Keep in mind that D.H. usually puts a tuned exhaust set up on his ZDZ'z and does not use a Bolly prop.

The first thing I would do is remove the Bolly. If you can find a Menz in the same size, and right now you might not, try that. Perhaps a 3W or RCS prop in the same size. A PT would be another good alternative. No Xoars!! They all seems to provide lower rpm counts and less thrust that I think they should. I don't believe the ignition has any influence in this case, either new or old style. You might also try using a little better gas. Try some premium just to see what happens. In any case, try only one thing at a time and see what each item does for the performance.