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ROTAX Carb Sync question

DaveWelch

Well Known Member
Installed new Throttle Cable and worked through the mechanical sync procedures.

Attempting Carb Sync with Carbtune (2-column) vacuum gauge marked in centimeters of mercury (cm-Hg). Have also double-checked for leakage opportunities in the hose connections.

This is what I?m seeing in the engine runs:
Left carb shows 2 cm higher vacuum at 2000 rpm idle (Left side ? 24cm, Right side ? 22cm)
Both sides show equal vacuum at 3500 rpm.
As throttle advances above 3500 the Left side vacuum decreases more rapidly until it?s about 1.5 cm less than the Right side at which point that the bars disappear off the lower end of the scale around 4800 rpm.

It's puzzling to see the Left Side vacuum Higher at idle and "cross over" to read lower at high power. Anyone else seen this? Nevertheless...

Question: How close is ?close enough? i.e. how much differential will the Compensator Tube between the carburetors ?compensate? for? According to my calculations a 2cm differential is about 2.5% of one standard atmosphere which doesn?t sound like much. However, if the gauge is accurate that 2 cm is around 9% of the lower of the 2 vacuum readings at idle. The 1.5cm differential would be about 20% of the 8cm indicated vacuum at high power.

Will appreciate any comforting words, advice, or recommendations to address the differences.
Sincere thanks as always.
 
Adjust to have it as close as you can at idle.
Don't worry about the rest (the documentation doesn't have adjustments at the high end as part of the procedure).
When you remove the gauge and reconnect the balance tube it will help a bit with the rest of the range, but the low end is what matters.
 
Electronic carb sync set ups aren't as good as analog dial gauges. The electronic carb sync set ups can split a hair, but once you take it off and reconnect the balance tubes all that hair splitting is gone. You can't diagnose with the electronic ones either. The analog gauges you can balance easier because you can see what carb to balance and by how much, but the big help is you can diagnose a carb issue or cable length issue as soon as the engine starts and settles down.
Set the high rpm sync first with the Bowden cable adjuster. The carb sync tool accuracy should make getting a good accurate sync equal. Then pull back to idle and set the idle rpm. They both should be right on and it isn't that hard once you've done it and understand what the gauges are telling you. Don't do the idle first because you most likely will have to change it once you do the high rpm sync.

The balance tube will help sync both carbs once hooked back up to about 3500 rpm, but after that it is too small to have any real affect at cruise rpm.
I prefer to set my high rpm sync at between 3300-3500 rpm and the idle down around 1650-1700 rpm.

Hope this helps
 
carb sync saga (you have to say it fast 3 times)

Thanks guys,

I meant to swap the lines tonight but forgot. Great suggestion.

The "CarbTune" (not CarbMate) is an analog tool sorta like having 2 columns of mercury to observe as you do the sync procedure. It is marketed for pre-EFI motorcycles but works well for our Rotax also. I checked into them after seeing Scott say that's what they use at the Mothership.

My plan tomorrow is to start over from the beginning with setting the idle mixture (2nd time), then mechanical sync (3rd time), and then the rest of the procedure again since I've made so many tweeks I've lost all reference to the starting point. The Van's procedure tracks the ROTAX procedure pretty closely. Rotax adds a step to equalize the vacuum at 2500rpm also.

The other thing I noticed again tonight is that if I get the idle vacuum equal, I see about 50-70 degree lower EGTs on the left side. With the idles mechanically sync'd it goes back to 2cm-Hg higher vacuum on the left but very close EGT readings. Note that all this is just after a ground warm-up with about 180F oil temps, EGTs in the 1100-1200 range. Warmed up enough?
 
Installed new Throttle Cable and worked through the mechanical sync procedures.

Attempting Carb Sync with Carbtune (2-column) vacuum gauge marked in centimeters of mercury (cm-Hg). Have also double-checked for leakage opportunities in the hose connections.

This is what I’m seeing in the engine runs:
Left carb shows 2 cm higher vacuum at 2000 rpm idle (Left side – 24cm, Right side – 22cm)
Both sides show equal vacuum at 3500 rpm.
As throttle advances above 3500 the Left side vacuum decreases more rapidly until it’s about 1.5 cm less than the Right side at which point that the bars disappear off the lower end of the scale around 4800 rpm.

