Why Does My Corvette 350 Small-Block Miss and Stall Out When the Engine Gets Hot?

C.R. Miller Asks…

- Marlan Davis

Q: I have a 1969 Corvette with a 300hp 350 engine. The engine was rebuilt several years ago with just enough bore to clean up the cylinders. It has the stock cam, stock heads with stainless steel valves, and a stock Rochester Quadrajet carburetor. It runs a 160-degree thermostat, a new heavy-duty water pump, and a nearly-new fan clutch.

When the engine temp nears 200 degrees Fahrenheit in heavy traffic, it starts missing and will shut down if I do not get off the road to let it cool down. It seems as if it’s experiencing vapor lock or the fuel is boiling out of the carburetor. In an attempt to isolate it from engine heat, the carburetor has three gaskets under it: fiber on the intake, a thick stainless steel plate, and another fiber gasket between the stainless and carburetor.

I went through the diagnosis steps in your recent engine vacuum troubleshooting article (“Your Vacuum Gauge is Your Friend,” Aug. ’19) and could not find an issue. I measured idle vacuum at just over 18 in-Hg and 22 in-Hg at about 1,500-2,000 rpm. I am at a loss as to where to go and what to do next. Can you please help with some ideas as to what will correct this issue?

A: You need to run any diagnostics checks when the engine is at 200 degrees and compare the results to near-ambient checks. Don’t wait for the problem to occur on the highway; try to duplicate the problem close to home or in your driveway. While the vacuum looks good under normal conditions, what is it when you’re having the problem?