Forget "making" a slipper clutch. The problem most people have at some time is stopping it slipping. If you have a problem with the rear when downshifting there are two better options for a bucket - 1:reduce crankshaft inertia/mass by fitting a smaller ignition flywheel from something like a YZ80, which will also get you a good ignition for when you hot it up and 2:don't downshift so bloody hard.
The problem I used to have when I first started was that I could slow down so fast that I couldn't actually blip the throttle quick enough for the downchanges causing a mismatch between engine and rear wheel. With heaps of compression and a light flywheel the motor got real responsive and the problem went away.
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