Note: I tried to send you a PM (to the PM you sent me) but it appears your PM function is turned off. Thus, a reply via open forum....
Your question to me: I bought my daughter a pipe for her Predator 90 for Christmas. I got it from xxxx a local guy who makes pipes for ATV's. I also have one on my 400 EX and it works great. The problem is that once he put the pipe on the Pred-90, it has absolutely no bottom end. We do most of our riding in the dunes sothis is a big problem.
Many say that when someone adds a pipe, they "should" also install a clutch kit. Especialy for soft sand conditions. A clutch kit (combination of different size rollers and different size clutch springs) allows the CVT system to rev higher before engaging, and revs higher at slower ground speeds. Same feeling as driving a car with automatic tranny in 3rd gear, instead of OD gear. Higher revs = more power and momentium up/down sand hills.
Many also say when more airflow can flow through a performance pipe, (and higher rev engine via clutch mods), one should also upgrade its air filter. Thus, reduce airflow bottlenecks in the front and back end (sort of speaking). Some install K&N Filter, some install unifilter and some install open foam filter. Others, like myself, install a bigger snorkel pipe on the factory air box. If you install an aftermarket air filter or modify the factory air box, carb jetting must be done inside the factory 16mm carb. The factory main jet is 82.5. If you go above 90 main jet in the factory Pred-90 16mm carb, you must "drill out" the hole behind the main jet. A special bit size must be used. For K&N & uni filter, some run up to 120-130 Main jet size. Many of the carb experts recommend that if jetting above 90, it would be best to replace the factory 16mm carb with a 20mm carb. The 20mm has a much better "impulse pull" for higher RPMs, takes more altitude changes and takes more weather temp changes. In other words, much less adjusting after you get its base set. If you install a 20mm carb, one needs to install a 20mm carb to engine intake and a new 20mm gas cable. If you install a larger snorkel on the factory air box, your jetting will increase to 87.5 or 90. Being under 90, no carb drilling is required. Yet, some air flow improvement is obtained - not as good as 3rd party air filter replacement but it works for us. For a picture of our modified snorkel (to keep our costs down), surf: http://community.webshots.com/photo/...29655958iCaQAK
If your young rider drives above 3/4 throttle NOW, making it rev higher via clutch upgrade will make it have slower ground speed. Thus, an upgraded CDI (one that is NOT Rev restricted) must also be installed to allow it to have the same (and a little more) ground speed.
For parts and validation of the above, contact:
Billy at http://www.wrhracing.net/contact_us
Rich at http://www.hetrickracing.com/contactinfo.htm
Ed at http://www.lucky7racing.com/
Brad at http://www.teamsimsracing.com/
Hope this helps....