The Q indicates radio noise suppression.
Champion L77-JC4 plug set to .030in.
The 1979 model Johnson 9.9hp uses a Champion L77-JC4 spark plug.
It depends upon the engine. Engines with higher compression require a narrower gap. If this is for a marine application (IE:outboard motor) 0.030 is the gap required for most, but you should double check.