k9copfl

Hi Bax,

Need some help setting up my OS 91 FS. Mounted upright in a Lainer Dart. New glow plug and fresh fuel. Starts great with good solid high end. I am having extreme difficulty setting the idle. I reset the low end needle to factory settings then adjusted from there. While running high idle, it runs great. Trying to get low idle (slow enough to control at taxi) it sputters and dies. Does not matter if glow igniter is on or off. This is a used engine out of a 1/4 scale cub. Side mounted in that. No problems then. I have gone through about 3 tanks of fuel trying to adjust the idle mixture with negative results. If I can get a reliable idle, the dart will take its madain flight. Please help this 4-stroke novice. Thanks.

rctrax

I think you are too lean on your low speed needle.

Laird SS

Have you tried the "pinch test" with progressively slower idle speeds? Start at whatever idle speed you can get, then pinch off the fuel line as close to the nipple on the carb. You should hear the idle speed increase slightly, then begin to drop off. If it drops immediately, you are too lean. If the idle speed takes a long time to rise, you are too rich.

Don't bother trying this at a high throttle setting because the engine will be drawing too much fuel for you to hear any increase in speed before it dies. The idea behind this test is similar to when we ran control line planes without a throttle. The engine rpm would always rise as the plane ran out of fuel because the mixture got leaner.

k9copfl

I did perform a pinch test and it died immediatly. It seems that I backed the low end out 1 1/2 to 2 turn with the same results. I will reset the needle to the factory settings again and try it again. Thanks for the help.

dcwolf

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]

KetRCFlyguy

My Feedback: (10)

OS 91 FS Surpass
Hey, I haven't done this much, but am hoping it is OK to ask this type question.
Does anyone "out there" know where I could get a complete carburetor for the OS 91 FS Surpass - non pumped. The engine is discontinued but mine worked great before the threads that hold in the needle valve apparatus stripped out So that part + needle valve was lost. Seems best to replace entire carburetor. Great engine, and doesn't have all that much time on it, so I'm not ready to "parts junk" it yet. I would appreciate any leads. My searches on internet are not expert, but I'm not finding anything for this vintage engine. Thanks for any advice and/or leads

Bax

My Feedback: (11)

You'd want to ask this in the general "Glow" forum because this forum is only for product support of O.S. engines. Unfortunately, we'd not know where you might be able to locate the carb or its parts. The last carb was sold in 2009.

KetRCFlyguy

My Feedback: (10)

Hello Bill,

Yes this forum thing is new to me, so thanks for the direction and for the advice and information.