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Since it does get its power from the alternator, I would expect it ti dim at lower RPMs. But I can be at high RPM and it does it.

OK. One more time. I finally got the correct piston today. I have the oil injector disconnected and plenty of pre-mixed gas  I'm putting in the fattest main jet I have while breaking in the rings. Wish me luck.

I had no idea "compression height" could be so different with same crank and rod?

That's my point exactly. Without changing the height of the cylinder, how can that be? That 17mm either is a mistake, or it's not meant to be used on a Kymco.

blue I may be able yo measure from my previous BBK that blew at 1000 miles due to user error lol.

It's because of user error I'm looking for this. I finally got mine running good, then installed the Technigas exhaust that seemed to double what power it had. A couple days later, it quit. Seems my oil injector wasn't working right. I have a severely damaged piston. It's easy to find Hoca BB kits, but not so just buying a piston. I've found a couple that says it's a 47mm piston for the Hoca 70cc BBK on a Kymco People 50. But it shows the compression height at 17mm. That can't be right. I've measured mine as close as I can. I get 28mm from top edge to center-line of the wrist pin.That 17mm dimension would put the top of the wrist pin hole just under the second ring. But, the picture of the piston looks right (and everybody uses the same picture). Anyway, if I ever have to but another one it would be easier to get a Malossi. That is if it has the correct compression height and is ported in the same place. I ordered a Hoca and got something else. The port is much lower and compression height is 24mm instead of 28. I put it in just so I could ride for now. It has much less compression so I have no low end power, although it will eventually do 45 on the flat. My new Hoca will be here Tues.

Wondering if you could increase the cooling capacity at all with a scoop of some sort or better fan. 

I had thought of going big bore until I read about your temps.

I have a Hoca 70cc BBK. You've seen the trials and tribulations I've been through. I finally discovered the exhaust was my problem, and managed to get it running pretty damn good. All this time my head temp was approaching 300. I installed my Techagas Next R yesterday. I have to say, I wasn't prepared for that. Over 50 on flat road. Haven't had enough time to play yet, but I'll let you know what the temp is later.

 Mine's an '07 with very low miles and Hoca 70cc BBK. I got it a little under a year ago from an auction. The paperwork with it indicated it had just top end job and a note stressing the importance of quality oil and proper mix. I didn't know it at the time, but it had been de-restricted. The first few days I road it, it ran much like it's supposed to with around 35 being top speed. Then it started going downhill fast. Getting slower and slower each time I road it. Everything I checked seemed to be fine on it. This is when I started realizing it had been de-restricted. My old compression tester had a leak in the hose, so I had a friend bring his hand held gauge over. As I cranked the motor over, he told me I didn't have any compression. I ordered the BBK because I had an idea of what 50cc would do and I didn't want to be in traffic when I can't get out of my own way. I now know there was nothing wrong with the 50cc jug and piston. It might have not needed the first top end job. This exhaust was so clogged, gas would only very slowly run through it. When trying to test the compression, it was building pressure in the exhaust. I've worked on a few 2 cycle engines before, but I've never heard of this extreme before. After cutting off the end of it, I found that's where the clog was, so while waiting on my new exhaust, I used a hammer and chisel open up that last baffle. I put it back on by wrapping it with some 3/4" 3-M semi hard rubber tape. I have no idea what it's purpose is, or where it came from, but it's been here for years and I finally found a use for it. I then wrapped a piece of 4" aluminium around that with 4-1/2" dryer vent clamps at each end. It doesn't make much more noise than stock, but it is robbing me of a little power. I can live with it a while. On flat ground it hits 45 MPH fast and will do over 50 downhill. This is with the stock 14mm carb. It has a #35 idle jet and a #105 main. The needle isn't adjustable, but I made a .040" washer and raised it. That helped a lot on take off.

Obviously, I've put this on the back burner. In the mean time, I've been working on jetting after disconnecting the rear brake light. A couple of days ago, I reconnected it and the noise is gone. I don't know what it was, but it'll probably come back. I'm now putting a 19mm carb on it.