100cc started rattling now wont run please help

[Terryosw]

Hi I installed a 100cc BBK and it ran good for a month and it sounded like the piston was nocking on take off and then stopped when riding and then it started rattling on going up hill the next day took it for a short ride pulled up in my driveway it started rattling and pinging bad so then it cut off so then I tried to start it and no good. So now it will turn over but will not run I've checked everything even put the original 50cc motor back on and it still won't fire up if anyone can please help? Thanks

[Shadowfire]

Engine won't start? The typical flow through is:
1. Check spark. (typically hold the end of the plug against the engine chassis with the boot so you don't electrocute yourself and kick it over. If you see a spark, ignition is good.)
2. Check fuel. (Spray starter fluid into the carburetor air intake while trying to start).
3. Check compression. You can find cheapo compression testers at Harbor Freight, and probably places like AutoZone.

Unfortunately, the most likely scenario is that you were running too lean, causing engine knock (the pinging sound), and hammering the entire piston assembly/crankshaft/crank bearings until something or things failed. You didn't say if it was a 2-stroke or 4-stroke engine; failure of the crank case/crank shaft seals or reed valve in a 2-stroke (because the rings had a complete failure and allowed explosive pressure into the crank case) would mean that you get no fuel into the combustion chamber, and the old top end wouldn't mean much without fixing the damaged parts.

[mmsscooters]

Also some big bore kits have a piston that sits entirely too high, seen in a lot or mortch and growsun pistons, once the connecting rod heats up and expands the piston actually starts hitting the head, if you have this problem you would hear a pinging and instantly lose power, took me over 2000 bore kits and 5 years to find a good brand that wouldn't do this. Are you sure the valves are adjusted right, and adjusted TDC AFTER the compression stroke?