cylinder 1 and 2 are burning much too dark

by milo avram in 912 / 914 Technical Questions 13 years ago

aloha Dears,

got a headache with a 912 S, (pusher on a trike)
cylinder 3 and 4 are burning normal,
but 1 and 2 are much too dark !?!,

did balance the prop but there's no change,

Â¿ valves ?,

cheers

milo

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Re: cylinder 1 and 2 are burning much too dark

by Roger Lee » 13 years ago

Roger Lee

128 5.28k 4.98k

Hi Milo,

Many people misconstrue what normal may be and it can be influenced by several factors. If you are only looking at your plugs after idle and they are a dry black velvet soot then that can be normal from a mixture (air/fuel)differential on different cylinders. The plugs do not look that way when running at higher rpms like cruise. This is usually only an idle development and not a big concern. It can even move from one plug to another at times. If it is a wet, oil and black then we have a separate problem. Dirty air filters can cause problems, to rich a mixture, but this one shouldn't have happened unless someone moved the carb needle clip down. Not syncing the carbs every 100 hrs and not setting the idle mixture screw at 1.5 turns. This last item may be a cause.
If you really want to look at what your plugs are doing in cruise then run it up there for 2-3 minutes and turn the engine off. Then look at the plugs for correct burn.

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

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Re: cylinder 1 and 2 are burning much too dark

by milo avram » 13 years ago

milo avram

2 20 0

hey Roger,

thanks, that is really helpful,

tomorrow I'll be checking the idle mixture screw at 1.5 turns,

Â¿ different cylinders fed by the same carburetor having different combustions !?!,
and this is happening since a short while only,
there seems to be quite some powerloss at cruise too, which is worrying me,

the plug is just wood carbon black, it is not wet, nor oily,

under no circumstances I found moving the carb needle clip, useful, never ever,

since many years rotax engineers insist in engine idle below 1 500 rpm,
and operation idle at ~2 500 rpm,
(where least vibrations are felt,
do You remember, 'when ever save and sensible'),
and although it is completely obvious why, few pilots around here do care about,

thanks a lot, hope to read from You,

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Re: cylinder 1 and 2 are burning much too dark

by shayne morris » 13 years ago

shayne morris

0 17 0

The "rifleing" of the intake maniflods has a lot to do with the efficiency of 1 and 2 cylinders.I have seen that if the engine normally operates above 5000 rpms,this problem tends to go away.Sub 5000 tends to bring this on..No cause for alarm..Hope that helps.

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Re: cylinder 1 and 2 are burning much too dark

by milo avram » 13 years ago

milo avram

2 20 0

dear Shayne,

I appreciate,

interesting,

Â¿ how come this occurring recently only ?,

I'm feeling a weird vibration beyond 3 500 rpm too, that's why I balanced the prop.,

thanks

milo

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Re: cylinder 1 and 2 are burning much too dark

by shayne morris » 13 years ago

shayne morris

0 17 0

Has air density changed drastically...Here's another trick...If the air screws are at 1.5 turns out now,try going to two or even 2.5.A definate change will occur if it is the carbs causing this.The vibration will fall off as well.Although this adjustmant is out of it's effective "range"I have seen where it works.(2200 - 3000).Let me know..

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cylinder 1 and 2 are burning much too dark

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