Pinking

What I'm saying is that your car has roughly the same bhp/litre and being of a similar design it's probably got a similar lbft/litre. Obviously the gearing is going to be different, but I'd suspect there's not really enough torque to pull the thing along at 30 in fourth. I can tell by ear if the engine's working too hard, and I expect you notice some difference in engine noise as the revs drop and the %age load on the engine increases. I still think you need to give the thing a good caning to clean out the engine... Bear in mind the engine will be running rich much of the time to help it run cold (ie, more fuel to less air, more carbon...).