do with emisions i.e. a quicker warm up will reduce emissions. I mentioned cats because they also have to do with emissions and seeing he was removing his cats, then why not remove the heat riser, if emissions regulations in his state allow. It's just one more piece on an engine that can fail and not required in the first place unless you live somewhere where it is mandantory to have a compliant vehicle.
I may just wire it open for now and see how it drives. The car doesn't go out in the winter, but will on a cool fall or spring day. I tried to find a spacer where the valve goes, but I couldn't find one for my car. I did find the spacer for the earlier heat riser valve. Other than the exhaust it is a stock L48, so I am not too worried about any lost flow from the wired open valve. I also need to check to make sure the diaphragm doesn't leak.
Last edited by JimLentz; Sep 19, 2014 at 01:36 PM.
That's why it's easier to just grind on the valve to remove it from the shaft (while it is accessible). Let the bi-metallic spring and counter-weight do whatever they want (and still look stock from the outside). That valve will never again be a problem.
P.S. In very 'cool' weather, it might take an extra minute for the car to smooth out the idle. Not much loss for the assurance that the heat riser is "open".
That's why it's easier to just grind on the valve to remove it from the shaft (while it is accessible). Let the bi-metallic spring and counter-weight do whatever they want (and still look stock from the outside). That valve will never again be a problem.
P.S. In very 'cool' weather, it might take an extra minute for the car to smooth out the idle. Not much loss for the assurance that the heat riser is "open".
The heat riser valve on my '77 is different as there is no spring or counterweight. This actuator connects to the heat riser valve and uses vacuum to control the valve.
That's not the 'heat riser'. That's just a gizmo for having heated air (coming off the outside of the exhaust manifolds) to supply warmed air to the carb at start-up. It has a vacuum-operated valve which opens when engine is cold and closes when engine is hot. The 'heat riser' is located between the exhaust manifold outlet and the downpipe for the exhaust system.
That's not the 'heat riser'. That's just a gizmo for having heated air (coming off the outside of the exhaust manifolds) to supply warmed air to the carb at start-up. It has a vacuum-operated valve which opens when engine is cold and closes when engine is hot. The 'heat riser' is located between the exhaust manifold outlet and the downpipe for the exhaust system.
Yes, but that has a rod that connects to the heat riser valve. I wish I had a picture handy.
On a '77 vacuum closes the valve. It should be open with engine off. The rod is adjustable. On mine the diaphragm is ruptured. I just put a plug inside the vacuum line that goes to the actuator and it stays open all of the time. There is a spring in the actuator that holds the valve full open with no vacuum. This is also a fail safe in case of vacuum switch, vacuum line or diaphragm failure. mike...
On a '77 vacuum closes the valve. It should be open with engine off. The rod is adjustable. On mine the diaphragm is ruptured. I just put a plug inside the vacuum line that goes to the actuator and it stays open all of the time. There is a spring in the actuator that holds the valve full open with no vacuum. This is also a fail safe in case of vacuum switch, vacuum line or diaphragm failure. mike...
Thanks everyone, I decided to reconnect it for now. For some reason it wasn't completely open when I put the new valve in. I removed the diaphragm to look at it. I changed nothing, but when I put it back on it is completely open with the engine off now.
Jim, this is what your '77 has on it now... correct?
This is what was used for many years on some of the C1's, most C2's and C3's. Here the spring would heat up and open the valve to allow the exhaust to flow through the pipe... or not, as they eventually tend to rust in place, frequently with them at least partially closed.
Another option is to use one of these to alleviate any chance of the valve not operating properly. https://www.paragoncorvette.com/p-357404-exhaust-pipe-spacer-rh-2-12.aspx
At the very least you should check that the valve operates freely periodically by reaching down and moving the weight (or the rod on the '75-'79 Corvette), just be sure the engine has cooled down before reaching down there and grabbing it.
Jim, this is what your '77 has on it now... correct?
This is what was used for many years on some of the C1's, most C2's and C3's. Here the spring would heat up and open the valve to allow the exhaust to flow through the pipe... or not, as they eventually tend to rust in place, frequently with them at least partially closed.
Another option is to use one of these to alleviate any chance of the valve not operating properly. https://www.paragoncorvette.com/p-357404-exhaust-pipe-spacer-rh-2-12.aspx
At the very least you should check that the valve operates freely periodically by reaching down and moving the weight (or the rod on the '75-'79 Corvette), just be sure the engine has cooled down before reaching down there and grabbing it.
Good luck... GUSTO
Yes, mine is the one at the top. Right now the rod/valve is where it should be and I will check it regularly. Since my plugs on the passenger side where the valve is located were fouled I think the previous valve was stuck at least partially shut. Thanks.
Yes, mine is the one at the top. Right now the rod/valve is where it should be and I will check it regularly. Since my plugs on the passenger side where the valve is located were fouled I think the previous valve was stuck at least partially shut. Thanks.
I got the exhaust finished yesterday as well installing the carb and a few other odds and ends. I took it for a 30-45 minute test drive to make sure it was fully warmed up. When I got home I left it running an opened the hood. The heat riser valve was closed. As soon as I disconnected the vacuum line from the actuator it opened. So, it looks like the TVS (Thermal Vacuum Switch) is bad. For now I am leaving the vacuum line disconnected and plugged.
Heat Riser on 77 is it Adjustable?
