Johnson 1980 4.5 hp J5RCS high speed problem

[gonefishin]

I recently bought this motor that had been in storage
since 1986. the motor is in showroom condition. it starts
with one pull every time and idles slow and smooth.
the motor runs great up to about 3/4 to 7/8 throttle but if
i try and go faster the engine starts to bang and it shakes
the whole boat. thanks for any help with this issue.

[boats.net]

Could need carb cleaned, or could be loosing spark. Check for spark with a timming light.