M600 running issue

Hi is anyone familiar with early monster 600 engines? I'm having an issue with throttle hang. It's nothing to do with the carbs or an air leak I am 100% about it. Also compression is excellent at 175psi hot and 150 cold. Valve clearances are good. Carbs are just rebuilt and balanced. Although the front cylinder is lower when hot. The throttle hang will only happen when the engine is hot. At idle when hot if I sprayed carb spray at where the head meets the cylinder the rpm would shoot up on the horizontal cylinder. Vertical one does not do this. I put it down to separating when hot. When I pulled the head and cylinder off there were no signs of loosing compression between the head and cylinder shock I thought was strange. At the same time I pulled the rear setup too. The rear piston is in better shape but I am putting that down to the piston slap in the front cylinder which they all suffer from. Should I swap the pistons on rebuild or leave them be? I will lap the heads to the cylinders before rebuild. Anyone ever have this issue? Thanks