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The rear tire finally seized up, so I went ahead and autopsied the gearbox. I was shocked by what I found. First of all, I knew it was going to be bad because of the unbelievable amount of metal particles in the drain oil. It was hard to drain too, because there were so many scrap shavings clogging the drain hole. Turns out one of the bearings was completely blown apart. It was the bearing on the gearbox cover side the holds the small 16 tooth gear that's pressed inside the other larger gear (I wish I knew the terms for these). The inner ring of this bearing is in pieces.

I have an idea of what happened. I bought a taller gear set after putting on my BBK. Most stock gears are 15/51 - % from what I've heard. Mine were 14/53 though. Not sure why. I remember when I installed my new 16/50 gear set, the small 16 tooth gear had a really hard time fitting into its bearing - the one that got destroyed. Both ends of that small gear were really tight actually. So maybe that extra pressure on the inner ring caused it to fail? Also strange: The stock bearings are labeled 6201/P6. Sources I've looked at call for 6201 (no P6). What does the P6 even mean?? Would that translate to a slight diameter difference of the inner bearing ring? The ends of new gears may also not have been sized to spec. I have no idea.

At least now I know which bearings I need to buy. I'm hoping if I get regular 6201 bearings, the new gear set will fit comfortably. I sincerely hope that metal in the gear lube didn't ruin any other bearings in the gearbox.

I hadn't thought to check that. Just looked at it briefly - it appears to be okay. The float moves freely and the valve comes out of it's seat. Could have been acting up with fuel in the bowl though.

I had had a problem plaguing me for quite some time now and I FINALLY solved it. This is something that almost made me give up on scooting. I had this thread going a while back if you want to read details itistheride.boards.net/post/27669/thread. In a nutshell, I was having major issues at higher rpms, the scooter would barely get over 35 mph without the engine repeatedly cutting out and dropping my speed suddenly. It would make a loud "braaap." It was like fuel was cutting out or the timing suddenly reversed, it was miserable. Getting up to that speed was laborious too, I could feel the engine hesitating. I had tried all kinds of things... new fuel filter, manual petcock, new CDI, new rings, back to the stock head, cleaned the carb, stock exhaust, different octane fuel, jetting and more (not in that order). Nothing made a difference.

Just got a new carb, slapped it on, and we're back in business. Easily hit 45, no fuel issues at all. I wish I'd tried that sooner. Can't wait to see what it'll do after I re-mod it. I have no clue what the problem could have been with the old carb. It was the stock carb, the scooter has less than 1,500 miles on it. The new carb is the same size - 19mm intake. My only theory is that the clip on the needle, which was not adjustable, was set fine for a stock 50cc but couldn't cope with the BBK.

Either way I'm just relieved it's working again!

That would be a relief, I guess. I was hoping I had discovered a silver bullet that would fix my persisting scooter problems. Maybe no such luck though. I researched jetsrus.com and other sites that sell jets and include detailed measurements and it looks like there is really only one size jet for these carbs anyway, i.e. there's no "shorter head" version of the jets I have. I just really think it's an odd carb to begin with, given the abnormal idle screw and the fact that the needle clip is not adjustable. I would have liked to buy a beefier carb anyway, like 23mm or something, except that then I think I'd need a larger intake manifold and a new 42mm UNI filter. But hey, if I were certain it would fix my problems, then I would jump right on that.

[replyingto=desmerwebb]PCB_GY6%27n[/replyingto]Heads with 64mm valves are often much less expensive than 69mm valve heads. If you find that you currently have a 69mm valve head, you can replace it with a 64mm valve head as long as you replace the valve rocker arms. You don't need a whole new rocker arm assembly, just the arms. You could probably find a set of those on ebay for about $10 and save yourself a few bucks in the end.

Any chance it could be the idle jet? I'm still running the stock #35 with a 19mm carb, so maybe it idles lean.