upgrade question on older 50s models...VL3/VB1 to VS5

[Scooterso]

customer of ours has called and is trying to work out some issues with his engine. we didnt build it, not sure who did but he has said he converted his VB1 (1957 150cc) from a deflector head to a dome head by using a VS5 piston. Technically, the numbers line up. 57.2 bore on both pistons. I think his problem is that the VS5 piston is for a "top of cylinder" piston port, whereas the VB1 is a "rear side of cylinder" intake.

the guy is saying the scooter will start fine when cold, but after riding for 5 miles or so, it just shuts down. hes going to try a new condensor (although he put new one in when he rebuilt it, along with new seals, etc).

anyone else done a VS5 piston conversion on a VB1? probably a VL3 would be the same set up. 1956 - 1957 wide body style engines...

thanks!