Unfortunately, I can't tell you about the 'add on kit'. I purchased my engine from BGF with both the centrifugal clutch and pull start installed ($169 + ship, if i recall).
Other than the issue mentioned, i've had no trouble at all. No vibration, no strange noises. As a matter of fact, the standard noise issue (coming from gears between the crankshaft and standard clutch) are not an issue with my engine. The Cent clutch seems to block a significant amount of this noise (similar to why some ppl put sound dampening material on their clutch cover). In my case, the clutch cover, which is significantly beefier, doesn't vibrate like the stock one does.
Another possible difference: I am using a Sick Bike Parts shift kit also, and always start out in 1st or 2nd on my 7 speed cassette. Clutch engagement is quick from a standstill, no slipping and no pedaling required. There is no slipping at all going thru the gears and in 7th on a mild hill with full throttle, there is no evidence of anything but full clutch engagement. I can climb the steepest hills in my area (pretty darn steep) at 8 to 10 mph in first with no pedalling and no clutch slipping.
The SBP shift kit adds a freewheel to the crank assembly, so when i come off throttle the cent clutch disengages from the driveline quickly which causes both chains to stop turning. To shift gears while coasting, I simply blip the throttle to get the chains turning, and turn my grip shifter. Very simple, and works like a champ everytime.
All i can say is, with my configuration, the centrifugal clutch works great! I can't tell you how long it will last because I only have about 300 miles on it so far... but even if it were to wear out soon, i'd buy new shoes and keep using it - it's so dang convenient.
SO, bottom line.... With a Sick Bike Parts shift kit, the centrifugal clutch works like a dream (at least in my case) and the whole package is exactly what i was looking for in a motorized bike.
Good Luck and Good Riding!
