Correct plugs are NGK BP7HS, which are colder than the ones for carbureted cars. It makes some difference. I've had good results with B8HS (colder yet), although they are not available with the "projector tip" (what the P stands for).

Assuming the D-jet is feeding the motor the right stuff, no leaks, distributor in good shape, etc., what's going to make the difference is the timing advance curve. There are two springs under the breaker plate that can be seen through holes in the top if you turn the motor to the right positions. There's a metal tab that holds the outer end of each spring. Carefully (!) bending those tabs outwards delays the onset/amount of advance at mid-rpm.

Experiment with that, and with the overall timing setting. I've had good results using plain 92-octane, idle timing 14 BTDC (less will feel boggy in the low end) without vacuum (leave it off), and bending adjusting the tabs so full advance doesn't happen until over 3500 rpm.

--Phil S.