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Mallard

Joined: 22 Oct 2019
Posts: 49
Location: Ireland

Posted: Tue Sep 14, 2021 1:53 pm Post subject: Carb Balancing

Has anyone had the experience of balancing the carbs on a Rotax 912 type engine and after a couple of hours of smooth running across all revs you suddenly get rough running at 3500 rpm.



If you experienced a similar situation, how did you deal with it?



I'd also be interested to hear from any 914 owners on the forum who have had the above 'Carb Balancing' problems & what was done to keep the Carbs Balanced?

Thanks for any contributions.

James

_________________
Seat of my pants

Area-51

Joined: 03 May 2021
Posts: 391

Posted: Tue Sep 14, 2021 11:44 pm Post subject: Re: Carb Balancing

What input data at 3500rpm makes you think its a carburettor related issue?

davidjoyce(at)doctors.org
Guest

Posted: Wed Sep 15, 2021 1:29 am Post subject: Carb Balancing

I would look seriously at the possibility that you have air leaking into the carburettor bowl. This now seems to be a fairly common issue with older 914s that still have the old paper gaskets. Typically folk spend months and a small fortune trying to work out what is happening before replacing the gaskets with the new Rotax rubber ones and &lsquo;living happily ever after!&rsquo;  Tends to produce very rough running at 3500+ rpm and reluctance to produce any greater revs - very disturbing if taking off from a short runway! Look out the article in Europa Flyer a year or two ago by Jim Butcher (assuming you are in the club it is available and searchable on the website). David Joyce, GXSDJ  

 

 

On 2021-09-14 22:53, Mallard wrote:  	  Quote: 	
		   --> Europa-List message posted by: "Mallard" <james(at)kingdom.ie (james(at)kingdom.ie)>Has anyone had the experience of balancing the carbs on a Rotax 912 type engine and after a couple of hours of smooth running across all revs you suddenly get rough running at 3500 rpm.If you experienced a similar situation, how did you deal with it?I'd also be interested to hear from any 914 owners on the forum who have had the above 'Carb Balancing' problems & what was done to keep the Carbs Balanced?Thanks for any contributions.James--------Seat of my pantsRead this topic online here:http://forums.matronics.com/viewtopic.php?p=503143#5bsp;--> http://www.matronics.com/Navigator=            &nbspf="http://forums.matronics.com" target="_blank" rel="noopener noreferf="http://wiki.matronics.com" target="_blank" rel="noopener noreferre========================http://www.matronics=====================

budyerly@msn.com

Joined: 05 Oct 2019
Posts: 286
Location: Florida USA

Posted: Wed Sep 15, 2021 2:13 am Post subject: Re: Carb Balancing

Attached is a Rotax Troubleshooting guide that was updated this year.



Most likely if smooth at idle with good carb balance you may have any of a few other issues.



Spend time on the Rotax Owners.com and the videos.  Rotax rebuild and tuning info is there.  My article is long and boring also but may shed some insight.



Best Regards,

Bud Yerly

Rotax Engine Troubleshooting Yerly April 2021.pdf

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h&jeuropa

Joined: 07 Nov 2006
Posts: 645

Posted: Wed Sep 15, 2021 3:18 am Post subject: Re: Carb Balancing

Bud's troubleshooting guide is also available on the Europa Club website.  You can use the search function on any page on the website to find information related to this problem.



Jim Butcher

Gilles

Joined: 16 May 2020
Posts: 33

Posted: Wed Sep 15, 2021 3:23 am Post subject: Carb Balancing

Le 14/09/2021 Ã  23:53, Mallard a       Ã©critÂ :

     

      	  Quote: 	
		          	  Quote: 	
		  --> Europa-List message posted by: "Mallard" <james(at)kingdom.ie> (james(at)kingdom.ie)



Has anyone had the experience of balancing the carbs on a Rotax 912 type engine and after a couple of hours of smooth running across all revs you suddenly get rough running at 3500 rpm.



If you experienced a similar situation, how did you deal with it?



I'd also be interested to hear from any 914 owners on the forum who have had the above 'Carb Balancing' problems & what was done to keep the Carbs Balanced?



	
     	
     



          

Hi all,     

Here is some info concerning our 914 UL and some other engines.     

We never had any carb synch issues, but     

1) We never disconnected the cables from the throttle levers.     

2) We have a mechanical stop on the throttle lever so as to never       exert any force to the carb lever stops. It is our experience that       many buddies and hangar mates complaining about carb losing synch       invariably happen to not have a stop on the lever, and so are       likely to distort the carb lever stops.     