It's puzzling to see the Left Side vacuum Higher at idle and "cross over" to read lower at high power. Anyone else seen this? Nevertheless...

Question: How close is “close enough” i.e. how much differential will the Compensator Tube between the carburetors “compensate” for? According to my calculations a 2cm differential is about 2.5% of one standard atmosphere which doesn’t sound like much. However, if the gauge is accurate that 2 cm is around 9% of the lower of the 2 vacuum readings at idle. The 1.5cm differential would be about 20% of the 8cm indicated vacuum at high power.

Will appreciate any comforting words, advice, or recommendations to address the differences.
Sincere thanks as always.

This was always confusing to me...so I went to the Rotax training class in Tucson. Whn syncing your carbs, you first sync the "idial" circuit/jet around 1650rpm (Adjustments are made with idial adjust screws).. Then run engine up to 3500 rpm and sync "main" jets (this is done by lengthening or shorting the throttle cable adjust bolts)...it is really that simple. The low and high sync points are totally different.

Check your gauges that they read equal (the same vacuum source); then you are merely balancing carbs to read equal vacuum at those two points.

Edit: I mis-spoke regarding syncing "main" jet...@ 3500 rpm, what you are syncing is the needle jet (intermediate rpm range); the fixed main jet (upper rpm range) doesn't require syncing.
 
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should be simple

Thanks Ric, that's what I thought too. But it isn't working out that way for me. Like "squeezing a balloon". I'm starting over again and will (again) follow the procedure exactly. Must have screwed up something in the process of "overthinking it" as I sometimes do.
'preciate it,
Dave
 
Adjust to have it as close as you can at idle.
Don't worry about the rest (the documentation doesn't have adjustments at the high end as part of the procedure).
When you remove the gauge and reconnect the balance tube it will help a bit with the rest of the range, but the low end is what matters.

Scott, you normally give good advice. However on this point you are wrong.

The sync @ 3500 rpm is very important! Just how much flying do you do @ idle? Suggest you read the Rotax manual.
 
Thanks Ric, that's what I thought too. But it isn't working out that way for me. Like "squeezing a balloon". I'm starting over again and will (again) follow the procedure exactly. Must have screwed up something in the process of "overthinking it" as I sometimes do.
'preciate it,
Dave

Dave...it doesn't normally take much turn of the idle screw or throttle cable nuts to effect change; also much easier to do while engine is running...just have someone you trust in the cockpit😳
 
Scott, you normally give good advice. However on this point you are wrong.

The sync @ 3500 rpm is very important! Just how much flying do you do @ idle? Suggest you read the Rotax manual.

Running rough (unsynched) at idle is what kills gear boxes.
Ground operations rarely have the throttles against the idle stops (on the RV-12 it is recommended to be at 1650 RPM this is below the green constant operation range) so they shouldn't be relied on to hold the carbs in synch.
If you do, that means if you keep the engine at above 1800 RPM for ground ops like you are supposed to do, but you have compromised your low RPM carb synch to have it perfect at 3500, you are being hard on your gear box and will probably pay for it ($$$).

We also do very little flying at 3500 RPM. Once you get the throttles near wide open, a small difference in vacuum values is undetectable.

I have been working with / on the Rotax 912 since Vans started the RV-12 development program in 2006 and I have been to numerous Rotax Service and Maint. training classes, so I am familiar with what it says in the manuals......
 
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Just another comment - -

Are you using the OEM supplied stronger throttle springs on the carbs ? Lighter duty springs may not be pulling the same and makes it harder to adjust at various ranges.
 
