vacuum advance idle issue?
Stuupid question...would vaccume leaks bad ebough to cause this present itself all the time, getting worse with increasing rpm?
Vacuum leaks under the manifold that effect fuel/air mixture gets worse with higher vacuum conditions. With the throttle wide open, the effects are minimum, since the vacumm is close to zero.
Having said that, for a non-wide-open throttle, the vacuum WILL rise with RPM, and such leakage problems effects could get worse for small to modest throttle openings.
But you can quit guessing: just spray WD40 around any sealing point to the manifold (under carb or at manifold to head gasket) and see if it effects the idle speed and smoothness at a particular location.
If I can make a suggestion: This troubleshooting over the internet works best if you follow a procedure of steps to investigate A, then B, then C, etc., and let folks know exactly the result of each step. In clear language..... for example, the report of "idle OK crap top end" is hard to understand plus vague....what is 'crap top end' exactly? 1500 rpm sitting still in neutral, or 5000 rpm pulling up a mountain at 100 mph?