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Rotax 912 Idle Speed Setting
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hgmckay

Joined: 23 May 2006
Posts: 397

Posted: Fri May 04, 2007 6:11 pm Post subject: Rotax 912 Idle Speed Setting

On a new Rotax 912 engine has the idle speed adjustment screw (throttle arm idle screw) on the carbs. been set by Rotax at the factory to limit the idle rpm to the number given in the Operators Manual (1400+/-), or does this have to be set by the engine installer as described in the Rotax Line Maintenance Manual, Section 4.1.2, page 18? Based on my understanding the 912 UL engine will not start unless the throttle is completely closed, and choke open (Cold Engine). If the throttle arm idle screw is resting on the carb. Idle stop as described on page 19 of the Maintenance Manual, the throttle is not completely closed, so how can the engine start in this condition (i.e. the completely closed throttle position is not the idle speed position)?   

<![if !supportEmptyParas]> <![endif]>  

I have done nothing to these carb. adjustments and my engine can be throttled back below 1400 rpm (i.e. it appears that the throttle arm idle screw is not in the correct position). I need some help with this matter. How do you reconcile a completely closed throttle to start an engine with a throttle arm idle screw set for 1400 rpm (i.e. throttle partially open)? Help!!!  

<![if !supportEmptyParas]> <![endif]>  

Hugh McKay in NC  

Allegro 2000  

Rotax 912 UL  

N661WW  

        [quote][b]

gfmjr_20(at)hotmail.com
Guest

Posted: Fri May 04, 2007 6:36 pm Post subject: Rotax 912 Idle Speed Setting

Hugh--

   With the throttle closed and the choke open for starting, the idle set 

screws WILL hold the throttle valves open just a bit----in your case 1400. 

Typically this has been roughly set at the factory, however, they still need 

to be set per the mechanical synchronizing as described in the Rotax manual. 

  This mechanical synchronizing can then be followed up utilizing gauges .

Based on your description of the current condition of your setting, your 

engine should start with no problem. you can then check for synchronizing.



George May

601XL  912s---70 hours

 	  Quote: 	
		  From: "Hugh McKay III" <hgmckay(at)bellsouth.net>

Reply-To: rotaxengines-list(at)matronics.com

To: <rotaxengines-list(at)matronics.com>

Subject: Rotax 912 Idle Speed Setting

Date: Fri, 4 May 2007 22:09:17 -0400



On a new Rotax 912 engine has the idle speed adjustment screw (throttle arm

idle screw) on the carbs. been set by Rotax at the factory to limit the 

idle

rpm to the number given in the Operators Manual (1400+/-), or does this 

have

to be set by the engine installer as described in the Rotax Line 

Maintenance

Manual, Section 4.1.2, page 18? Based on my understanding the 912 UL engine

will not start unless the throttle is completely closed, and choke open

(Cold Engine). If the throttle arm idle screw is resting on the carb. Idle

stop as described on page 19 of the Maintenance Manual, the throttle is not

completely closed, so how can the engine start in this condition (i.e. the

completely closed throttle position is not the idle speed position)?



I have done nothing to these carb. adjustments and my engine can be

throttled back below 1400 rpm (i.e. it appears that the throttle arm idle

screw is not in the correct position). I need some help with this matter.

How do you reconcile a completely closed throttle to start an engine with a

throttle arm idle screw set for 1400 rpm (i.e. throttle partially open)?

Help!!!



Hugh McKay in NC

Allegro 2000

Rotax 912 UL

N661WW


	


_________________________________________________________________

Download Messenger. Join the i’m Initiative. Help make a difference today. 

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Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Sat May 05, 2007 8:01 am Post subject: Re: Rotax 912 Idle Speed Setting

Hi Hugh,



With a brand new engine you will need to set the idle rpm. 1400 rpm is way too low. It should idle at approximately 1800rpm. 1400 will cause excessive vibration to the gearbox and eventually cause you clutch to start slipping. This is taught in all the Rotax classes. You can download the "Light Maint" and the "Heavy Maint" manuals on the Rotax/Kodiak web site.  It is listed under "tech info". They explain how to do it.

