I bought and installed an 80 cc engine I found on eBay. I rode the bike maybe ten or so times on short trips then it became very stiff and I couldn't make it work. It would throw the chain and at first I thought there was something wrong with the chain idler. But after a few more checks I discovered the engine was very tight and was the reason the chain was coming off when I tried to start it. I was so happy with the bike and now I'm distressed and don't know what to do. An obvious cause is not enough oil in the fuel mix but I thought I was careful to make it twenty to one as suggested in the manual. Do I have a burned out engine? How can I check for this condition? Any ideas out there as to my next step?