Hey, I have a china made 80cc bike engine running a 20-1 oil gas ratio. I am using a HP carburetor from Top Corporate and also a high performance exhaust. I understand that when the spark plug is black and oily it means you are running too rich but I don’t know how to adjust it. I’ve seen on the forums that you adjust the need but my carborator doesn’t have that. I am at high elevation so I installed a smaller jet but it didn’t help much. I would really appreciate it if some one could help me out because I get to about 23 mph but only about 3/4 of the rpm I know it can get. If I can adjust the air to fuel ratio I think that would fix it.