so it's only at part throttle cruise on the highway....check your timing (sounds repetitive, huh?! ) it could be the vacuum advance is pulling too much timing and you get spark knock, but with less throttle, there's not enough air to get spark knock, and with more throttle there's not enough vaccum to advance the timing that far.
) it could be the vacuum advance is pulling too much timing and you get spark knock, but with less throttle, there's not enough air to get spark knock, and with more throttle there's not enough vaccum to advance the timing that far.