My brother has a .53. It spins a large prop for that size four stroke and should last forever.

I have 2 R155's. They run well and make more power than a Saito 150 but the mufflers are very loud and carbs leave a little to be desired. It is the most unusual carb I have seen. It looks like an air bleed but they channel the air to oppose the fuel jet. The channel size appears to change in relation to the position of the throttle barrel.

To choke the engine you slide the barrel out to cover the intake. There is a captive spring loaded detent ball that holds the barrel in place and also allows the choke to work, this must be adjusted properly in order to get the carb to work smoothly.