Problem with consistent running 2004 Piaggio Zip 50cc 2T

Vehicle: - 2004 Piaggio Zip 50cc 2T, 5,600 miles

History: - Vehicle came as a barn find. Hadn’t run for 12 years. A sports exhaust (ZR-X) had been fitted. Circular brass insert in carburettor inlet side is still in place. Has reached 45 mph so assuming variator restrictor ring has been removed. Carburettor has been dismantled and cleaned with carb cleaner (noting of the size of the main jet is 53, so the standard). Standard air filter which has been cleaned, oiled and refitted. Plug has been cleaned, gapped and refitted. Fuel has been drained and fresh fuel added. Choke seems to work ok as when it starts the revs are high but returns to normal speed after 2-3 minutes.

Problem: - Vehicle starts no problem and ticks over fine. Will rev fine. Take it out on the road it will accelerate up to 45mph and pulls strongly. When I slow down then try to accelerate again it will go up to about 15mph then bog down. Will stick at this for about 200 metres then seems to clear and accelerate back up to full speed then it repeats i.e., won’t accelerate for a bit then clears and goes to full speed.

Any suggestions?

 

It's the carb jetting. You have an aftermarket pipe with the standard size main jet in there.
What you are feeling when it first starts is the extra fuel, and less air, created by the choke being 'on'.
As the choke comes off, the mixture gets lean, i.e more air than fuel, the engine will die a bit (bogs), then the fuel amount (because the engine isn't using it) catches up with the amount of excess air and off it goes again.
Simple upjet of the carb required