No bump start, with the z-Start; well in an emergency you can put in an extra drive plate and turn it into a manual clutch but that's a pain... If you're trail riding a YZ-450, 5 tungsten carbide balls works very well. For a WR-450, I think 5 tungsten carbide balls is too abrupt for the stock gearing but if you like to ride around in 3rd gear all the time, the 5 tungsten carbide balls would work well. 10 tungsten carbide balls will essentially turn the 450 into an on/off clutch, not good IMO. I don't think either the YZ or the WR 426s needs the tungsten carbide balls for trail riding but again, if you really like to lug the motor in 3rd gear, then it's a good addition. We try to setup each bike to give a good adjustment range for it's intended use. The TC balls are just another tuning option. One more thing... I was just doing an install on an '04 WR-450. The '04 seemed to have a lot less clutch cable slack and more clutch cable friction than the last '03 I installed one in. Make sure the clutch cable slack is pulled up towards the bars (there is a rubber grommet on the frame that you can pull the cable through) so the cable can move freely into the the z-Start perch adjuster. Also, if you run the z-Start perch adjuster, don't forget to check for proper cable slack from time to time. This seems to confuse quite a few people... The z-Start perch adjuster only adjusts engagement speed and engagement rate. The original perch cable length adjuster must still be used to adjust for proper cable slack. The tricky part is to adjust your cable slack with the bike in neutral and the engine revved to at least 3000 RPMs (so the z-Start is engaged). You're looking for about a nickels worth of play between the lever and perch (just like your Yamaha manual tells you).