If its off an HP500, it should have an electric choke. Is the choke wired up??
I would disconnect it and use a zip tie to hold the choke so the throttle blades are fully open.
Was anything done to the carb or did you just bolt it on??

No it's manual choke so made it's not the original as i brought the engine second hand already got a cable tie in holding it off

It's been cleaned and rebuilt but no change to jetting etc