Idle mixture screw on bing 64

by Kevin Machniak in 912 / 914 Technical Questions 12 years ago

Hi,
I have a 912 ULS , I just completed my first inspection cleaning on the carbs.
I turned in the idle mixture screws on both carbs to find where they were set, they were at 2 1/2 turns out from full in. I know the recommended is 1 1/2 turns out from full in . I assembled and installed the carbs like I removed them 2 1/2 turns out. What would be the need for the previous owner, mechanic to have them at 2 1/2 turns out ? I believe this would account for the engine never really ever running smooth between 2500-3000 rpm, below and above it is real nice but nasty vibrations in the mentioned rpm range . The jet needle has c-clip second slot from bottom , has always been there summer(+25`C) and winter( -20`C).limited winter flying so far so no data on temps.

I did a carb sync using a carbmate , but did the setting at 2500 rpm. I know Roger likes at 3500 rpm. I did verify thru the range of rpms up to 4000 and sync is good.

I would like to eliminate this vibration, rough running between 2500-3000 rpm , Ideas?

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Re: Idle mixture screw on bing 64

by bill martin » 12 years ago

bill martin

10 757 0

Kevin,

I do my carb sync at 2500 also, and my idle screws are set at 1 1/2 turns out. Another thing you might check are the enrichener circuits. If they are not closing fully after starting you could be running too rich.

Bill.

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Re: Idle mixture screw on bing 64

by Kevin Machniak » 12 years ago

Kevin Machniak

6 167 40

I plan on taking the carbs apart again and changing all the o-rings and gaskets,there is no record of them ever being changed within the last five years, parts are coming, when I reinstall I am going to have the idle mixture screw at 1 1/2 turns out.

Any one know what size wrench fits the idle mixture screw's hex head ?

I have drip trays under the carbs so it is next to impossible to adjust the idle mixture screws when all is mounted, I possibly could if I had a wrench to fit the screw's hex head and get enough turning before hitting the drip trays.

I just wonder if that 1 turn in from 2 1/2 out will make that noticeable of a difference ? I have checked the starter carbs for return but will recheck that as well. and then a pneumatic sync to finish off.

Here's another question , with an inflight adjustable prop does it matter what prop pitch you sync the carbs at ? I have been using a fine setting .(static runup 5500 rpm pitch setting)

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Re: Idle mixture screw on bing 64

by Roger Lee » 12 years ago

Roger Lee

125 5.12k 4.53k

Hi Kevin and All,

It doesn't matter what the prop pitch is. Fuel and air flow will be the same. It does make a difference where the mixture screw is set and 1.5 turns out is the best all around spot.

Another item is you should not only sync the carbs at idle, but you should sync them higher than 2500 rpm. If you set the sync at 2500 rpm then when you rev up to 3500-4000 they will be out of sync. We don't fly that close to idle rpm (2500) and that is too low on the flow spectrum to do a good job. You need to get those carbs flowing enough air and fuel to make a difference and closer to a cruise rpm.
Sync those carbs up to at least 3500 rpm. It does make a difference.

I have seen a major dealer set the high rpms at 2500 (too low) and then back off the idle stop screws and not even sync the idle. Don't do this. If you think it is important that the carbs be synced so the right and left halves of the engine run at the same rpm why on earth would you want the right and left to be out of sync at idle and run at opposing rpms.

If a mechanic tries to balance your carbs and not engage the idle screws and not sync the carbs at idle tell them you want it your way and to do the sync right at the upper and lower rpms.

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

Thank you said by: YEN NIEN YU

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Re: Idle mixture screw on bing 64

by bill martin » 12 years ago

bill martin

10 757 0

Kevin,

You can fine tune the idle mix screw using the 1 1/2 turns as a starting point. See chapter 12-20-00, section 10.5) of the maintenance manual.

I think though, you will find that the 1 1/2 turn setting will need no further adjustment. Just be sure they are both exactly 1 1/2 turns out.

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Re: Idle mixture screw on bing 64

by Kevin Machniak » 12 years ago

Kevin Machniak

6 167 40

Thanks Bill and Roger,

I am waiting for the o-rings and such . I am looking forward to getting all new rubber inside the carbs and doing a test and sync with the idle mixture screws at 1 1/2 turns out , a sync at 3500 rpm and at idle stops.

Will I need to change settings on the carbs going from + 30`C to -20`C ?
That's the range of climate from summer to winter where I live !

Right now ground temps are +5`C and latest flight yesterday engine temps were

At 3500 ASL, approx +2`C
with prop in cruise 5000 rpm manifold pressure 25.5 inHG
Oil 206`F
CHT cyl 2 = 190`F cyl 3 =195`F
EGT cyl 1= 1338`F cyl 2 = 1363`F cyl 3= 1312`F cyl 4 = 1369`F
Coolant temp 171`F ( coolant sender is just above rad in, approx 1 1/2 ' away from expansion tank)

I had half to 2/3's of the oil cooler blocked off.

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Idle mixture screw on bing 64

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