A few thoughts about your installation. First, the FS-91 is at the top end of the power range for the Lanier Dart. As such, it is going to be more difficult to get the engine to idle down low enough to keep the plane from moving. We normally expect this engine to idle around 2,500 rpm with a 12x10 or 13x8 prop. If you are getting this type of performance, there are a few things you can try, but don't expect the engine to idle much less than 2,000. The key to getting the lowest possible idle is to add weight to the prop/spinner to increase the flywheel effect. Use the largest diameter prop possible that will provide a similar load to the engine. A good choice in this case is a 14 x 6. Also, use an aluminum spinner and add some spinner weight if possible. Another thing that well help reduce the idle rpm is a glow driver. There are several aftermarket companies that make drivers which will light the glow plug below a given throttle position.

If your engine will not idle below 2,500 than the issue most likely lies in the fuel mixture, valves or compression. Adjust the valve clearances per the instructions. For the mixture, the pinch test, as Laird SS suggested, is a good way to go. If you still cannot get it resolved, the engine may need to get serviced.

Hope this helps.

David Wolf, Senior Product Support Supervisor
Great Planes Product Support
3002 N. Apollo Dr. Ste. 1 Champaign, IL 61822 USA
Support Phone: 217 398-8970
Email: [email protected]