why does my Idle Needle need to be set so Lean ??
10-17-2016 | 01:42 PM
#1
Thread Starter
My Feedback: (1)
why does my Idle Needle need to be set so Lean ??
Baxter, on my 61Fx the Idle Needle is not even 1 turn open, I would say it's about 75% to 80% of 1 turn open,
could it be because the SlimLine Pitts is so restricting of the exhaust ? I ask about the muffler because it will only spin to 11,800 tuned for flight with an APC 11X8,
fuel is Magnum Plane 15% Nitro, plug is the OS 8,
it will idle great at about 2,400,, it will set and idle for maybe a half minute and not sputter when advancing to full throttle, but idling any longer than that she will sputter badly when advancing to full throttle,,, tuning it richer or leaner doesn't help,
my engine has had about 7 gallons burned threw it and it has very good compression, never flown lean
thanks Baxter !!
Jim
10-18-2016 | 09:31 AM
#2
My Feedback: (11)
The idle needle isn't "lean". It's just set where it needs to be to get your engine to operate properly. There is no such thing as setting a needle "too lean" or "too rich" if the engine's running the way it should be. We've had engines with main needle valves at 2 turns out and at 1/2-turn out and it's just how that particular engine needed to be set. The actual position of the needle is irrelevant when the engine's running as it should.
HIgher fuel pressure may require you to turn the needle in farther than if you have lower fuel pressure. However, it's all in how the engine runs. The idle mixture is also influenced by the high-speed needle. Most non-pumped O.S. engines have the main needle set between 1-1/2 turns out and 2 turns out when running at full throttle on the ground. The idle needle is adjusted after that.
A basic setting procedure would be to start out with the idle mixture set to "stupid rich" and the high-speed needle about 2+ turns out from closed. Start the engine at 1/4-1/3 throttle and advance to full before removing the starter battery. Adjust the high-speed needle to just rich of peak RPM and then retard the throttle. Adjust the idle mixture as needed until you get the best idle RPM that gives you a good throttle transition.
HIgher fuel pressure may require you to turn the needle in farther than if you have lower fuel pressure. However, it's all in how the engine runs. The idle mixture is also influenced by the high-speed needle. Most non-pumped O.S. engines have the main needle set between 1-1/2 turns out and 2 turns out when running at full throttle on the ground. The idle needle is adjusted after that.
A basic setting procedure would be to start out with the idle mixture set to "stupid rich" and the high-speed needle about 2+ turns out from closed. Start the engine at 1/4-1/3 throttle and advance to full before removing the starter battery. Adjust the high-speed needle to just rich of peak RPM and then retard the throttle. Adjust the idle mixture as needed until you get the best idle RPM that gives you a good throttle transition.
