77orange

Current heat riser that is working but sticks. Does not appear to be a 1977 design.

Working on my 1977, quadrajet, 2 inch pipes heat and have the incorrect heat riser according to documentation. I think this is a 1974 year heat riser. The current set up works fine other than it is sticking and needs replaced. Before I ordered I thought I would ask a few questions to confirm the correct unit and also a little bit about the difference between what is installed and what SHOULD have been installed. Car came like this to me last year - no history known.


Question 1: Is this in fact a 74 style unit?


Question 2: is the adapter flange that was between it and the exhaust pipe needed? This adapter pipe was hitting the flapper in the heat riser and stopping it from functioning! I removed it a month ago and haven't noticed any exhaust issues. Other than the car runs much better with the heat riser operating


Question 3: My current choke heater tube comes from the intake manifold up to the choke and works great. Was this likely a retrofit from the earlier models?


Thanks in advance, Jim.

Alan 71

Hi Jim,
The heat riser in your photo is controlled by a simple bi-metallic coil.
The original heat riser for your 77 is controlled by a small vacuum actuator controlled by a temperature sensor in the coolant neck.
Regards,
Alan


(Thanks to mds3013)

77orange

I see. So the heat needed to advance the choke is the same (metal tube that draws heat up from the intake manifold). The change then is in how the heat riser is butterfly is controlled. If this is correct, which is better?

TimAT

The 77 heat riser is operated by vacuum that is controlled by a thermostatic switch mounted to the thermostat housing.
The one you have is controlled only by the heat at the valve itself. Unless you're wanting to go back to OEM, there's nothing wrong with the one you have. If it's a little sticky, there's a really high temp lube called "Mouse Milk". Lube of choice for light aircraft turbocharger waste gates.

77orange

Thanks everyone. I will order a replacement one from earlier 70's.

One remaining question if you have time. When operating correctly, should the heat riser heat spring pull the flapper open and hold it there when the engine gets hot? Mine opens and closes with throttle response (now) but never stays open at idle, even when hot. I think this too is another reason to replace it but curious as to exactly what it is supposed to do. I would think it would hold open when hot and that's what the metal temp spring is for.

0Willcox Corvette

The spring should hold the flapper partially closed while the car is cold and open fully when the spring heats up. (if you are using an older style riser).

Willcox

77orange

Well, that was a wasted $65!!! I bought one from CorvetteMod via Amazon and installed it. It stays more closed at idle when hot than the one I took off. In fact, I thing this one sticks. Got back from a 15 minute ride and it was closed. Touched it with a screwdriver and it went open.

Wrote CorvetteMod via Amazon for advice and they said "we have no technicians, check your installation...". No technical help at all???

Should have ordered from Wilcox... maybe I will