It is not hard. It is very important that your carb run in sync. You don't want the right side running at 4800 rpm and the left side running at 4900 rpm. They are then fighting and opposing each other. The manual sync will get you in the ballpark. The pneumatic sync will put you dead on for idle and at full rpm sync. Some people aren't very good at keeping up with this, but it is important. Yes your engine will run without doing this, but you paid a lot of money for a fine engine. You should treat it as such and you want it to last and be healthy. 



Go to the manuals and they will explain and/or you can call me and I will explain the process and walk you through it.



Roger Lee

Tucson, Az.

520-574-1080 home

520-791-5286 for work May 5th. up until 7pm tonight.



p.s.

you will need aset of vacumm gages. easy and cheap to make.

I think there is a picture here on the forum.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Sat May 05, 2007 8:05 am Post subject: Re: Rotax 912 Idle Speed Setting

Hi Hugh,



Carb sync tool picture

carb_sync_150[1].jpg

Description:

Filesize:

120.66 KB

Viewed:

1603 Time(s)

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

hgmckay

Joined: 23 May 2006
Posts: 397

Posted: Sat May 05, 2007 10:19 am Post subject: Rotax 912 Idle Speed Setting

Roger, Thom, George:



Many thanks for the advice. I suspected exactly what you have described. I

have the Rotax "Line Maintenance" Manual as well as the vacuum gauge kit

(sold by Lockwood aviation), and will perform the mechanical and pneumatic

carb. synchronization per the Rotax Maint. Manual instructions with all of

your advice taken into consideration. Many thanks again!



Hugh McKay



--

jkuehn(at)mountaintime.my
Guest

Posted: Sat May 05, 2007 12:03 pm Post subject: Rotax 912 Idle Speed Setting

As far as balancing the carbs, Phil Lockwood wrote two articles in the 

EAA light sport magazine (or whatever the titles was/is) on the topic 

that is much easier to follow and contains more practical information 

than the Rotax Manual.  Both together comprise a really complete 

manual.  I photocopied this series of articles and have them with my 

engine documentation together with all the service notes and updates 

from Rotax that apply to my enging.  This has been covered before in 

this forum and you can probably look up the dates publication of the 

articles.  I don't have it right here!



Jack



Hugh McKay III wrote:

[quote] 



 Roger, Thom, George:



 Many thanks for the advice. I suspected exactly what you have described. I

 have the Rotax "Line Maintenance" Manual as well as the vacuum gauge kit

 (sold by Lockwood aviation), and will perform the mechanical and pneumatic

 carb. synchronization per the Rotax Maint. Manual instructions with all of

 your advice taken into consideration. Many thanks again!



 Hugh McKay



 --

ami(at)MCFADYEAN.FREESERV
Guest

Posted: Sat May 05, 2007 12:47 pm Post subject: Rotax 912 Idle Speed Setting

Has anyone tried the Twinmax  electronic carb balancer (which electronically 

senses differential pressure between the two intake manifolds) and how does 

it compare to the mechanical tube-type instruments?



Duncan McF.

---

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Sat May 05, 2007 1:57 pm Post subject: Re: Rotax 912 Idle Speed Setting

I don't particularlly like the tube type because you have to keep them oriented up and should be verticle and not leaned over. I like the dial type like in the picture. These can be any orientation and are easy to read. These are just gages you can pick up at an auto parts or online. The gages depending on how much you want to spend are $19-$30. Mine are liquid filled. The rest of the gage parts set is something you can pick up for about $10-$12 at your local hardware store. You do not need any special vacumm gages to do this carb sync. Something easy to use and read is the best. 



Once you do a carb balance you will find it quite easy from then on. Just remember that the gage with the highest vacumm is the carb getting the least fuel. If you need more rpm adjust the carb with the most vacumm which gives it more fuel and the vacumm will reduce (i.e. vacumm is 12, adjust the carb to get more fuel and the vacumm goes down let's say to 10). If you need to decrease rpm then pick the carb with the least vacumm and raise its vacumm number by reducing the amount of fuel it gets. These adjustments are done by turning the Bowden cable. That's the main throttle cable that comes into the carb when you use your throttle. You need to use the idle stop screw and the vacumm gages to make sure your idle is set at it's correct rpm and the carbs are equal when they are at idle. 



