383 Cruising Issue
I wouldn't cruise there because;
A) that's a lot of wear and tear on the engine About 70% more.
B) NVH. Noise,Vibration and Harshness.
4000 at 65 is friggen loud. And
C) the driveshaft is whipping around something fierce.
D) the rear end is running a lot warmer, and is gonna want a better oil.
E) the automatic is gonna run warmer too..... and it puts it's heat right into the coldest part of the rad...... which then immediately goes into your engine. So your cooling system has to be up to the task.
F) RPM Sucks gas.
Dredging up an old thread because I found it interested.
6kevin8 - One of the first things I did was replace the exhaust when I acquired the Scamp. When I did so, I added a bung to each side's exhaust. Then I put an AFR gauge in (just one, swap from side to side when I need / want to, just to balance or confirm balance). It has been the most satisfying thing I have done to the car in 1.5 years worth of a long list of things I've done to the car (mainly all maintenance and repair after the car sat for 15 years).
I tell you this for 2 reasons. 1) The sound you tried to recreate on this thread, the "k-thunk, k-thunk, k-thunk, k-thunk" sounds like what I have grown accustomed to as a lean sound. I would have never guessed that before the AFR gauge. I might be wrong, because of the differences between mine and your interpretations as to what we hear. And, 2) if you have the ability at all, I strongly suggest putting an AFR gauge on it. It will answer so many questions.
I wonder if those long drives on the interstate are building up just enough heat to start boiling gas in the lines. But the gas is moving fast enough that most of the time the gas doesn't fully vaporize before it gets to a the carb. Don't know if hills would make it better or worse. I would generate more heat, but it would use fuel more rapidly too. But, some level of vapor lock might be slightly starving the carburetor causing it to go lean. Just a theory.
It's been ~4 months, maybe you've figured out what the problem really was. I would be interested to know.
7milesout