Kohler 12 hp acting sluggish

I'm working on a model k301 12 hp cast iron engine
it seems slow on throttle response takes like 5 seconds + to go from idle to full top rpm on governor
also in side air cleaner it's has black film and making a lot of noise out carb when you open it up
i cleaned the carb and it doesn't flood anywhere
but It runs best at idle with idle mixture screw all the way in
the main adjust as normal any idea were to start looking for problem
or are the Kohlers always a little lazy throttle response wise
Thanks Ferris

May check and adjust the valve clearance . The cover is behind the carb, and doesn't require removing the head or valves. This engine had adjustable tappets to make thing easy. Also if the mower is hydrostat it could take longer to wind up then the newer mowers but shouldn't take that long.

Thank you I will start there
Will let you know after weekend if that was it
Thanks again

Update
Check the points no - little gap and badly burnt
Found someone had hook coil up wrong and condenser was on wrong side
Switched points and gapped right
Rewired coil and condenser and runs a lot better
Thanks again
Ferris