A mechanical sync is done first and all it does is get you in the ballpark to do the pneumatic sync. Once done it usually doesn't have to be done again for subsequent syncs unless you remove carbs or cables and need to start from scratch. If you keep up on the sync at every annual/100 hr. inspection then the cabs should only need a tweak to keep them synced.
The reason to use 3300-3500 rpm over the recommended 2500 rpm is that 2500 rpm is still too far down into the idle circuit using only the idle jets which are out of play at higher rpms and because it flows little fuel and air. Even though we don't fly at 3500 rpm it is more representative of air and fuel flow and it is getting off the idle circuit into the main jet side. If you only balance at 2500 rpm and then advance the throttle you are much more likely to get your gauge needles to split again. If you sync at higher rpms that usually isn't the case unless you have another problem. Syncing too low an rpm is like using a torque wrench at its top or bottom 10% of its range. It isn't recommend.
The reason to set the high rpm sync first is because if you set the idle first you will most likely have to do the idle again after youturn the Bowden cable adjuster for the higher rpm sync.
You can set the idle at 1600-1800 rpm if you want. Pick your own rpm. We should all know that you should never just shut and idle at low rpms under 1800 and for that matter you are always better of at 2000 rpm or higher. It is much easier on the gearbox and engine components due to the engine's high compression. The only reason to have a lower idle rpm is for shut down only. When you pull all the way back and the idle is dropping to let's say your 1700 idle rpm then shut down the engine.
 
The only reason to have a lower idle rpm is for shut down only.

Not so Roger

As has been discussed and debated here in the past, an idle speed set at 1800 RPM (or higher) will have a pronounced impact on landing performance of an RV-12.

I strongly recommend owners follow the recommendations of the RV-12 maint. manual regarding idle speed adjustment, and then operate the engine within Rotax's recommendations by manually positioning the throttle so that the engine is never idling below the green range.
 
You can set the idle at 1600-1800 rpm if you want. Pick your own rpm. We should all know that you should never just shut and idle at low rpms under 1800 and for that matter you are always better of at 2000 rpm or higher. It is much easier on the gearbox and engine components due to the engine's high compression. The only reason to have a lower idle rpm is for shut down only. When you pull all the way back and the idle is dropping to let's say your 1700 idle rpm then shut down the engine.

Where would one find a reference for this information? I've always pulled the throttle to 1800 (I think it is -- the bottom of the green arc on the tach) and shut down. I have my idle set to roughly 1600, because otherwise it can be a challenge to get slow enough on final.
 
The stick controls air speed on approach and landing and the throttle altitude or glide slope. All these articles say the same. There are hundreds more.
Landing in your chosen idle rpm speed is controlled by the stick and the nose attitude.

http://www.boldmethod.com/learn-to-fly/navigation/pitch-and-power-on-a-glideslope/


https://www.pilotworkshop.com/tips/final_approach_speed.htm

http://www.110knots.com/airspeed-and-altitude-controll-two-schools-of-thought/

http://flycasey.com/landing-pitch-and-power/

Quote from Beyond PPL book on flying:

"In days of yore, instructors always taught that on the approach you should control airspeed with pitch and maintain the correct glide slope with the throttle.
The technique taught was (and still is) a good device for getting students to co-ordinate properly their applications of pitch and throttle."
 
The stick controls air speed on approach and landing and the throttle altitude or glide slope. All these articles say the same. There are hundreds more.
Landing in your chosen idle rpm speed is controlled by the stick and the nose attitude.

http://www.boldmethod.com/learn-to-fly/navigation/pitch-and-power-on-a-glideslope/


https://www.pilotworkshop.com/tips/final_approach_speed.htm

http://www.110knots.com/airspeed-and-altitude-controll-two-schools-of-thought/

http://flycasey.com/landing-pitch-and-power/

Quote from Beyond PPL book on flying:

"In days of yore, instructors always taught that on the approach you should control airspeed with pitch and maintain the correct glide slope with the throttle.
The technique taught was (and still is) a good device for getting students to co-ordinate properly their applications of pitch and throttle."

Roger (Pretenting to be someone else because of previously being banned),
Please don't start this debate all over again (for those interested there is a lot of discussion in the forum archives on this)

Bottom line.... I agree that Roger is on the Rotax engines but on at least this one point ignore what he says.
 
Where would one find a reference for this information? I've always pulled the throttle to 1800 (I think it is -- the bottom of the green arc on the tach) and shut down. I have my idle set to roughly 1600, because otherwise it can be a challenge to get slow enough on final.

Dale...there has, for some time, not sure how long; various views regarding "correct" idle rpm (and landing rpm's)....I would follow Scott's advice.

