Hi there im looking at an 0.s 120 surpass3 fourstroke. I see that they are available with or without a pump? I havent come across pumps before? What difference does it make, what do they do?
Any help would b great thanks

The pumps make them run more steady throughout the flight.

Years ago OS had some issues with their pumps....but not now. I have four OS pumpers and really like them. A person can lean the motor out and leave it at that setting. Will run the whole flight at that setting without any problems. Also move the tank back to the C/G without any draw problems. I have read on here the repair cost might be excessive...can't say, as I haven't crossed the bridge yet. Some of mine are three years old and still work good. Just use a filter before the pump.

The big issue with the O.S. pumps is replacement parts. You can't get parts for 120 surpass II pumper now. Neither pump or regulator. The regulator is the biggest issue as it means a full carb replacement if the diaphram goes. If the pump goes, you could go with an aftermarket I'm guessing, but the regulator is an issue. One that I found out after buying a used Surpass II and then found I couldn't get parts for it. I've been trying to find out if the III unpumped carb will fit, but I think the answer is no.

Don