This doesn't tell you how to do it, but maybe a little insite as to what is going on when you look at the gages or your trying to set the idle when the gages are connected.



Again if you have any questions call me anytime.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

scrimm

Joined: 09 Jan 2006
Posts: 81
Location: Gilbert, SC SC99

Posted: Sat May 05, 2007 3:02 pm Post subject: Rotax 912 Idle Speed Setting

Is there anyone on the forum in the Atlanta area that can do the idle and

carb sync?



I am in need of both.



Thanks,



Steve Crimm

--

_________________
Steve Crimm
N42AH

davestapa

Joined: 13 Mar 2007
Posts: 30
Location: Woodstock, GA

Posted: Sat May 05, 2007 4:36 pm Post subject: Rotax 912 Idle Speed Setting

Steve, where in Atlanta are you? I live in Woodstock, but plane is based

in Calhoun. This morning we balanced carbs on a Zenith 701 with a 912ULS.

We balanced carbs on my Kitfox Series 5, 912ULS a couple of months ago. I

have the gauages. 



C. David Estapa 

Woodstock, GA

Kitfox 5, N97DE, 912ULS



Do not archive



On Sat, 5 May 2007 19:01:31 -0400 "Steve Crimm"

<steve.crimm(at)stephenscott.com> writes:

[quote] 

 <steve.crimm(at)stephenscott.com>

 

 Is there anyone on the forum in the Atlanta area that can do the 

 idle and

 carb sync?

 

 I am in need of both.

 

 Thanks,

 

 Steve Crimm

 

 

 --

_________________
C. David Estapa
Woodstock, GA
S5TD N97DE

rampil

Joined: 04 May 2007
Posts: 870

Posted: Sat May 05, 2007 5:08 pm Post subject: Re: Rotax 912 Idle Speed Setting

Just to re-iterate what Roger said.



Idle speed should be above 1800 if you hope to preserve the function

and integrity of your gearbox.  Low RPM apparently allows a sort of

torsional resonance in the dog gears which will chew them up in short

order



Ira

_________________
Ira N224XS

Thom Riddle

Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

Posted: Sun May 06, 2007 3:54 am Post subject: Rotax 912 Idle Speed Setting

Hugh,



A few more points, some different from what others have said, based on 

personal experience synching carbs on our Allegro w/ 912UL and on a 

customer's Diamond Katana w/ 912F, also 80 hp.

1)	The idle can be made smooth in the 1400-1500 rpm range.

2)	Since the Allegro is such a great glider you will have difficulty 

getting the airplane to come down on final with the idle speed set at 

1800 rpm.

3)	Idle above 1800 rpm CAN make the starting carburetor (what some 

erroneously call choke) not function properly or at all.

4)	With the balance tube disconnected during the pneumatic synch 

process, pay attention only to the vaccuum readings and the idle speed. 

The engine will seem rough even when the numbers are perfect but will 

smooth out when the balance tube is reconnected. Rotax schools don't 

always tell you that. Just make the numbers right and it will be silky 

smooth with the balance tube reconnected.



Thom Riddle

_________________
Thom Riddle
Buffalo, NY (9G0)

Don't worry about old age... it doesn't last very long.
- Anonymous

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Sun May 06, 2007 6:04 am Post subject: Re: Rotax 912 Idle Speed Setting

Hi Thom,

I mean no malice here, but only to debate some points.

The 80 hp v.s. the 100 hp engines are different. The compression is lower in the 80 hp. You might get away with a little lower rpm? All my training and experience is with the 100 hp.



My Flight Design CT has a glide of 14:1 and at 1800 rpm landings and losing altitude are not a problem. Setting your rpm down at 1400-1500 makes the gearbox take a beating form our higher compression and porp inertia. If the engine feels rough then it is rough. Both my Rotax instructors said not to run them down this low. Your engine should run as smooth without the crossover as with if the carbs are balanced exactlly. The crossover tries to keep equal pressure between them during all running conditions. I have hooked up my gages with the crossover connected. The lower the rpm the crossover does the least and they will stay out of balance just not as far. The higher I ran my rpm the more the crossover helped. It made them close, but still out of balance some. If you balanced the carbs well you could fly with out the crossover. After time the carbs slowly can get out of balance, usually not much, but still out. The crossover was set up to help combat the out of balance carb and to help under normal conditions to maintain equality between the two. These are constant pressure carbs and that's why the crossover is so benifical.



