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OS91 Surpass II w/pump Mixture Adjustment Problem - 8/12/2011 2:55 PM   
GRandolph



Posts: 64
Score: 100
Joined: 9/14/2008
Last Login: 5/22/2013
From: Paducah, KY, USA
Status: offline 		I have a new OS91 Surpass II w/ pump that was broken in per procedure and now has about 7 flights on it. I am running 10% nitro with 18% oil(synth/castor mix), a 14x6 MA, and an F plug with the engine installed upright and a 14 oz tank at engine level directly behind the engine with no inline filter although I do filter the fuel when filling the tank.

The problem is that the last few flights the full speed mixture needle valve has been very sensitive. The engine is so bad now - either too rich or too lean - that I dare not fly it. I start the engine with the needle at about 2 or 2-1/2 turns out and start to lean the engine out. The RPMs at full throttle are about 7500 at this point. I slowly turn the needle valve in but the engine stays rich at 7500 rpms until all of a sudden with just one click of the needle valve the engine goes lean up to 9800-10000 rpms. If I then start opening the needle valve to richen it I usually can get the mixture just right at about 9000-9300 rpms but when I go to idle and back to full throttle the engine is too rich again and runs at 7500 rpms. I have adjusted the idle mixture screw and it has good transition with no flame outs when advancing the throttle rapidly. The idle mixture screw is turned 90-120 deg CCW from too lean (where it cuts out when advancing to full throttle). The full throttle needle valve is at about 3/4 to 1 turn open when the mixture transitions from too rich (7500 rpm) to too lean (9800-10000 rpm).

I took the engine off the plane (Hog Bipe) and ran it on a test stand with a different tank but same fuel (10% nitro) and the mixture was easily adjusted to 9200 rpms - no problems at all. Put the engine back on the plane with the tank from the test stand and it did the same thing - would not hold the mixture at 9000-9300 rpms but ran either too lean or too rich. The fuel tubing is not pinched either. Fuel tank does not appear foamy when engine is running.

Any ideas on what I need to try next??? Any help is appreciated!


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