i have a 1981 kz750e and when i drive it will only get to 55mph. after that i can hold it down full throttle and the rpms wont get above 5000. i have been slowly fixing it up but i am at a loss when it comes to this. and i have to have the choke on all the way just for the bike to run an idle speed of 2000 if it drops below that it dies on me. it gets only like 15 mpg which it should get like 53 according to the manual. i normally work on cars and am new to bike so any help would be appreciated.

Without knowing more I would say check on the float level being to high and a very dirty or completely clogged air filter. Also check that your exhaust is clear and not clogged/ blocked up.