Summarized some info here :       http://contrails.free.fr/engine_rotax_stops.php     

Among the carb issues we heard of, are the change in float bowl       gasket thickness. An engine rough running was driving a buddy       mechanic mad, until we discussed carb gaskets, and it appeared       that some Rotax dealer-supplied gaskets were too thin, raising the       bottom of the bowl too close to the main jet and impairing the       flow into the main jet.     

FWIW,     



          -- 

       Best regards

       Gilles

       http://contrails.free.fr

       http://lapierre.skunkworks.free.fr

_________________
Best regards,
Gilles
http://contrails.free.fr

wdaniell.longport(at)gmai
Guest

Posted: Wed Sep 15, 2021 4:57 am Post subject: Carb Balancing

I agree with David - I had air being sucked into my bowls but only above 11500.Â  Â  Nitrile is the way to go.

William Daniell



LONGPORT



+1 786 878 0246

On Wed, Sep 15, 2021 at 5:32 AM <davidjoyce(at)doctors.org.uk (davidjoyce(at)doctors.org.uk)> wrote:



 	  Quote: 	
		   

I would look seriously at the possibility that you have air leaking into the carburettor bowl. This now seems to be a fairly common issue with older 914s that still have the old paper gaskets. Typically folk spend months and a small fortune trying to work out what is happening before replacing the gaskets with the new Rotax rubber ones and â€˜living happily ever after!â€™ Â Tends to produce very rough running at 3500+ rpm and reluctance to produce any greater revs - very disturbing if taking off from a short runway! Look out the article in Europa Flyer a year or two ago by Jim Butcher (assuming you are in the club it is available and searchable on the website). David Joyce, GXSDJ Â 

 



 

On 2021-09-14 22:53, Mallard wrote:  	  Quote: 	
		   --> Europa-List message posted by: "Mallard" <james(at)kingdom.ie (james(at)kingdom.ie)>



Has anyone had the experience of balancing the carbs on a Rotax 912 type engine and after a couple of hours of smooth running across all revs you suddenly get rough running at 3500 rpm.



If you experienced a similar situation, how did you deal with it?



I'd also be interested to hear from any 914 owners on the forum who have had the above 'Carb Balancing' problems & what was done to keep the Carbs Balanced?

Thanks for any contributions.

James



--------

Seat of my pants





Read this topic online here:



http://forums.matronics.com/viewtopic.php?p=503143#5bsp;--> http://www.matronics.com/Navigator= Â Â Â Â Â Â Â Â Â Â Â &nbspf="http://forums.matronics.com" target="_blank" rel="noopener noreferf="http://wiki.matronics.com" target="_blank" rel="noopener noreferre========================http://www.matronics=====================

Duncan McFadyean

Joined: 18 Jan 2011
Posts: 219

Posted: Wed Sep 15, 2021 7:29 am Post subject: Carb Balancing

Or Viton.    	  Quote: 	
		      On 15 September 2021 at 13:57 William Daniell <wdaniell.longport(at)gmail.com> wrote:     

     

               I agree with David - I had air being sucked into my bowls but only above 11500.    Nitrile is the way to go.     

                                                                                 William Daniell              

            

LONGPORT           

          

                     +1 786 878 0246          

         

        

       

      

     

     

    

    

               On Wed, Sep 15, 2021 at 5:32 AM <davidjoyce(at)doctors.org.uk (davidjoyce(at)doctors.org.uk)> wrote:       

     

      	  Quote: 	
		               

I would look seriously at the possibility that you have air leaking into the carburettor bowl. This now seems to be a fairly common issue with older 914s that still have the old paper gaskets. Typically folk spend months and a small fortune trying to work out what is happening before replacing the gaskets with the new Rotax rubber ones and â€˜living happily ever after!â€™  Tends to produce very rough running at 3500+ rpm and reluctance to produce any greater revs - very disturbing if taking off from a short runway! Look out the article in Europa Flyer a year or two ago by Jim Butcher (assuming you are in the club it is available and searchable on the website). David Joyce, GXSDJ        



       

On 2021-09-14 22:53, Mallard wrote:        	  Quote: 	
		                   --> Europa-List message posted by: "Mallard" <james(at)kingdom.ie (james(at)kingdom.ie)>          

          

Has anyone had the experience of balancing the carbs on a Rotax 912 type engine and after a couple of hours of smooth running across all revs you suddenly get rough running at 3500 rpm.          

          

If you experienced a similar situation, how did you deal with it?          

          

I'd also be interested to hear from any 914 owners on the forum who have had the above 'Carb Balancing' problems & what was done to keep the Carbs Balanced?          

