I have an OS .91 Surpass 4-stroke. Engine runs beautifully from 1/4 to full throttle, but wants to die when idling. Will only idle for 15 -30 seconds, then dies. If I give it enough RPM's so that it won't die, the plane wants to taxi down the runway. The engine is mounted 90 degrees from vertical and the carb is above the center of the fuel tank. Any ideas out there? I haven't touched the low-speed needle adjustment. Afraid to!!

Sounds like that's what you need to do though.(adj. the idle mix.) If it sputters, and spits fuel out the exh. then stops, then it's too rich. If it just kinda sags smoothly, and/ or shuts off suddenly, it's too lean. If you haven't touched the idle mixture though, chances are good that it's too rich. Most needles are set rich at the factory, and you should try turning it in, clockwise, an eighth of a turn at a time, until it smoothens out. Keep doing that until you get a good idle and transition. If the engine is new, it will take some running-in before you can really get a good, reliable idle. Also, make sure you have an O.S. type F plug in it.

It certainly sounds like the low end is just a bit too lean especially given that it will accelerate well from 1/4 throttle on up. If it was too rich then it would bog/hesitate when accelerating. Open up the low end just a tiny bit at a time and test for long term idle vs instant acceleration. When you get the proper mix then reset your top end RPM with a tach but the max ground based RPM you'd ever want out of this engine is probably around 9600-9800 as it will, like most engines, unwind a bit in the air so proper setting of the high end will avoid the possibility of over reving in the sir at full throttle and the resultant often disastrous valve float.