OS 45F won't idle?
 

I have an older OS 45F 2 stroke in my Harrier 3D. First I had bubble in my fuel line and fixed that. Now it runs at high rpm and high idle, but won't idle down for crap. It still has excellent compression. It also loads up too. If I adjust the low speed for decent transition along with the high speed it gets better, but the lower idle just isn't there like it was when mounted upright. I previously had it mounted upright in an old trainer and it ran great. Now it's inverted and running like a dog. I bought it for next to nothing, but would still like to use it. I run Wildcat 10% with 18/20 oil content. Is the fact that I inverted it making it run poor or just a tuning issue. The fuel tank is inline with the carb as far as I can tell. I'd really like to get this plane in the air but haven't because of it not running reliably.

Thanks,
Tim

RE: OS 45F won't idle?
 

Tim,


Did you try leaning down the low-speed circuit?
Is the idle stop-screw backed out enough for the throttle to close sufficiently?

OS engines generally come adjusted pretty rich on the low-end, to the point of dowsing themselves to death while idling.
The 'factory settings' must be changed for the engine to run reasonably,


Do the 'pinch test', i.e. set the engine to the lowest speed and pinch the fuel-line firmly.

If the engine just dies immediately, it is too lean.

If it speeds up considerably and takes a couple of seconds to die, it is too rich.

If it speeds up a bit and dies after 0.5-1 second, it is close to the right setting.


Accessing the low-speed needle is done with a 2-3 mm flat screwdriver, in the middle of the throttle arm, on the right side of the carburettor.


Adjusting the low-speed requires that the engine be broken-in, at least 30 minutes of running at a rich two-cycle (if it is an ABN engine), or 45 minutes richer for a ringed engine.

Do the adjustments in small increments. Initially 1/4 turns and then fine adjustments of 1/8-1/16 of a turn, until a perfect transition (from idle to full throttle) is obtained.

RE: OS 45F won't idle?
 

Since it's inverted I would fit it with an "F" glow plug and run 15-20% nitro. I'd bet any amount of money that the plug is getting too cold.

RE: OS 45F won't idle?
 

G'day Nathan,
A type "F" glow plug is too hot for 10% nitro fuel in a 2 stroke, the heat range of a glow plug is dependant on nitro content of the fuel, not if the engine is upright or inverted.
As the nitro content goes up, a colder plug is required.
As Dar stated, low speed mixture is the problem.
Once that is set properly, it will run great with a OS #8 plug, or similar.

RE: OS 45F won't idle?
 

Yes, as nitro content goes up the plug should get cooler; however, I know many people that run "F" plugs in two strokes with enhanced engine stability and no side effects (advanced timing, detonation, shortened engine life). I would not recommend using a hotter plug as a "bandage" for the problem, but for added stability and lower idling. Perhaps I didn't make that clear. Oil can really cool off a plug when your engine is inverted, even if you are not running rich.

RE: OS 45F won't idle?
 

People are running the Saito SS plug in 2 strokes.

RE: OS 45F won't idle?
 

Thanks for all the helpful suggestions. I'll mess around with it some more once I get back in town on Tuesday.

Tim