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  #1  
Old 06-12-2017, 12:33 PM
DaveWelch DaveWelch is offline
 
Join Date: Mar 2005
Location: Huntsville, AL
Posts: 273
Default ROTAX Carb Sync question

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.
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RV12 N951DZ
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  #2  
Old 06-12-2017, 12:48 PM
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 6,577
Default

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.
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  #3  
Old 06-12-2017, 12:55 PM
Ralph Inkster Ralph Inkster is offline
 
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Location: Calgary, Alberta
Posts: 281
Default

Just wondering, did you switch the vacuum lines to see if the variance was in the gauge set-up?
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  #4  
Old 06-12-2017, 08:20 PM
Jolly Jolly is offline
 
Join Date: Nov 2016
Location: San Francisco, CA
Posts: 16
Default

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
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  #5  
Old 06-12-2017, 09:21 PM
DaveWelch DaveWelch is offline
 
Join Date: Mar 2005
Location: Huntsville, AL
Posts: 273
Default 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?
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RV12 N951DZ
First Flight 9/23/16!
No paint yet. Working on a scheme with John Deere colors or accents.
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  #6  
Old 06-12-2017, 09:24 PM
Phantom30's Avatar
Phantom30 Phantom30 is offline
 
Join Date: Jul 2009
Location: Coeur d'Alene, ID/Casa Grande, AZ
Posts: 525
Default

Quote:
Originally Posted by DaveWelch View Post
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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307 (CAB) Phantom
Search and Destroy (Can
Tho RVN)
Distinguished Flying Cross Society Member
CH-47 & UH-1H "Driver"
Rotax 9 Series Service IRMT

RV-12 Kit#729 "N312RD" is now a full functioning fun machine!! Thanks Van for fulfilling my dream😎
2017 Dues Paid

Last edited by Phantom30 : 06-13-2017 at 12:44 AM.
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  #7  
Old 06-12-2017, 09:46 PM
DaveWelch DaveWelch is offline
 
Join Date: Mar 2005
Location: Huntsville, AL
Posts: 273
Default 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
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RV12 N951DZ
First Flight 9/23/16!
No paint yet. Working on a scheme with John Deere colors or accents.
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  #8  
Old 06-12-2017, 09:46 PM
Phantom30's Avatar
Phantom30 Phantom30 is offline
 
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Location: Coeur d'Alene, ID/Casa Grande, AZ
Posts: 525
Default

Quote:
Originally Posted by rvbuilder2002 View Post
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.
__________________
Ric Dickison
307 (CAB) Phantom
Search and Destroy (Can
Tho RVN)
Distinguished Flying Cross Society Member
CH-47 & UH-1H "Driver"
Rotax 9 Series Service IRMT

RV-12 Kit#729 "N312RD" is now a full functioning fun machine!! Thanks Van for fulfilling my dream😎
2017 Dues Paid
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  #9  
Old 06-12-2017, 09:55 PM
Phantom30's Avatar
Phantom30 Phantom30 is offline
 
Join Date: Jul 2009
Location: Coeur d'Alene, ID/Casa Grande, AZ
Posts: 525
Default

Quote:
Originally Posted by DaveWelch View Post
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😳
__________________
Ric Dickison
307 (CAB) Phantom
Search and Destroy (Can
Tho RVN)
Distinguished Flying Cross Society Member
CH-47 & UH-1H "Driver"
Rotax 9 Series Service IRMT

RV-12 Kit#729 "N312RD" is now a full functioning fun machine!! Thanks Van for fulfilling my dream😎
2017 Dues Paid
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  #10  
Old 06-12-2017, 11:11 PM
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 6,577
Default

Quote:
Originally Posted by Phantom30 View Post
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......
__________________
Any opinions expressed in this message are my own and not necessarily those of my employer.

Scott McDaniels
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")

Last edited by rvbuilder2002 : 06-12-2017 at 11:13 PM.
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