I upgraded the Mag II (2speed) by adding a Polini 64 cc reed valve kit w/ 15 mm Bing carb and 88 jet. Got me a motomatic pipe, and downsized the front sprocket from an 18 stock to a 15! (did re-gearing after the problem described below, and it worked great by the way, but engine still had a metallic knock so I had to take it apart) During breakin around the neighborhood, the bike had low end power, and at about 28 mph, it felt like the bike grew a 3rd gear?!@?! I never topped it out b/c of the breakin period, but this was easy a 40+mph combo. Fastest I took it was about 32/33 mpg. On Day 2, I was approaching a stop sign at about 15 mph, and the bike went into a high wine and then rolled to a gradual stall. Thot I seized it, but I kicked it over and started right up?? But, then it had a metallic ting-ting-ting when running. Took apart the cylinder, and found black patch at 6 and 12 o'clock with some minor longitudinal scratches, I sanded down the cylinder and piston black markings with 800 and 1000 grit (a minor amount of sanding sanded very little but until cylinder felt smooth), popped in a new ring (chrome coated), and put it all back together
It now idles out of control, bogs to a stall under startup load, and the metallic knock is still there. The idle is scary way too fast, and not adjustable with the mix screw on the bing carb! Totally different than it ran before. Does sanding mess up the compression? I left in the 15 mm bing with 88 jet and motomatic in place because it worked so nice when i first got the kit. using 50cc high compression head treats recommended. Engine temp under idle was clear in the 300 degree F plus range at idle when giving it some gas after I put it back together.
I think rather than the kit failing, it probably was some debris pulled into the cylinder. We found the intake manifold had lots of metal flakes from the cast process, could have sucked a flake into cylinder causing this pain? Before I put it back together, I filed out the flakes and cleaned the mani all good so there is now no chance of additional metal into cylinder.
What to do? Trash the cylinder? Re-jet? WHat jet? Needle position? Could it be transmission? I know you know! Do you know? I don't KNow. ugh. thanks