912 Carb Balance

[ChrisBolkan 130]

Can someone tell me why Rotax chose to set up the 912 carbs the way they do? My prior experience with carb balancing was with the 582. Very simple to balance, the procedures make sense, and the carbs stay in balance.

 

Now I have this 912. Not flying it yet, but have helped a few freinds balance their carbs and what a PITA that is! Now I have one myself I am wondering if anyone can give me a good reason why not to set the carbs up to spring shut like the 582 and have the idle balance be set on the idle stops instead of on tensioned cables? What a mess the tension open scenario produces, and how easily they can get out of sync!

 

Has anyone converted a 912 to spring close and cable tension to open instead of the standard spring open and cable tension to close?

 

I figure the engineers who designed the engine know a LOT more than I do, but for the life of me I can't make any sense of why they would do it this way.

 

I have heard that it fails full throttle instead of idle. OK great. But If that is the only reason, I'd rather have it fail closed throttle.

 

What do I not understand here?

 

Thanks!

 

ChrisB

[Av8r3400 1179]

That is the exact reason, Chris.

 

If the fail was to close, that would mean and engine out crash every time a throttle cable failed.  Fail to open would mean a difficult landing, but one where power is available thus more survivable.  Liability.  Lawyers have a lot of power.  This is the same reason there are Bing carbs used instead of Mikuni carbs.  This is also the same reason it took Rotax several decades to finally market a fuel injection system.  

 

I know people who have put second springs on the arm to "neutralize" the return.  This is not smart IMO.  That makes your throttle cable a push-pull system.  Not good.

 

Certificated engines do not have a throttle return spring to either direction.  Their carburetors and controls are, as you know, a far different design, though.

[herman pahls 9]

Some where today I noticed that a Kitfox or Avid was back flying again after an incident in which a carb cable broke and that carb going to full throttle apparently did not help preventing damage to the airframe

The Bing on the Jabiru I flew for a year also had the spring set up to go full throttle if the cable failed but it only had one carb.

How does a 912 run when one carb is full idle or full throttle and the other one is still controlable with throttle?

Herman

[Av8r3400 1179]

I've never tried. Hopefully never will, either.

I would rather have an engine that runs at high power and gets worse as you pull power back, than an engine that idles fine but cannot make flight sustaining power...

[wypaul 448]

I have added the neutralizer spring to mine and would be interested in why you think that this is a bad idea.  My setup allows the carb springs to still maintain slight pull.  By the way AV8r3400 I didn't catch your first name.

 

I don't think that the engine would run good with 2 cylinders on one side doing the work.  My thought is to do what several others have done and buy the pricey split cable that ACS sells.  Others might sell this as well.

http://www.aircraft-spruce.com/search/search.php

 

I have the Jabiru 3300 on my Q-2 and the spring was removed so it works as a push/pull setup.  The throttle throw is very short and the movement of the throttle is short also, about an inch or so.  This could be increased but haven't seen a need to do so.  It now has over 600 hours and works great.  It has the standard friction aircraft cable.

 

 

Dang, I talked myself into ordering one today.

 

Doug I don't know why the Rotax and Jabiru are setup that way.  At least with the Jabiru it is a single carb so it won't shake the engine off the front of the airplane.  I wonder if Bing may require it or maybe the lawyers had a hand in it as was mentioned already.

 

Paul

[wypaul 448]

Good point on the full throttle AV8, saw your post after I posted above.

[Guest]

So if the reason is safety, that is that Rotax feels a full throttle failure is safer that an idle failure, why does the 582 have idle return springs instead of full throttle springs?

ChrisB

[dholly 718]

If my 582 went to full throttle, my KF-3 would still be in my garage instead of the parts bin!

[Guest]

Hi Paul,

 

I would think that adding a "neutralizer" spring would result in poorly balanced carbs throughout the throttle travel, because the cables are not in tension therefore would not be assured of positive throttle position relative to each other.

 

The way I see it both carb cables have to be tensioned whether tensioned to idle or tensioned to full throttle in order to maintain proper balance. Seem to me that operating the cables as push pull would produce slop and differences in throttle position of the carbs relative to each other, unless the neutralizer spring affected the mixing mechanisim as opposed to 1 neutralizing spring per carb. Maybe thats the way it works. I don't know. It's all new to me.

 

 Chris

[SuberAvid 617]

I decided the fail "on"  for the Bing 64's was not what I wanted and changed mine to fail "off" on my Subaru.  It just was not normal to me to have them work that way and it was easier to utilize the throttle cable linkage to pull the carbs on rather than off.

[wypaul 448]

Chris,

I will be the first to admit that I am new to the business of carb balancing so my views are open to change as I learn more about this. What the CarbMate indicated after balancing the carbs with the neutralizer spring in place is the balance remained in balance through the range of throttle movement with slight flicker to the right at one point. My guess is that the balance tube will handle that slight change. When I get a chance I will check with the Mate hooked to the primer lines and the balance tube in place to see what happens. One other note is that the spring does not neutralize the throttle springs totally but does reduce the pull substantially. The carbs will still go to full if one of the throttle cables going to the carbs fails. Will check to see what happens if the cable to the splitter fails by disconnecting it.

Paul

[Guest]

Hi Paul,

 

Each carb has a return spring. In the case of the 912 it pulls the carbs wide open. The way I see it is if the neutralizer spring(s) are either attached 1 per carb, or one spring affects the mixer which affects both carbs. I think if the neutralizer spring is set up one per carb it could be problematic. If there is just one spring that affects the mixer, it should be fine.

 

How is your neutralizer set up? Do you have a photo?

 

ChrisB

[wypaul 448]

I have one spring that is attached to the mixer. Next time I go to the hanger I will get a picture of the spring setup. One per carb would not work with the sort of cable arrangement the plane has. With the new cable it can be operated with springs in the idle return position, full throttle, or my preferred method of no spring at all.

Paul

Edited 10 Apr 2013 by Paul S

[Guest]

Hi Paul! A photo would be great. I would like no spring like the Facet carb and vernier on my Lycoming. As a second choice I would like return to idle. I do not like return to full throttle they way my 912 is set up, but I suppose I will be getting used to it.

P.S. Sorry about the underlining. Finally figure out how to stop it.

ChrisB

[wypaul 448]

Hey Chris, finally made  it to the hanger today and have that picture.

 

Paul