I have several of these engines in my collection, at one time we were of the opinion that it should be the engine standard choice for my scale sport models.
This one was not running very well and a few flips of the propeller answered my question; the valve timing was off and would have to be reset before the next run.
I discovered this before: valve timing cannot be accomplished on surpass engines until the valve rockers and push-rods are removed are removed and the head bolts loosened, to allow the timing gear clear the cam follower. This is a very annoying reality with this design; since the timing operation is essentially a hit or miss task it usually requires several tries to engage the correct tooth; having to remove all those screws is most frustrating procedure indeed.
As soon as the valve cover was removed we also discovered the valve gap was far too large (estimated at .090 - .120 inches): another reason why its performance was so poor.
Engine run after timing set-up and valve tappet clearance adjustment:
Watch another of my OS 48 4-C engines running after I had performed a rather unorthodox repair when it threw its connecting rod in flight and punched out the bottom of the crankcase.