I have an OS F plug. I have run it with 20/20 fuel. It seemed to run fine the first 3/4 gallon of fuel (except that it flamed out in both flights). But after that I cannot crank the throttle or get a consistent idle. I sent the engine in a few days ago. I adjusted all 3 of the things shown in the instructions: needle valve, idle valve, and tappet clearance. There was nothing wrong, and the tank was pressurized and working fine. Since they have nothing useful on their website as a troubleshooting guide, I sent it in.
A pattern pilot whose opinion I respect said that you need three YS engines to be competitive: one in the plane, one on hand as a backup, and one in the service center. With all the spare parts he carried, he may have had a fourth hidden in his van.
For what it's worth: I own 4 YS 4 cycle engines and all have required adjusting the regulator toward the lean side before they would run correctly. Unfortunately, this is not mentioned in the instructions and is actually discouraged by many YS users. If your regulator is set incorrectly, this would cause exactly the symptoms you describe as the engine will require leaner mixture as the ring seats (break in period) but is actually running much too rich, at least through the mid-range.
Here's the method I use with great success. Open the H.S. needle too far and start the engine. Adjust the H.S. needle until the engine runs cleanly but still on the rich side at W.O.T. (wide open throttle). Then close the throttle to approx. 1/3 open and adjust the regulator until the engine again runs cleanly but perhaps also a tad rich. Then go back to W.O.T. and readjust the H.S. needle. Adjusting the idle is done last, the least important and the least sensitive of the three fuel adjustments. Odds are that it too will be set on the rich side from the factory but remember, it works 'backwards' from the other two adjustments: turn the screw to lean the mixture. At this point the engine should run correctly and require nothing more than setting the H.S. needle to the conditions of the day. If you change fuel, you may have to adjust the regulator again but odds are that the idle needle won't need to be touched again. Go fly the plane and note the throttle response and smoke output- the throttle response may not be quite razor sharp and there should be a very visible smoke trail throughout the entire RPM range. You can further lean the regulator in about 1/8 turn increments to perfect throttle response.
Assuming the engine is mechanically sound and not defective, it should run like no other model engine currently available. Whether that is better or worse is in the eye of the beholder :-) My opinion is that they are the finest engines available at any price but I do sympathize with those who have difficulty with them; I've worked on quite a few at the field. They are different from other model engines and non- YS experience actually seems to hurt rather than help.
One last thing...this past weekend, we were getting our planes ready for an upcoming race and one guy was having problems with his .91. He finally noticed the throttle barrel stop screw was missing. Of course, his problems weren't limited to the low end but just thought I'd throw this tidbit out in case some haven't used Locktite on theirs.
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