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With a little massaging there is more available, without spending too much money on parts! (No NOS).
I got 141 hp at the rear wheel, (measured on the dyno that was used to tune Mick Doohan's 500 2 smoker) by mild porting of the heads inlet side, taking 20 thou off the head, cutting up the standard down pipes and welding them into a 4 into1, and therefore deleting the exup. The carbys were modded by filing the taper on the mainslide needles to a finer point, and polishing them, in a drill stand to a fine finish with 600 grit wet n dry. (They were made identical with a micrometer, by marking them at 2mm intervals with a fine marker pen). The carby mainslides were drilled oversized to 150%, in order to liven up the slide response to the low pressure. Bigger main jets are fitted. Cutting away the top of the inlet horns, and all of the plastic inside of the air cleaner end, improves flow into the airbox, but transforms the lower side of the fuel tank into a lid for the airbox. Jacking up the tank at the front tunes the airflow. Offsetting the the ignition trigger to operate 2 to 4 degrees earlier, improves power further up the rev range. It takes some fiddling to get it all balanced! Good luck!
 
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