          

          

Thanks for any contributions.          

James          

          

--------          

Seat of my pants          

          

          

          

          

Read this topic online here:          

          

http://forums.matronics.com/viewtopic.php?p=503143#5bsp;--> http://www.matronics.com/Navigator=            &nbspf="http://forums.matronics.com" target="_blank" rel="noopener noreferf="http://wiki.matronics.com" target="_blank" rel="noopener noreferre========================http://www.matronics=====================

Mallard

Joined: 22 Oct 2019
Posts: 49
Location: Ireland

Posted: Wed Sep 15, 2021 11:02 am Post subject: Re: Carb Balancing

Thanks for your assistance David but fortunately the problem isn't as bad as that, at least not yet! The engine is producing normal power all the way from idle up to & including turbo boost. It just runs a bit rough (at) 3500rpm. We didn't notice it while flying & only spotted it while sitting on the ground & holding the throttle steady (at) 3500rpm. 



Btw, during the last annual inspection the Carb floats were changed by my engineer using the Rotax supplied (foc) kit which I'm fairly sure included the Rubber gaskets. 



Also,

When we balanced the carbs we stopped all the vibrations which were present pretty much throughout the full rpm range. The vibrations were so bad it initially felt like the prop was out of balance.



Regards 

James 

 	  davidjoyce(at)doctors.org wrote: 	
		  I would look seriously at the possibility that you have air leaking into the carburettor bowl. This now seems to be a fairly common issue with older 914s that still have the old paper gaskets. Typically folk spend months and a small fortune trying to work out what is happening before replacing the gaskets with the new Rotax rubber ones and &lsquo;living happily ever after!&rsquo;  Tends to produce very rough running at 3500+ rpm and reluctance to produce any greater revs - very disturbing if taking off from a short runway! Look out the article in Europa Flyer a year or two ago by Jim Butcher (assuming you are in the club it is available and searchable on the website). David Joyce, GXSDJ  

 

 

On 2021-09-14 22:53, Mallard wrote:  	  Quote: 	
		   --> Europa-List message posted by: "Mallard" <james>Has anyone had the experience of balancing the carbs on a Rotax 912 type engine and after a couple of hours of smooth running across all revs you suddenly get rough running at 3500 rpm.If you experienced a similar situation, how did you deal with it?I'd also be interested to hear from any 914 owners on the forum who have had the above 'Carb Balancing' problems & what was done to keep the Carbs Balanced?Thanks for any contributions.James--------Seat of my pantsRead this topic online here:http://forums.matronics.com/viewtopic.php?p=503143#5bsp;--> http://www.matronics.com/Navigator=            &nbspf="http://forums.matronics.com" target="_blank" rel="noopener noreferf="http://wiki.matronics.com" target="_blank" rel="noopener noreferre========================http://www.matronics=====================

_________________
Seat of my pants

Area-51

Joined: 03 May 2021
Posts: 391

Posted: Sun Sep 19, 2021 4:55 am Post subject: Re: Carb Balancing

Thanks for the pdf write up Bud; itâ€™s very thorough informative and extensive, and yes will save many going through all those experiences to find a solution...



On the issue of this thread, have you used a dual meter to observe left/right manifold pressure readings through operating rpm range and while the vibration at 3500rpm is active, flickering needle on either gauge will indicate sticking valve; thereâ€™s also small chance one of the choke valves is not fully returning to correct position, or a carburettor piston may be binding intermittently on the slide... then again it may not be fuel/carbi related.. if you have access to two verified good carbis then swap them out and see if the issue disappears... will be interesting to see what it ends up being !!

budyerly@msn.com

Joined: 05 Oct 2019
Posts: 286
Location: Florida USA

Posted: Mon Sep 20, 2021 3:36 am Post subject: Carb Balancing

Sorry I missed your post,

You know, in my experience I've never had a sticking valve.  Good point.  In our automotive troubleshooting that was always a well documented issue, but I've just never had one in my Rotax.  However, you bring up something that is really important.  If the valve on the dual manometer gauges is wide open, the needles swing just like a sticking valve due to the valve timing and short intake.  Great point.  To use the Rotax balancing setup, one has to close the valves to smooth out the needle movement to get anything close to accurate.  John Hurst had an old dual manifold gauge from a multiengine plane he used for balancing and it worked perfectly without any valve because of the small orifice in the gauge itself for smoothing.



The choke valve is another issue I've not had because I made sure I had properly adjusted my linkage cables.   I've also never had a leaking starting carb gasket.  I did find out from testing the carbs run lean above about 4000 if the choke cables are pulled.  

