Great run

Could be congealed castor oil in the regulator which the new fuel is dissolving.

little power house. good score

Nice video, as always! It should happily run an APC 13x6 prop. The gue in the pressure line is just old fuel from the crank case. And the top end sounded way ritch. Propably up to a whole turn. After setting the high speed needle, set the low speed. And this adjusts the air, not the fuel. Clockwise is richer. Easy to get wrong 😅

The pressure comes from the cranckase via an orifice that pulses every 2 revolutions using the pressure generated when by piston moves down and the rotary disc closes the port, with every pulse of pressure the diaphgram of the regulator opens the presurized fuel and allows to flow by pulses to carburetor, this is a mechanical fuel injection, the old oil in the cranckase between rear and front bearings pass with the air used to pressurize the tank, if there is old oil you should see that in the pressure line, if it's on the regulator to carb line, maybe you have old fuel residue inside regulator, that causes fuel flow issues, I'm not an expert but i used to service others fellows engines, specially YS that not so many people take the time to read the manual to adjust correctly.

I just obtained a used YS 91 and am trying to figure out how the plumbing and all works, so this is helpful,

hey David, I don't know if you can hear it but there is a weird hum in your audio feed for this video!

How will you plumb this for refueling on airplane? Would be nice to use the Dubrois Kwik fueling valve and probe somehow.

It's need a break in, slowly.