My Vette is a 1968 327/300HP/automatic. The engine is original with 108,000 miles. Installed a new performance cam, lifters, etc and a Mallory electronic ignition at 93,000 miles. Engine has always run perfect.

Over the last six months I've noticed a slight hesitation when driving at speeds above 40 mph. It does not cough, backfire, stutter, etc. It is just an instantaneous hesitation and then it continues to run fine. Below 40 mph it runs fine with no evidence of a hesitation. Engine starts and idles fine.

Any thoughts would be appreciated.

 

How long does it continue to "run fine" once the hesitation occurs? If it was a fuel issue, it would offer consistently poor performance at the speed you reference. It would seem to indicate an issue in the main metering circuit but I have my doubts you'll find the gremlin in the fuel circuit. But that's hard to say with absolute certainty. In many cases, rather than reference speed, it can be far more relevant to use load. An engine going up hill at 40mph has a very different load than an engine running on flat land at the same 40mph. Fuel issues in the main metering and enrichment circuits have a load component.

If the problem is very intermittent, you are probably dealing with an electrical issue. Could be a poor connection in the ignition primary, a bad component such as your ignition module or the coil or even in the ignition switch itself. Unfortunate for you, an intermittent electrical issue can be very hard to track down. You can try a wiggle test on the associated wiring. All it takes is a multimeter. If you find no issues with the wiring/connectors, you'd be looking to the module or the coil.

Then there is also the issue of cyclic variation in the engine. You may have an issue developing in the ignition secondary, such as a plug or plug wire breaking down. And just because they may be new parts, you can't entirely rule them out. But without a more consistent misfire, it can be very difficult to track down without a sophisticated tool like an oscilloscope.