1983 Johnson 50hp running rough after rebuild

[KellerS]

So to took several Johnson outboards from 1981-1985 to make one motor. I had the block bored 20 over, the head surfaced, and all new piston, rings, gaskets, thermostat, etc. I rebuilt the carbs, then after I had it all together except cables hooked up I brought it to a company to do the finishing touches. he did a great job hooking everything up got it running and hardly charged me anything. The problem I am having is that it runs rough at idle speed. When I get to quarter throttle or more it runs great, and it starts instantly. The mechanic said he got it as good as he could, but with an old crank and rods, the motor just won't ever run great, it is about 90%. What are your all thoughts, do you just think it needs to be broken in? Any input would be appreciated.

Thanks

Scott