It's shaking because your setup is wrong. The magic number is 25 because your gear ratio is probably close to a 8:1 which puts you at about 100rpm per mph and 25mph is about the speed that would be your clutch locking up. Put the green spring in it and it will get better. A stock clutch works great because most karts and minibikes that use a Max torque have gear ratios set for about 30mph. To go faster on one gear your clutch needs to slip a lot more. Think of it as driving a manual car in 1st gear vs 3rd only. In 1st you lock it pretty much at 10mph, but starting in 3rd you need to slip it till about 25mph or else risk under revving the motor. Also when you slip it starting in 3rd you rev the motor to about 2800 to 3000, because your foot instinctively knows the best torque is right there. This is why you need a heavier spring. Setups that have worked for me have been black spring with 3 heavy shoes, black spring with 6 heavy, and green spring by itself. My local karting shop doesn't carry heavy shoes so I just get the one spring. Then when your clutch shoes and bell get really worn like .050" each, you can go back to stock spring for almost the same effect. Get a short piece of pvc about 1.25" ID to make the reassembly easier. The acceleration will still be slow in general terms, you might not beat a Prius or even a bus but you'll beat a bicycle off the line. You need horsepower to accelerate fast, and to go fast, and a 79cc doesn't have much of it so the setup needs to be tits. Basically; gear the bike for the speed you want to go (and if motor can handle it) and set your clutch to lock up at around the torque maximum of the engine. For example, my bike is geared 7:1 to top out around 55mph at 5k with the clutch locking up at 30mph (green spring). Even the green spring is a bit too close to stock for me, but it's good enough and my motor has enough power to force the clutch through its chatter. Also cancel your order for that #41 crap and get the #420 at your local mc shop.