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Could be either bearings or piston slap or piston melted slightly and you now have aluminum welded to the cylinder wall (which seems to be pretty common for this little air cooled motor in my experience). I am rebuilding my blaster right now because the piston siezed for just an instant about a year ago and now its bad enough it wont even start. The piston is scarred on the sides , left aluminum on the cylinder and the rings finally got welded in the piston , not enough compression now to run. This is the 3rd time ive rebuilt this motor due to piston siezure or piston slap. If it has piston slap it might run a long time before any major damage occurs depending on how bad the clearance is and how hard you ride. It will start fine and run fine but will have a knock and the knock is hard to hear at times when the rpm's go up.
If you want to know what the problem is just take the cylinder off. You can leave the head still on the jug , it only takes about 20 minutes to tear down and pull the jug off and you can look at the piston and cylinder to see whats going on.
 

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yikes... haha. knocking is usually a sound you hear when your engine detonates. detonation will happen when your gasses around the outter edge of the combustion chamber randomly explode after normal ignition. heat is the driving factor in detonation... the gasses around the outter edge cant take the heat and spontaneously combust without an ignition source. this can be caused by to low of octane, lean mixture, or excessive engine abuse (high rpms for a long amount of time).
 

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i have the same noise happening it sounds like from the crank area but it is really piston to cylinder wall clearence. bout 3 hours after i heard it it blew the rings were completly gone. mine was b/c of lack of oil in the gas mix ( blocked oil injection off).
 

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I have a 98 blaster. when I bought it 3 years ago, it had a knocking noise on higher rpm. The bike was totally stock. I was certain it was piston or rod issue, so I tore it down checked the crank and con rod pearing forplay, none. I had the cyl bored .10 and installed a new wiseco piston and fmf exhaust. To my surprise that dam noise is still there. Pisses me of,but no problems.Knock on wood.
 

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Blaster don't have valves and they just rattle. I've had 3 different cranks and two topends on mine and it still rattles. Most of them I've been around that are not stock have that 'rattle'
 

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One of the worst problems with Blasters is broken piston skirts:


Long before these break off, the engine makes a slapping noise.
This is the good Lord telling you to save up for a new piston before the old one breaks.
 
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