Re: "Newer" 25/35hp OMC compatibilities

[Don]

OMC started messing with the ports (block differences) during the middle 80s when hp ratings were measured at the prop instead of the crank. The later 20hp models actually had a restricted intake manifold and smaller carb.
Be careful swapping those early 25/35hp models around, although the powerheads might have been similar, the crank/driveshaft splines were different.
What size prop are you using on your 78 35hp? What type of tachometer are you using when you see 4500RPM? Sounds like there is a legitimate problem if it is surging at high speeds. You might want to remove/check/clean that high speed jet one more time, perhaps some stray crud got lodged in the jet after you cleaned the carb.