I, personally like a lower idle rpm (1550), and I go there only for engine shut down. Landing, I use around 2000 rpm, as I like to use a higher approach speed fly onto the runway. I pull back to idle just before main wheels contact; then to 1900 for run out/taxi...just my way.

Jolly's sync procedure is right on....use them! And you will be happy.
 
Just to clarify (even though it has been covered numerous times and is in the archives)

Even if you use 1600 RPM for an idle setting, with the throttle pulled to idle in a glide for landing, the engine will not be idling at 1600 RPM.
Because of relative wind the prop is unloaded and the engine will be turning at more like 1900.
Only when you come to a complete stop will the engine idle down to 1600 RPM and at that point the pilot should advance the throttle and get the RPM back in the green range
 
Just to clarify (even though it has been covered numerous times and is in the archives)

Even if you use 1600 RPM for an idle setting, with the throttle pulled to idle in a glide for landing, the engine will not be idling at 1600 RPM.
Because of relative wind the prop is unloaded and the engine will be turning at more like 1900.
Only when you come to a complete stop will the engine idle down to 1600 RPM and at that point the pilot should advance the throttle and get the RPM back in the green range

Yes, I'm aware of that. I honestly couldn't tell you what my actual RPM is on final -- I pay no attention to it until rolling out. Throttle is all the way out and my eyeballs are over the nose with an occasional glance at airspeed until the AoA starts beeping. I have the idle set for 1600 or very slightly above on the ground.
 
Thank-you John,
I remember noticing that difference before and had forgotten.
I'll pay more careful attention during my third, hopefully final, mechanical sync!
Clear Skies,
dave
 
Power at idle and pitch for speed, got that, Jolly er Roger, whoever you are driven to be, thanks for your expertise. I'm newish to the ELSA world. Lots to learn and refresh on. Know it all or not, you come off as wanting to inform us newbies with thoughtful referenced material. Hope the Blog police lighten up on you and reconsider your value to us. Doug in IL

The stick controls air speed on approach and landing and the throttle altitude or glide slope. All these articles say the same. There are hundreds more.
Landing in your chosen idle rpm speed is controlled by the stick and the nose attitude.

http://www.boldmethod.com/learn-to-fly/navigation/pitch-and-power-on-a-glideslope/


https://www.pilotworkshop.com/tips/final_approach_speed.htm

http://www.110knots.com/airspeed-and-altitude-controll-two-schools-of-thought/

http://flycasey.com/landing-pitch-and-power/

Quote from Beyond PPL book on flying:

"In days of yore, instructors always taught that on the approach you should control airspeed with pitch and maintain the correct glide slope with the throttle.
The technique taught was (and still is) a good device for getting students to co-ordinate properly their applications of pitch and throttle."
 
I've done BING carb synch dozens of times, albeit mostly on BMW airhead motorcycles.

Two different adjustments are in play:

1) At idle, the throttle cables should have enough "play" that the idle is determined solely by the position of the idle screws. Get that right first. Make absolutely sure the throttle arms are firmly against the idle screws at idle. If not, adjust the barrel adjuster involved to make sure they both are.

2) At whatever midrange setting you choose -3,500 seems about right - it's the cables that determine the synch. Adjustment for that is with the barrel adjusters. As long as that adjustment does not eliminate the "play" in the cables at idle - it can - idle speed should not be affected by the midrange synch.

Finally, make sure your "chokes" are fully off. On my Sky Arrow the "choke" cables are really long and the return springs seem marginal at best to close them all the way against the stop. I think I'll look for slightly stronger return springs while I have everything apart for my annual.

As an aside, "choke" is put in quotes because the BINGS don't utilize a choke, per sé. Rather than "choke" the flow of air to the carb to enrichen the mixture, the BINGs use a separate starter circuit. I think most pilots/owners are aware of this, but some newbies might not be.
 
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1) At idle, the throttle cables should have enough "play" that the idle is determined solely by the position of the idle screws. Get that right first. Make absolutely sure the throttle arms are firmly against the idle screws at idle. If not, adjust the barrel adjuster involved to make sure they both are.

This is not relevant for the carbs installed on a Rotax 912

The cables on a 912 are under maximum tension when the throttle is at idle so no slack can be provided.

