Quick questions about my engine!

I have an 80cc 2 stroke bike engine made in china, and I have a great curiosity as to why when i reach top speed it feels like my engine has a governor on it???
I just got through my first 2 gallons of gas (4 1/2 gallon tanks) and my manual says that my break in is done. I just tried to hit full speed today and when i did, i got it up to 34 MPH but was only able to hold it for maybe about 2 seconds before It sputtered back down to 30 MPH. What causes this? How can i better my top speed without risking hurting my engine and still keeping gas mileage up? Right now i have an average of about 80 miles to a tank. Does anyone have a shifter kit that they have a picture or movie demonstration of? If not Can someone that has one make a youtube video so i can see how it works and if it is really worth dropping on my 30 year old Univega? thanks guys i know you will be of great help if you respond to my post!!!!

80 miles to the tank and 30 -34 mph top speed is doing pretty good with the stock setup and within its performance range. You could invest in a smaller 36 tooth sprocket if you want speed but a shifter kit would be the best.

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I just did a shifter kit a few weeks ago. With my old bike it's a 5 speed and I went with a 11 tooth counter sprocket. If a 12t were available I'd go with that. The first 3 gears are still too low, so I shift 1-3-4-5. It finally gets to 31 mph comfortably with a stock engine. (80-66cc?) Soon I will start to try to tune it. I dropped the carb needle down to the top notch and it helped. It still tries to 4-cycle a lot, so after I get this tank of fuel emptied, I'll go 24-30:1 and try that. It may do a comfy 35.

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Thanks for your statement. I am curious about the shifter kit. I've had trouble with my chain idler, loosening and then the chain coming off and damaging the spokes of my back wheel. Also, just keeping the chain in proper tension has been a problem. Would the shifter kit alleviate this problem?

Nice motor mount, did you fabricate those? You may have a secondary income if you want to sell those here.