And what carburetor(s) and have you spent time tuning them?Along with the spark advance advise above.... Been my experience most guys run a far richer cruise fuel ratio mixture than necessary because they read spark plug color. Plug color is for full throttle...Cruising at 1800 rpm the carb is generally just coming onto the main jets from the off idle circuit.Lean down about two jet sizes and try that.If no lean steady speed surge or slight hesitation as you very gently feed the gas pedal at about 45 mph,lean down a bit more.Use direct drive at 45 mph.If you jet down more than 3 or 4 sizes you may have to enlarge the high speed or power valve circuit if that's possible.Your engine's spark plugs will likely be bone white at light loads,no problem if the engine responds ok and no detonation.Check it out on the hiway at speed in OD also.
But always jet for the way you drive,a easy driver can run a leaner mixture than a gas pedal jabber
