Ace9

I have purchased a brand new 91 Surpass II with a pump unit. The engine has been broken in on a test stand and has also flown my aircraft for over a dozen flights and is broken in nicely. It has always ran very rich right from day one. I can't seem to get it to lean out using the high speed needle valve at wide open throttle. In fact I now have the needle valve fully seated all the way in. I can back it out one, maybe 2 clicks (not turns) but it slows down quite a bit to a point were if you don't close the high speed needle valve completely it won't pick up speed again and simply keeps laboring from being pig rich. I have managed to improve idling quality immensely by turning in the mixture control screw in about 1/2 turn from were it was set. The throttle bore is only open approx 1/16" of an inch now compared to 1/8" before. You could hear the engine pick up speed when idling while I was turning it in. It idles at 2200 RPM all day long now compared to continuing stalling as before I began adjusting it. It still smokes lots from idle to 80% throttle without hesitation but the RPM's drop off slightly from 80% to 100% throttle. It is still to rich on the high end. I know I could make the low to midrange better yet if I could lean out the high speed needle more. Believe it when I say that the needle is fully seated. I checked my fuel lines and they are the recommended 2.5mm inside dia. and 5mm outside dia. I also took out the Needle Valve holder assembly to check for foreign debris twice now and it is spotless. Even at wide open throttle and the High speed needle valve fully seated the engine still seems to surge slightly from being rich. It's weird because from full power back to 80% the engine RPM increases as it starts to lean from switching over to the midrange circuit from the mixture control screw's setting. I am using a new OS type F plug, 10% Omega fuel, Inverted pitts style muffler from slimline and a master airsrew 14X7 3 bladed prop. Please help, Ace9.

Bax

My Feedback: (11)

If you have the high-speed needle turned all the way in, and the engine still runs, then you have a damaged needle or a damaged needle seat in the carburetor. A damaged needle seat will require a new carburetor body to fix it.

Needle seats can be damaged by over-tightening the needle valve (you close it, not tighten it), or by having a long extension on the needle valve with a knob on the end. That will vibrate around, causing the needle to beat against the needle seat and open it up.

You should be able to close the needle and stop all fuel flow.

Ace9

Thanks very much for the reply Bill; I figured that it was either a incorrectly set fuel pressure regulator or something wrong with the needle/seat mating surfaces. I also thought that with the seat fully closed it should stop the engine and the fact that it doesn't had me leaning toward the needle/seat mating surface. I will check it out and will reply to this post once it is resolved. I definitely did not over tighten the screw which would cause damage, however that does not mean that someone else did not before I received it. I am a certified auto technician and am currently working in a engine rebuilding shop so I consider myself having a good feel for over tightening things like this. I also have 3 other OS engines and have never had a problem yet. In fact that is all that I'll ever buy as I appreciate and recognize the satisfaction of a well built engine. Ace9

Ace9

Update; Ordered a new needle assembly (OS# 27681900) which contains the needle holder, needle, spring and O ring all together. Installed it and problem is solved. Close the needle and the engine dies. Seems to like only about 1/4 of a turn opened from closed but runs great, Ace9.