The so called choke only adds more fuel for start up. It does not change anything else.



The rough running will beat up your tranny. Kind of like smoking, won't get you today, but the future is bad.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

pilot(at)ip6.com
Guest

Posted: Sun May 06, 2007 11:31 am Post subject: Rotax 912 Idle Speed Setting

I have given up with that tool you show in the picture and bought a



Carbtune 2-col and Toolpouch



http://www.carbtune.com/carbdtls.html.



Much superior.

As a matter of fact I have a brand new tool from Lockwood that I want to

sell 

If you are interested contact me.

-- Silvano



Roger Lee wrote:

 	  Quote: 	
		   



 Hi Hugh,



 Carb sync tool picture



 --------

 Roger Lee

 Tucson, Az.





 Read this topic online here:



 http://forums.matronics.com/viewtopic.php?p=110993#110993





 Attachments: 



 http://forums.matronics.com//files/carb_sync_1501_110.jpg

scrimm

Joined: 09 Jan 2006
Posts: 81
Location: Gilbert, SC SC99

Posted: Sun May 06, 2007 12:12 pm Post subject: Rotax 912 Idle Speed Setting

David,



I am in Roswell



Steve 



--

_________________
Steve Crimm
N42AH

Thom Riddle

Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

Posted: Mon May 07, 2007 3:44 am Post subject: Rotax 912 Idle Speed Setting

Roger,



Nor am I arguing, merely discussing what I know from personal 

experience.

1)	I mentioned 80 hp engines because I know the difference in 

compression ratios between the 100 and 80 hp does make the gearbox 

abuse worse if the engine is not idling smoothly. Also because it is my 

understanding that Hugh's Allegro is also running the 912UL.



2)	The CT has much more effective flaps than the Allegro, is about 100 

lbs heavier than the Allegro empty and thus the behavior comparisons on 

final are not really valid. This behavior also depends somewhat on the 

pitch of the prop. What I said about behavior on final approach in an 

ALLEGRO, I said from personal experience in an airplane essentially 

identical to Hugh's.



3)	I've never seen a Rotax 912 series engine idle REALLY smoothly 

without the balance tube connected. This includes the CT with Rotax 

912ULS at Lockwood being balanced by the instructor Dean Vogel on April 

14th of this year. It includes our Allegro w/ 912UL. It includes the 

Diamond Katana w/ 912F. It includes my old Titan Tornado with  912UL. 

All of the above with vacuum gages exactly matched and idle at 1800 

rpm. Incidentally, when setting the idle in the 1400-1500 rpm range and 

the carbs are synchronized properly there is no difference in the 

smoothness compared to 1800, at least not on the 80 hp engines.  

Roughness is the thing we are trying to avoid. If it idles smoothly at 

1450 rpm then there is no damage being done to the gearbox. At 

Lockwood, Dean set the idle on the CT at 1800 rpm so I don't know how 

it would have been at 1500 rpm or lower. I do know for absolute 

certainty that the gages were reading identically, the rpm was 1800 and 

without the balance tube the engine was running rougher than when the 

balance tube was reconnected. The balance tube is an integral part of 

the intake system, not a nice to have option. If it was not needed it 

would not be there. Unless one has a dynamic balancer at hand, the 

degree of roughness we are talking about is a subjective thing. Your 

idea of smooth running may be different than mine. Unless we can 

quantify our subjective assessments, then were are not going to get any 

further in this part of the discussion.



4)	In THEORY, the starting carburetor does not work much above 1800 rpm 

because the vacuum at the starting carb fuel exit port is insufficient 

to draw fuel into the intake manifold. That is the reason not to set 

idle above this rpm. It can cause difficult or impossible starting when 

cold. Have you ever activated the starting carb IN FLIGHT with the 

engine running at various rpm? If not you may be in for a surprise. Try 

it at varying rpm from 3500 - 5500 and see what happens; you might 

learn something. Hint: The response is different at different rpm.



