here is my first well first in a long time motor bike build ( when i was 12-15 i built a few chainsaw wakker bikes lol ) it called the leathal threat and took a few days and about 300$ to build its a single speed 26 inch beach cruiser with hub brakes the back pedal kind, i chose that style to keep the handle bars free of cludder. the motor is a grubee gt5 66 cc when i first got it in the mail i was not impressed with the quality of it at all !!!! and thought it would barly run ! and the mix ratio of 16:1 was insane who runns a motor that lean and full of cheap oil ? but i put it together as is and rode it stock on a crappy mountain bike to break it in and test it out and tune it up. well i found that it wouldnt idle for nothin and it had harldy any power and smoked like hell and it was fouling plugs like crazy ! so i said to hell with this thing and pulled it from the test bike and began the tear down. i am a great and pretty damn expearianced mechanic when it comes to 4 stroke motorcycles and atvs i restored a honda act 200 3 wheeler (big red) a few years ago... new cam wizco piston high comp head cluch mods the works and the thing is a beast ! so i will aply mu knowage to this and give it all i had with as little $ as possable. first i tore the top end off and found tons of carbon and real crappy casting of the combustion chamber dome and the ports were so bad i cant believe this thing runs ! even the intake z manifold had huge burd and casting flakes protruding into it ! - i decked the head down about .1 and polished the chamber and cleaned up the finns - i ported the heck out of the jug taking about an 1/8in off the bottom of the intake and 1/8 off the top the exaust and widend bolth by a 1/4 and decked down the jug a little cleaned up transfer ports - port matched the intake and exhaust - ported intake manifold nice and smooth - modded the exhaust baffel removed cat and driled 4 1/4 in holes in the cap - lengthend the intake with a short tube - new piston ring (mollybendium) -cut 5/16ths off the entire intake side of piston skirt - ground 4 ramps in the top of the piston to make a 4 leaf clover pattern alowing air fuel to travel over the piston at bdc easyer and out the jug at down stroke faster... the piston is way lighter now ! and the decked head make up for the compression lossed from the grinding -made a thin copper head gasket about half the stock thickness dindnt measure lol just knew it would help comp. - nkg b5hs plug hard wired to ignition (no plug boot) - adjusted clutch properly and oiled gears - replaced clutch ball bearing and rod with brass rod a little longer about 1/16 and new bearing 5/16 the old one had a flat spot it was so soft !!! and greases that hole with 80w lucas gear oil. - silicon caulcked the wires coming out the stator to protect them from elements - replaced all nuts and bolts with stainless steel ones - made custom front motor mount that bolts strait through the frame for a more solid mount. - rebuilt the carb and tunned it to new setup. - painted to my likeing with flat black vht engine paint the results ? the bike has an unbelievable amount of more power than before ! i can cruse at 30 mph no problem for long periods of time and it has massive tourqe for a little engine as it pulls me up even the steepest new england hills at 20mph no problem also it rev's up way quicker and has a much broader power band !!! and i hardly spent any money at all with the mods i did !!! the only thing i bought was a new plug and a 2 inch piece of hose !!!!!!!!!!!!! if you have a dremal tool then you should have at your motor !!! srs power is waiting for you to unlock it ! if you do not have a dremal borrow one or buy one for 30$ the power you get for that 30$ will be more than any bolt on upgrade you can get for this engine in that price range !!! i now run my engine at 40:1 full synthetic 93 octaine and my plug looks perfect and the bike idles like a purring kitten !!!! the only thing left to do is make a custom built expantion chamber but that is a whole thread on its own with all the mesureing and math involved !!! with a good custom made expantion chamber and the right length header i think i can get 40-45mph out of this motor with just a swap out fromm 44 to 40 teeth on the sprocket !!! right now i cruse at 25-30 right at the power band but she will pull 35 on a flat. 45 is possable with a good tunned exhaust and a slightly smaller sprocket.