Hi!
Inner diameter on fuellines has no effect on engine performance or behavior if you just keep using popular sizes (1,5-2,5mm for engines from COX .049 to big OS .300 twins!
Most of the time when an engine quits .....it's because you the owner has made a misstake and set the highspeed needle (or idleneedle) to lean!!!

Regards!
Jan K
Sweden