Top-end stutter after big bore upgrade

[dhph]

Recently I did a 70cc Polini upgrade to my son's Mongoose 50 (2 stroke) along with a 19mm Arreche carb upgrade. It pulls great from idle to about 3/4 throttle, but from there up it stutters and won't make any more power. My first thought was it was running too rich up top (although the plug didn't really look like it) so I started to reduce the main jet from 95, to 90, then to 85. All that happened by the time I got to the 85 main was a noticeable decrease in power throughout the rpm band, but no affect on the top end stutter. So now I'm back at a 90 main jet with good performance to 3/4 throttle but hitting the stuttering-wall from there on up. Running without the airbox lid does not help, the cooler fall temperatures recently do not help, and taking out the exhaust baffle also does not make a difference (exhaust is stock). At this point I'm inclined to think it may not be a fueling issue but maybe a CDI issue. The green wire loop has been cut but maybe the CDI itself just can't cope with the increased power/RPMs? Any thoughts?

[quadnana7]

Good aftermarket pipe should make a huge difference.

[Hutchracer]

We did this same upgrade to my daughters Mongoose and had the exact issues you are having. Adding a pipe won't make a difference as we tried that with our quad we are running a 70 Polini cylinder with mild port work and the motor cases were done as the kit says to. I put a 24mm carb on with the injection pump blocked off and were running a 4.5" Uni filter with an outerware over it with no air box and we run at 50:1 premix on 94 octane gas. After much fighting and trying different jetting we finally tried a aftermarket CDI box and it finaly would rev out fully. I would say to look into running a different CDI box and I'm pretty sure that will cure the problems you are having. I need to remind myself sometimes that building this quad is a lot harder than some of the others as I'm sure you know. We have been running flat track with ours and it will pull the DRR and Apex 90s on the straits just fine it gives up nothing to those bikes at all. My girl is 9 and was clocked at 44mph this summer on hers the fastest in her class was 47mph but that bike has a $2500 motor with lots of port work and 210psi of compression and is running a PVL.

[Yuk33]

Darin Wentz should be able to help with Kymco info. Moped Hospital in Key West Fl. mess with Kymco motors as well.

[Hutchracer]

I have worked with Darin before on our quad and to say he was a big help would be a huge understatement... He is very helpful and I only wish more mini quad people were as helpful as he was and supportive of the brand.

[jerkyboy]

I had the same issue when I installed the polini on ours and
what your hearing is it hitting the rev limiter. I put a pvl on it and no more problems.