Black spark plugs

by Peter Varmfors in 912 / 914 Technical Questions 9 years ago

I have an 912 uls with 210 hours, I have noticed that the spark plugs are black which indicates to rich mixture.
I’ve tested to run the engine at 2200 rpm and then raise the choke slowly, the rpm raises up to 3000 rpm an then falls back. (I do NOT have an EGT gauge)
I’m thinking of moving the needle one notch (second from top) and see if it’s getting better.
How drastic is the leaning by dropping the carb needle down one notch?
Another thing could be to open the idle mixture screw (maybe 2 turns instead of 1.5) to get more air into the fuel.
The carburettors are recently served and also balanced.
Anyone who’s got an idea about this?

Regards
Peter Varmfors

Re: Black spark plugs

by Roger Lee » 9 years ago

Roger Lee

126 5.13k 4.53k

Hi Peter,

Black dry sooty plugs are normal for many in the idle rpm range. This is why when going to check plug color you should run the engine up for a few minutes and then idle back with an immediate shutdown. I would leave the carb needle position alone for now. Make sure the idle mixture screw is 1.5 turns out. Then run the engine around 4000 rpm for 5 minutes. My bet is the black soot is gone. The engine mixture can be off a tad with unequal lengths on the air intake at low rpms with the fuel/air mixture. The soot goes away at higher rpms. This affects different cylinders and plugs for everyone at idle rpm.
Unless there was a special needs situation the stock carb setup should works for the majority of the engines.

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

Thank you said by: Peter Varmfors

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Re: Black spark plugs

by Rob Seaton » 9 years ago

Rob Seaton

3 1.32k 240

As the carbs have been recently been serviced you should look at what was last done to them before making any adjustments that may mask the true problem:
Are the slides perfectly aligned in the throat? do they move freely with finger pressure?
What notch are clips at?
Is the choke assembled correctly? Are the dots on the brass shaft aligned with the cable end? are the chokes on the correct carb? look at the letter stamped on the inside of the choke shaft: "L" is on 1-3 carb and the "R" is on the 2-4 carb.
Does the float valve hold pressure?

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Re: Black spark plugs

by Peter Varmfors » 9 years ago

Peter Varmfors

2 3 0

Hi Roger and Rob
Thanks for your quick answers.
The sooty spark plugs have been, I think, since the engine was new. This is the first 200 hours.
So it’s not a phenomenon that has suddenly occurred just now. I’ve done the service of the carbs my self. All things that you mention Rob is ok. I’ve also checked the float valve pressure which is ok.
If found the tip of changing the needle on ROAN, operational checks.
Maybe I’ve just should leave it as it is then.
The engine sound and runs well, although I think it consumes more than according to recommendation.
Regards
Peter Varmfors

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Re: Black spark plugs

by Rob Seaton » 9 years ago

Rob Seaton

3 1.32k 240

try going for a hard run and shut it down with out a prolonged idle time and see what the plugs look like. they should be almost white.

Thank you said by: Peter Varmfors

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Black spark plugs

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