ORIGINAL: wcmorrison
If it has a smoke trail that is okay. But I do not worry too much about smoke trails. Smoke trails indicate to me that there is some Castor in the fuel mix and that is a good thing.
If it has a smoke trail, that indicates there's oil in the fuel, period. Castor or Synthetic, Olive oil, or peanut oil.. Doesn't matter. If it smokes, there's oil present. Using the smoke trail for anything other than reference is redundant.
ORIGINAL: blw
Same here with using acceleration/performance tests and a good tach. I never regretted the money spent on a TNC.
I also use the ear test. You have to listen to what your engine is telling you.
Some of us can't afford the luxury of owning a TNC tach. For us, a Hobbico works fine.
For a few of my engines, setting the needle 300rpm rich of peak using the tach is nowhere near enough. I have to set the engine closer to 800rpm rich of peak for it to not sag at 2/3 tank and be scary lean by 1/3 tank. Half the problems I encountered setting up my SuperTigre S90K involved using the tach to set the needle only, and being way too lean and having the engine quit. (Setting 300rpm rich of peak per the suggestions I received here) Going back to the "old school method" of doing a quick pinch of the fuel line gave me FAR better results than using the tach to set the needle. Maybe its the 800' elevation, or maybe the sun and moon weren't aligned... I better consult the Mayan calendar.
I only use the tach for reference - finding peak. The pinch test is a far more tell-tale sign of where the mixture is set than anything else.
Of course, this is all in my humble and modest opinion.