Carburator leaning

by Michael Sweeley in General Tech Discussion 9 years ago

I recently did a 100 hr inspector on my 912 ULS, while checking and replacing spark plugs I noticed that the old plugs were dark brown not greyish as I would expect...this seems to indicate to me it is running a bit rich. Can some one point me to some instructional videos or articles which will give this neophyte some direction as to how to adjust the mixture setting. I believe that one does this by moving a needle valve to a lower setting but I am not sure and I am differently unsure as to its location the method used to do this.
Thanks
Mike

Re: Carburator leaning

by Roger Lee » 9 years ago

Roger Lee

125 5.12k 4.53k

Hi Mike,

I wouldn't rush to lean the carbs.90% of the time they are fine at the factory setting. Usually only people with special needs like flying out of high elevation airport and flying the majority of the time at high altitudes need to consider a change.

The plugs can be a dark brown to a dry black soot after idling.

Make sure the plug gap is between .023 - .027.
Then run the engine up to around 3.5K -4K on the ground for 4-5 minutes at operating temps. Then pull the throttle back and immediately shut the engine off. Now check the plugs. My bet is they will be a more normal color for you. The dark brown or dry black soot goes away at normal operating rpms. Too much idling causes a slight imbalance in the air/fuel mixture and can cause dark plugs.

Given the right circumstances and having a lean engine could get you into a detonation problem. At WOT (i.e. take-off) it is better to be a tad rich over being too lean. Guessing wrong will cost you $3500 - $4500.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: Carburator leaning

by Michael Sweeley » 9 years ago

Michael Sweeley

8 25 0

Thanks Roger...as always good advice. Plugs were just dark brown not black and no indication of carbon build up. Guess I might just be a little to paranoid.
Mike

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Carburator leaning

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