> 90% of the problems with vintage motor scooters are either carburators or ignition.
> > Howard
>
> Now I have a problem with my grammar. Is it correct to say "90% are?" I think I was letting my verb agree with the object of a preposition.

I think this is a case where the linking verb "to be" means that the ambiguous subject is clarified by the "object." "90% is a good grade," but "90% [of some group of people] are idiots."

See the explanation I gave to Ayako a while back about whether "It is you who is wrong" or "It is you who are wrong" is correct.