Tillotson Carburetors
The Dennis Carpenter link works fine for me.
The Tillotson ML5B that came on the H and some of the J models has a really small venturi, so yes, you do need a larger carb to allow the poor thing to catch its breath.
All the MT Tillotson carbs look identical externally with the exception of the float bowl cover - the fuel line enters the float bowl cover on top and some models and on the side of the cover on other models. I am aware of at least 35 different MT model carbs. Internally the carbs can differ in venturi size, main nozzle design, as well as the other different size orifices - idle mixture adjustment galley, idle bleed passages at throttle plate, air correction orifice size, etc. They also differ in the needle taper used for idle and main jet adjustments.
Some MT's where employed on 2-stroke engines and these were configured very different than carbs set-up for 4-stroke engines. The vacuum signal on a 2-stroke is very weak relative to a 4-stroke engine, thus the internal passages and main nozzle are designed/sized differently for these 2 applications.
The MT venturi size is given in 1/32nds of an inch and is cast into the bottom, inside, of the float bowl as a whole number, usually with a circle around it. The largest MT carb venturi dia is 0.656", which will have a "21" cast into the bottom of the bowl to indicate the venturi is 21/32" dia. I believe your MT 61C has a 0.656" venturi, as I know that the MT 61A has the large venturi and it is usually a constant in a given carb series (like the "61" series). But let us know if you see a "21" in the bottom of your float bowl. I'm quite certain the MT61 series were configured for 4-stroke engines, so with an adjustable idle and main circuit the carb should work well for you.
The MT 12A was used on many of the later style Whizzers with the large ports (the later 300 and 500, 600 and 700 series engines), and had a venturi size of ~ 0.563" to allow better breathing. Any of the MT carbs employed on Cushmans usually work well on Whizzers. Many of the Cushman carbs had a 0.625" dia venturi, with the later ones having a 0.656" venturi.
Hope this helps.
