I love this site & have spent many hours reading the forums etc. I have recently built a "Happy Time - Starfire GT4 67cc" based Mountain Bike. I only want to use the bike off-road, as the area I live is mainly forest & very hilly. After fitting the engine, I found the gearing was way too high for most of the terrain. I've now geared it down by making a jackshaft between the engine & rear wheel sprocket. Now I can burble up the steep tracks with my trusty Collie (Cody) leading the way!!! I've done a few other mods/improvements also, but now I'm keen to try & develop a centrifugal clutch type engine to suit my needs. It's quite funny really that everyone wants to go faster & I just want to go slower!!! I do think that the shifter kit from sickbikeparts is brilliant, well designed & engineered to a very high standard. That kit gives you the best of both worlds - fast top end & great hill climbing ability. I'm saving my pennies at the moment to get the kit cause I want to be able to go faster as well!! Best Wishes to you all Shaun.