I have a 27 0z Ringmaster with an OS FP 20 on it .about how much fuel will it take to do the OT Pattern
 
what is a good prop for it

  9-4 APC, accept no substitutes.  If you have a pitch gauge, depitch it to about 3.6-3.7".  An alternative is 9.5-4.5 graphite prop, depitched to about 3.7. The problem you will likely encounter is that it wants to go too fast, indicating excessive power, rather than too slow. Lines should be around 60'-62' .015 (.012 makes me nervous at this speed/amount of pull). Expect around 3.9-4.0 seconds a lap on 60' lines, stock APC 9-4, and a proper setting

   It will take about an ounce and a half to do the OTS pattern, but I would recommend making provision for at least 2.5 ounces, because set up properly, with this engine, it will do a respectable Classic flight if you are careful on the controls.  The tank can be no more than 1.5" wide (from the plane of the mounting lugs), so make it taller instead of wider. Set it up with a suction setup, and put muffler pressure to the vent.

    As in many previous posts, run the engine *dead stock*, exactly how it comes out of the box, including the stock muffler, make no modifications of any kind to anything. Set the engine on the ground by peaking it out lean, then richen it up until you get the first distinct drop in RPM, about 5-6 clicks if you are using the front needle version, about 3 clicks or so if you are using the rear-needle version (which is fine and should be left alone, and not modified). It should run in a medium 2-stroke in level flight, going leaner in the maneuvers. If it goes over the top lean in the maneuvers, open up one click, try again, until it peaks out at the leanest point. If that setting causes it to 4-stroke any, at all, in level flight, put on an air filter (panty hose or Bru-line coarse), start over, until it gives you the right variation in flight.

    Assuming the ABC engine, almost any 10-15% castor/synthetic mix will work (I have run it on gasoline and heated Wesson oil...). If it is an iron-liner engine, use more castor.

    This is an entirely solved problem, it will work as described and the engine-to-engine variation is negligible, so we have set up a dozen of them for exactly this application with universally good results.

     Brett

my kind of engine run, just like the good old K&B green head ,thanks Brett,and Tim

Stock small, with the stock spray bar (and remote needle).  If you change the spraybar size, you need to adjust the venturi diameter.

thanks Brett