I have several ys engines and love them,however I bought a used 53fz and it is giving me fits.Its runs strong and idles like a Swiss watch,but every time I change throttle settings I never achieve the popper high end needle setting. It can be all leaned out and running great,then shut the throttle and when you open it again it is too rich,do it again and then its lean .All I do is chase the needle around.
Fuel is 20/20 plug is OS#f prop is a 12x6. I have two 63 that run perfectly on same fuel.

Did you clean out the whole fuel system including the HS needle and seat?
Also look at the HS needle and make sure it isn't worn. Sometime guys will put long pieces of metal rod on them to clear a cowl and that extra mass vibrating will accelerate the wear of the needle and seat.

I haven't checked needle and seat, but it did have an extension on it when it came.I did clean out the regulator which had a small piece of something in it. Will check the tank too.
If I could fly at one throttle setting it would be fine,but after my first maneuver it all goes to pot.

Thank's

I may have fixed it.Found a really small fuel leak on the needle valve o ring. Couldn't even see it in the plane,but put engine in test stand and saw it.Replaced needle valve with one from my 63 and ran engine.It ran fine on test stand so it hopefully will run good in plane.
Thank's, Petec.