On our pred 90 i have done the following: WRH race pipe, .016" off the head, rev-less cdi, 27 tooth rear sprocket,running 40:1 klotz and premium, lighter roller in primary clutch with only three rollers, single throttle cable, polished and ported exhaust, 30 pilot jet and 90 main jet. I'm going to check the pipe muffler packing but i have very little if any low end. Could you insert a washer with a smaller inner diameter into the muffler section of the pipe to create some low end? what would work good but not cost an arm and leg at this time.
thanks
hunt

 

With the pipe you definitely need to modified the clutch engagement. Change the springs in the rear clutch. This will allow the motor to rev higher towards the pipes 'power band' before it engages. Clutching is critical with a performance pipe. Billy at WRH knows exactly what springs you need. In addition, once you get your clutching corrected you'll be able to add roller weight back to the front variator. This will give you more top end speed as well as the bottom end your looking for.