Prop, just for kicks I dragged out my .30, it is silver not black as I indicated above. You need not worry about the carb barrel it will throttle like a rheostat when the needles are adjusted properly. I ran a Graupner 10.5x6 on the .30.

Merco .30
Fuel== Fox 5%/20% castor
Plug==Fox IdleBar
The engines from left to right, .40, .50, .61 gasoline, .61 glow. As you can see all the mufflers are the same except they get progressively shorter as the engines get smaller. There is one Merco I'm still trying to get and that is a .61 Diesel, Ed Carlson thinks he has one but has not found it in 5 years, maybe some day.