I believe the distributor on a B20F has a vacuum retard, same as on the B20E. If that's so, you'd have to reason the other way around. Try to run it without the vacuum hose -- plug the manifold and the vacuum control unit -- and maybe retard the initial timing a degree or two. Make sure the centrifugal weights inside the distributor are moving freely. You can increase the tension on the centriugal weight return springs inside the distributor by bending out the tabs they are attached to. With the hose detached, this will lower the revs at which the advance starts to come in and increase the revs at which you reach maximum advance. You can increase the spring tension even more by opening up the distributor and fiddling wit the screws that position the weights. At any rate, it's not a bad idea to take the distributor apart and to give it a good cleaning. I've found an incredible amount of crud in the three I've dealt with, and have the impression that they worked better after the cleaning.
Bob S.