Question regarding exhaust heat riser

[Kurt G.]

Just put a new exhaust on my '72 small block, along with a fresh overhaul on the q-jet. The previous owner had installed a SS exhaust and wired the heat riser open. Replaced with steel. My question is, what is the purpose of the heat riser, and what functions does it control? I've noticed a couple of things since the initial start up. First, the right side exhaust pressure and volumn are considerably less than the left side, both at idle and warm up and after I have driven the car a few miles. Second, during acceleration, both from a stop and during gear shifts the engine hesitates and seems to be loading up, with very poor acceleration. I noticed the heat riser was not opened up after driving with the engine at idle. Normal? Thanks in advance to all the engine guru's out there.

[Patrick H.]

Re: Question regarding exhaust heat riser

Kurt,

Be sure you dodn't install the hear riser upside down. This is not tough to do incorrectly. Be sure that when you apply a bit of pressure to the weight it drops DOWN.

The purpose of the heat riser is to direct exhaust heat back through the intake manifold to warm the carb on cold starts. As the engine warms up the bimetallic spring on the heat riser expands and the heat riser opens. This then allows the exhaust to flow out both sides of the car. The right side will be closed (almost non-working) on cold start, but with my cars it doesn't take too long for the riser to open and the exhaust to work.

Patrick

[Jim T.]

Re: Question regarding exhaust heat riser

Right side muffler will last longer with the heat riser wired fully open.

[Edward J.]

Re: Question regarding exhaust heat riser

Kurt, If you did not have any problems before the carburetor was rebuilt, the may be a internal adjustment problem,Float adjustment( maybe to low), hung up primary metering rods. take a look down the throat of carb. and see if there is a good shot of fuel when you push down on throttle. should be a steady stream of fuel coming from the nozzles . if the engine is cold you may have to open the choke butterfly.

Does the hesitation disappear after the engine warms up?

[Duke W.]

Re: Question regarding exhaust heat riser

The purpose of the heat riser is to create a "hot spot" under the manifold plenun to promote fuel vaporization for good start off driveability after a cold start. In some cases there is also a "slot" that
allows exhaust gas to flow directly under the base of the carburetor idle circuit.

Even when warm the heat riser may stay closed at idle, but blipping the throttle should open it up since the shaft is slightly offset. The thermostatic spring tend to weaken with heat cycles and the valve can become sticky due to wear and corrosion, so some may not have enough tension to open the heat riser even at cruise, which will cause poor performance.

Heat riser action allows the right side muffler to rust out early because most of the right side exhaust is routed through the heat riser to the left side exhaust, so a lot of water condensation occurs in the right side muffler. This is especially the case if most trips are short.

Since most of us only drive our vintage Corvettes in mild weather, wiring the valve open will promote better exhaust system life and mitigate fuel percolation problems without creating driveability issues. Some also "gut" the heat riser housing or, on small block, use the the blank housing that was installed on fuel injection engines.

The heat riser passage can also be blocked by installing thin stainless steel shims across the inlet manifold gasket opening. Many years ago I did this on my '63 340 HP engine, but I removed them because of a severe start off stumble for the first 15-20 minutes of around town operation. This can also cause problems if the choke coil is mounted on the inlet manifold above the heat riser passage - not enough heat for the choke valve to fully release - however, I have recommended that one side be blocked - the side opposite the choke coil. This creates a dead end passage that allows hot exhaust gas up the passage to heat the choke coil and provide some heat under the manifold, while mitigating the fuel percolation problem that the heat riser can cause.

Duke

[Kurt G.]

Re: Question regarding exhaust heat riser

Thanks guys. Me thinks me sees more wire in my future. Haven't driven it enough to see if it stops hesitating when warm, and right now don't plan to. Maybe once all the sand on the roads is cleaned up.

[Patrick H.]

Re: Question regarding exhaust heat riser

Thanks guys. Me thinks me sees more wire in my future. Haven't driven it enough to see if it stops hesitating when warm, and right now don't plan to. Maybe once all the sand on the roads is cleaned up.

Kurt,

Assuming it's installed correctly, not binding and actually is opening, you shouldn't have to wire it. It may be a poor quality repro part, or it may be that the hesitation is in the carb as Ed mentioned early on.
Wiring it open only fixes one of the symptoms, not the problem itself.

Patrick

[Don L.]

Re: Question regarding exhaust heat riser

Kurt,

Be sure you didn't install the hear riser upside down. This is not tough to do incorrectly. Be sure that when you apply a bit of pressure to the weight it drops DOWN.

Patrick

I knew that!!
Ya made me look, Patrick.

I did wonder though if the force of exhaust, when RPM's are higher than idle, will hold the flap in the "all open" position. I've never been sure mine opens all the way, because when warm, I've been able to open it a bit more by hand when parked/running.

Good thread subject. Thanks!