Also, Rotax specifically specifies that a secondary idle stop must be used because the carb. stop that hits the screw will flex under load when someone is pulling on the throttle to the idle position.

For the RV-12, the stops are small blocks that slip onto the cable and get clamped with a set screw. they are adjusted so that they bump the barrel adjuster when the throttle is at idle position.

They are supplied with the throttle control.
 
This is not relevant for the carbs installed on a Rotax 912

The cables on a 912 are under maximum tension when the throttle is at idle so no slack can be provided.

Scott,

You are, of course, right. My Sky Arrow is the same, as I assume all ROTAX 912 installations are.

I blame, in no particular order...

1) Negative transfer from my motorcycle carb syncing days.

2) Mature Onset Dyslexia.

Thanks for setting the record straight.
 
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I have not yet even pulled the cowlings, but cannot understand your post,
is there a reference that would make it more clear to me?
I am installing a Vans Vernier throttle, but in the meantime I am entertaining my airport bums by pulling the throttle out, letting go and watching in disbelief as it goes to full throttle! (us old people are quite easy to entertain actually)

This is not relevant for the carbs installed on a Rotax 912

The cables on a 912 are under maximum tension when the throttle is at idle so no slack can be provided.

Also, Rotax specifically specifies that a secondary idle stop must be used because the carb. stop that hits the screw will flex under load when someone is pulling on the throttle to the idle position.

For the RV-12, the stops are small blocks that slip onto the cable and get clamped with a set screw. they are adjusted so that they bump the barrel adjuster when the throttle is at idle position.

They are supplied with the throttle control.
 
I have not yet even pulled the cowlings, but cannot understand your post,
is there a reference that would make it more clear to me?
I am installing a Vans Vernier throttle, but in the meantime I am entertaining my airport bums by pulling the throttle out, letting go and watching in disbelief as it goes to full throttle! (us old people are quite easy to entertain actually)

The engine install section of the KAI covers it.
 
My first encounter with th ROTAX 912 was on a Searey I built. On that setup the springs were set to shut the throttle. I think the RV-12 is a much safer design (spring to WOT).
 
Safer

My first encounter with th ROTAX 912 was on a Searey I built. On that setup the springs were set to shut the throttle. I think the RV-12 is a much safer design (spring to WOT).

It makes sense I suppose if you have a cable failure in cruise. It's a real pain when you're trying to slow down and get down to a short strip as the RPMs creep up on you.
 
On my first home built I had a Continental O-200. I installed springs to make the throttle go wide open if the cable snapped. Well one day it did on the ground which sent me rocketing toward the FBO lobby window. Killing the mags stopped it. I limped back to the hangar using mixture for a throttle. Sometimes getting experience really sucks!
 
As a follow up, I just did my carb balance as part of my Sky Arrow's annual condition inspection. It's not on my checklist, and I did not get around to it last year and figured it was about time, though my engine did feel about as smooth as always.

Everything is a little tougher with a high-mounted engine and a pusher prop. This time, I hooked up long extension hoses using some oxygen hose I had and ran it up to the cockpit:

35556253912_74b5843b52_z.jpg


35593858221_c8249f9d11_z.jpg


It appears I was overdue:

34883685174_844d103dbf_z.jpg


Anyway, the Law Of Primacy is still strong with this one - in spite of the previous discussion in this thread, needing more throttle on the left side led me to adjust the barrel adjuster to take slack out, which, of course, is backwards - you need to screw the adjuster in to allow the springs to pull the throttle lever towards a more open position. All opposite to my BING experiences on BMW's.

I was wondering why my adjustments - in both directions - seemed to be having little or no effect. I'd loosen the lock nut, screw the adjuster in or out and tighten the locknut again. I was missing the fact that the adjuster on these carbs are not threaded into the support arm - I thought they were. After that "Aha!" moment, it only took two tries, adjusting the nut on the barrel side, to arrive at this:

35725172605_55a9320713_z.jpg


It appears about 1/2" off in the photo, but the two vacuums were tracking pretty perfectly throughout the rpm range. There may be some parallax error in the photo as well.

Thanks for the input and corrections - I think it finally got through to me!
 
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