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912 uls idle mixture and balance

  • jrd
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1 week 15 minutes ago #22179 by jrd
I have a 912 uls with 800 hrs. checking carb balance at 4000 rpm is spot on but as rpm lowers balance falls off. At 2000 it is off by a good bit. Does one adjust the idle mixture screws to balance low rpm?
Thanks

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6 days 22 hours ago - 6 days 9 hours ago #22186 by garrett
Replied by garrett on topic 912 uls idle mixture and balance
No you adjust the cables and or linkage. They pretty much self balance once the throttle plates are opened up it is in the lower rpm's that they need precise synchronization.
Last edit: 6 days 9 hours ago by garrett.
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6 days 16 hours ago #22190 by Roger Lee
Replied by Roger Lee on topic 912 uls idle mixture and balance
Sync the carbs between 1600-1800 with the idle stop screw. If you do set the carb sync for idle down below 1800 you can do that with no issues so long as you remember to not sit and idle there on the ground. Use the lower rpm setting for shutting down the engine or final approach. Due to some windmilling the prop will not turn that slow on approach as it would on the ground. This isn't done with the Bowden cable adjustment. The Bowden cable adjuster is used for the higher rpm sync. The book says 2500 I far prefer 3300-3500. Your 4k wasn't an issue. You are also correct that if you sync let's say at 2500 many times the needles will split again is throttle is increased. This is why I prefer 3300-3500 sync vs 2500 as the book states.
Remember that in low rpms you are in the idle circuit and jets and at high rpms you leave that and are using the needle and main jet. This is more representative for a carb sync at higher rpms to get out of the idle circuit. Plus not all cables and throttle behave perfectly from the idle setting to higher rpms so it should at least be looked at.

I know of only one company that tries to set carb sync by backing out the idle stop screws all the way and only syncs the carbs at 2500. Doing this is not correct and the carbs have to be off somewhere if this is how you do it. With your own results showing a difference between 4K sync and out of sync at idle you just proved that point. The cross over balance tube helps the carbs in the low rpms, but it is not a cure all.

Sync your carbs properly at at every 100 hr. and or annual inspection. Once done right doing it at the next inspection only takes a few minutes.

Be good to your engine. It needs to live, run and be happy for a couple thousand hours.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell

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6 days 10 hours ago #22192 by Eugene
Replied by Eugene on topic 912 uls idle mixture and balance

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6 days 9 hours ago #22193 by alananpat
Replied by alananpat on topic 912 uls idle mixture and balance
Eugene, I've been flying behind a homemade version of this for years now. Nothing not to like about it!

Alan
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6 days 8 hours ago #22194 by Eugene
Replied by Eugene on topic 912 uls idle mixture and balance
Here is my homemade balancing tube. I did it last year with very good results.

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6 days 8 hours ago #22195 by Roger Lee
Replied by Roger Lee on topic 912 uls idle mixture and balance
A larger cross over tube is still not a substitute for a carb sync. It can only do so much especially if one carb is opening at a different setting vs the other. Never fail to sync carbs. Doing so can cause gearbox issues down the road or cracks in other places like mufflers, exhaust pipes, mounts, ect....

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell
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6 days 6 hours ago #22199 by Eugene
Replied by Eugene on topic 912 uls idle mixture and balance
Here is better design with common large plenum and equal intake distance to every cylinder. Situation with 2 carburetors feeding 2 opposite engine sides requires precise balancing. Larger balancing tube is simply attempt to create this large common plenum. And in experimental aviation we can do this legally with very good result and very low vibration at Idle.

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6 days 1 hour ago #22210 by Roger Lee
Replied by Roger Lee on topic 912 uls idle mixture and balance
This is a whole different animal from carbs.

Just a side note for many with an experimental certificate. Operating Limitations written by different inspectors when the plane was certified are not all created equal. They have guidelines, but can make additions or subtractions.

I have read in some that any change in the aircraft must be approved by a FSDO or inspector in writing. Before you make changes in the US read ALL you Operating Limitations front to back.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell
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5 days 22 hours ago #22223 by Eugene
Replied by Eugene on topic 912 uls idle mixture and balance
drive.google.com/file/d/0B9ZeEB_79LEPdTV...Vms/view?usp=sharing

In ELSA your Operating Limitation is not set in stone. At local FSDO I changed from VFR daytime only to night VFR. Of coarse after installation needed lights. Also installed LED landing light.

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5 days 17 hours ago - 5 days 17 hours ago #22228 by Roger Lee
Replied by Roger Lee on topic 912 uls idle mixture and balance
You're held to your individual operating limitations in ELSA or SLSA in the US. You can't make them up or disregard them at will or there would be no use to even have them. If you have an incident you will be held to that those standards written in your limitations. That's why it's call Operating Limitations. This would also be a point for any insurance claim.

If you were never accountable you could do anything with no consequences. Like some do with the IRS. You can claim anything you want, until,,,,,,,,, you get caught.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell
Last edit: 5 days 17 hours ago by Roger Lee.

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5 days 9 hours ago #22232 by alananpat
Replied by alananpat on topic 912 uls idle mixture and balance
My Experiential Operating Limitations clearly state that any Major Change requires a return to Phase 1 for 5 hours. And, of course, an appropriate logbook entry. Building and installing a larger balance tube was not a major change. Going from the original 2 carb setup to a single fuel injection system probably would be. That really looks fine by the way. Not gonna do it though. :-)

Alan

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5 days 8 hours ago - 5 days 8 hours ago #22236 by Roger Lee
Replied by Roger Lee on topic 912 uls idle mixture and balance
"Building and installing a larger balance tube was not a major change."

Why do you think this isn't a major change. My question would be who gets to make that decision? It altered the engine from the MFG. Only a few people in the world have done this.
I have never ever seen any test data from a reliable source for the change ever published. Many in their experimental Operating limitations have to get approval from the FAA. I've read it many times in owners paperwork.

Plus the fact that yours doesn't come right out and say you need approval shows that there is no consistency from inspectors issuing these limitations and why everyone should read theirs word for word before any modification.

Using one brand of hose over another for a fuel line may not be a major change, but this tube alters the engine performance which people have claimed. Altering the engine performance is a major change.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell
Last edit: 5 days 8 hours ago by Roger Lee.

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4 days 10 hours ago #22247 by alananpat
Replied by alananpat on topic 912 uls idle mixture and balance
I asked my DAR before flying the airplane. I do agree about the lack of consistency.

Alan

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