Without it being here to examine, I would do a compression test, check out
the exhaust system, look at the timing chain to see if it is off a tooth.
Touch the rear breaks to see of they are heating up. Then do a vacuum test
on the advancer assembly and hose to the carb. Place a timing light on the
pulley and see where it is when idling. Something is not right or it would
be doing the speed.
Bill