Originally Posted by Secondaries

Sorry to repeat something that already has been said, but please double check your throttle linkage and your bowden cable.

Make sure the little round roller is at this position at rest:



And that your bowden cable is adjusted so that this little tab has a slight gap here:



FYI, in this picture I am moving the linkage so that all the slack is removed and any further I pull will move the pump throttle. With slack this gap will be a little bit bigger (which can be seen in the first photo).

When I got the car, that round bushing was halfway through that slot, and the transmission was shifting a bit later than it should. After making these adjustments, the car feels "correct".