I have an OS 70 surpass engine that's been in use for about 4 months. Lately I notice that when its at just below half throttle it sounds like it's surging. Anybody have any thoughts?

Have you adjusted the valves? That almost always needs to be done after the first hour or so of operation.

Another possible answer is that you've got the engine under-propped. It will turn a Graupner 14x7 with authority, and you don't want anything less than a 13x6. If the engine is overspeeding (surging is one of the indicative sounds), the valves are floating.

Just my opinion from running three of these. Around here, anyway, guys seem insistent on under-propping four strokes. I have no idea why.

Steve

I'm running a 20/6 on my Satio 180, and it works great. I read guys running 16/8's and 17/6's on this engine, and I don't understand the logic. A carbon fiber 20/6 turns 7900 static and pulls hard, also it brakes hard on down throttle. Another benefit is engine life. I constantly try to get people to prop it up, but unless the engine is screaming, they don't think it's providing power.

Steve

Thanks for the info. I will start with a valve check as that is something I have not done. This is my 1st experience with a 4 stroke. Also I have been running a 11X7 prop on it.

Again thanks and I will see if this helps.

Bob

Like Steve said, you definately are underpropped. I think it is over-heating. A 13 X 6 out to get you in the RPM range good for 4 Stokes.

To add to the above posts you may be running the low speed needle too lean. If the
engine only surges at less than half throttle like you say, you might try richening up the low end.
I run a 13x6 on my 70 at minimum.

Sam