One last question for you, Roger:

Why does the Rotax factory say the acceptable range in idle rpm is 

between 1400-1800 if anything under 1800 rpm causes damage? Does your 

instructor know more about Rotax engines than Rotax?



Thom in Buffalo

FAA Powerplant Mechanic since 1972

Rotax afficionado for only six years and still learning

Retired mechanical engineer and machine designer

_________________
Thom Riddle
Buffalo, NY (9G0)

Don't worry about old age... it doesn't last very long.
- Anonymous

h&jeuropa

Joined: 07 Nov 2006
Posts: 645

Posted: Mon May 07, 2007 4:23 am Post subject: Re: Rotax 912 Idle Speed Setting

Duncan,



We use the Twinmax and it works just fine.  



Jim & Heather

Europa XS

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Mon May 07, 2007 6:38 am Post subject: Re: Rotax 912 Idle Speed Setting

Hi Thom,



My instructor was Eric Tucker, he is Rotax's main man for engines. He is the top dog for Kodiak out of the Bahamas. If you want the straight scoop on anything he's the guy to ask. I have found a number of Rotax dealers who don't know what they are talking about and you just have to shake your head at some of the answers.

I agree that the crossover is necessary. There is a difference because of the way the 80hp is made v.s. the 100 hp., that's why the difference in idle rpm's for the 1800rpm recommendation. Try and set a 912uls at 1400-1500rpm and watch it shake. The 80hp is quite different. It is taught in all 912uls classes and taught by Rotax/Eric Tucker. It has not just been bored and stroked as many think. Your engine should be almost as smooth when balanced right without the crossover as with the crossover. Over the last 5 years I have balanced 912uls's dozens of times and I'm very anal about making the sync perfect. 

I would recommend the 3 day Rotax 912uls class to anyone that owns a 912uls. It was a very informative class and an eye opener. The class is offered around the US and it cost $300. What I learned was that some of these long time 912uls owners and A&P's didn't really know what they were talking about when I went to some of the Fly-In's or called a dealer for info because they had never had any formal training. There attitude was if it didn't break right away it must be right. 

You ask why the Rotax factory says 1400-1800rpm? Because of the many different applications, prop inertia, mountings, ect.. Rotax has changed a number of things over the last 5 years because they finially get enough engine hours from field use to understand why it would be better done a different way. They even teach some differences in class v.s. the manual. When I was in class I had 2 pages of changes to make in my manual to bring it up to the newer way of thinking from Rotax. Look at all the service/AD bulletins over the last several years. Rotax makes changes all the time, some we never know about because we don't need to make the change in the field they just decided there was a better way.



All the so called "choke" does on the 912uls is a fuel bypass. It introduces more fuel at start-up.



Bottom line I have learned with any engine be it auto, ATV, motorcycle, ect, people are going to do what they think is right or was told to by another person. I have learned that has a pretty big error factor. That's why I went an took a couple of the Rotax classes so I would know the difference.



Thom,



Please do not take any offence as we are only chatting and having a good open debate. I respect all your opinons and ideas.



Have a good day.



Roger Lee

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

hdwysong(at)gmail.com
Guest

Posted: Mon May 07, 2007 11:41 pm Post subject: Rotax 912 Idle Speed Setting

Howdy Thom and Roger,



Just wanted to say "Thanks!", guys.  If you two were having 

this dialog in a hangar somewhere I'd be one of many folks 

crowded within earshot, scribbling notes as fast as possible!



We FINALLY convinced our 914 to run like it should ONLY by 

listening to the most knowledgeable folks we could find.



Some experts preached from the manuals, others preached from 

the classes/forums, others preached from their experience. 

It was ALL word of mouth, and we found all of our answer(s) 

buried in the overlap.  Now, we're "experts", too.



So... PLEASE keep preaching!  You're providing more answers 

than you realize.



D

DO NOT ARCHIVE

Thom Riddle

Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

Posted: Tue May 08, 2007 2:45 am Post subject: Rotax 912 Idle Speed Setting

Roger,



No offense taken. As we both said, just discussing the subject from our 

own experiences.



Thom in Buffalo

do not archive

_________________
Thom Riddle
Buffalo, NY (9G0)

Don't worry about old age... it doesn't last very long.
- Anonymous

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