You have given me a lot to consider. First as to muffler pressure and gas. This test was just that, a test. I am concerned about muffler pressure and gas. I purchased a Perry pump that runs on crankcase pressure. This pump could be used for fuel or smoke. There was too much pressure. I thought about the oscillating pump but have not gotten to that yet.
As for the ST-51 carb. I have run that carb on a 25cc weedeater with an adapter made of .500 aluminum. This is what I used muffler pressure on. The ST has a sharp point on the hi end needle, just one "click" on the hi needle would make quite a difference. The low needle was not a problem and the engine responded very well. This engine had the "TIM" ignition (SAM). The ST carb is, I believe, the same as on the 61 or even bigger. (Tower info on carbs.) BTW the tim units require "dwell", so 3 .250 magnets all south in a row at 1.3" diameter. The ability of the ST to supply plenty of gas is why I took it off my ST. It is now back on the 51 with gas and CH ignition. 20% synthetic oil to 80% gas. Idle has been a little of a problem so I put the Syncro spark on it. I have not had time to try it again. Turned 11.6 on an APC 10/7. If it looks like I am going in too many directions at one time, I am! Also, the 20% oil to 80% gas was the result of an e-mail to an engine expert at an R/C magazine who's opinion I respect.