 

I know that the pistons in the Bings, when out of sync, will be a bugger to find with only a vacuum gauge set.  I went nuts trying to figure out my slight roughness problem about 15 years ago.  I made up that plumbers piece to test and finally found the issue in my plane.  I tried cleaning up the walls and pistons, new diaphragms, and such to no avail.   Lockwood advised that I needed two new carbs.  Later when talking to the Rotax factory reps, I was advised that they actually match their sets of Bing carbs to ensure this was not a problem with a new engine.  I broke down and bought two new matched carbs and it has been fine ever since.  I know a couple of owners that only changed one carb and it balanced fine.  Were they lucky, or is there a wear issue in the Bings we don't know about?  



I hope one day one of you guys stumbles on how to repair/adjust a set of carbs that fail to work in unison because of bad pistons.  Changing out diaphragms and gaskets, cleaning and smoothing didn't work for me.   Is it geometry, weight, passages, clearances or what is the actual cause, and what solution is needed to ensure the carb pistons work in unison has surely escaped me.  I never did figure it out.  I threw money at it after nearly six months of tinkering and my two new carbs worked on the bench in unison and were smooth running.  



All one can do is check the compressions are all consistent so you know the mechanical is sound, then on to the carbs, to ensure they are rebuilt and work in unison and then on to the ignition.



You know, it is always the last thing you check that finds the issue.  Or you can do the car dealer repair method, just change components until the engine runs right and charge the customer.



Thank you for that,

Bud Yerly 



--

Area-51

Joined: 03 May 2021
Posts: 391

Posted: Mon Sep 20, 2021 11:43 am Post subject: Re: Carb Balancing

I hear you there Bud... getting the dual vacum gauge to function may require blocking off the balance pipe between left/right manifolds, otherwise backpressure may interfere with true gauge reading; inline restrictor would dampen needle movement.



The bing carbi is not too different to the stromberg; age/grime/carbon blowing back into the inlet manifold from the combustion cylinder will eventually result in wear between carbi piston and its bore; so over time there would be slight discrepancies between operational height at various rpm due to slight vacum differences ; so the more carburettors used the greater the opportunity toward variation in mixture; and the way each carbi is balanced off against the others is by raising each piston manually and observing changes at idle or set rpm (as with SU or Stromberg tuning)... It doesnâ€™t take much to setup a minor harmonic vibration. This is why a dellorto can offer superior performance; from dellorto its onto the world of mechanical or electronic injection and all have their endemic nuances.



Another usable diagnostic is monitoring all individual exhaust headers at tuned lengths from the exhaust valve; compare and contrast...



Personally, to get multiple constant depression carbis to run in perfect unison, other than buying s new matched pair, everything needs to be set up equaly on a bench down to .0005â€ accuracy, all pistons need to be checked and equally balanced, and then piston heights need to be equally set at a uniform air flow, and then it all needs to be setup for smooth idling... It can literally take weeks; and you might get 200, or 2000, hours of joy... somewhere you just need to draw the line at Cost/Time/Benefit. After 40 years making internal combustion engines run smooth first thing I would do is throw both carbis in the bin, unless you are a diehard purist, and fit good quality injection.



All things equal the rotax is a really really sweet powerplant which has left me very very impressed by... as is, could probably benefit by having an O2 sensor and readout retro fitted for inflight diagnostic.



And Bud is right there; get your fuel supply done and move onto ignition ðŸ‘

rparigoris

Joined: 24 Nov 2009
Posts: 792

Posted: Mon Sep 20, 2021 12:44 pm Post subject: Re: Carb Balancing

Hi Area 51 I added a mixture monitor with O2 sensor on 914 with a mixture control. Not yet flying:

https://1drv.ms/u/s!Ah1S270Nwg9V5VC2OuaYiCuGYBtz

https://1drv.ms/u/s!Ah1S270Nwg9V5WTC-lrqsfXAr8mm

Dial controls a needle valve that leaks float bowl pressure to balance tube. Rich switch energizes enrichment solenoid and takes my leaning circuit out of picture, it has a diode to isolate the TCU from getting back fed.

Ron P.

Area-51

Joined: 03 May 2021
Posts: 391

Posted: Mon Sep 20, 2021 4:55 pm Post subject: Re: Carb Balancing

Thatâ€™s an intelligent and novel idea!!



Sometimes the piston spring can be swapped out for different tensions to affect mixture under acceleration at different altitudes; other times the needle profile can be changed to affect mixture settings at specific engine loads or rpm; its not uncommon to see some really old guy playing with a watchmakers lathe doctoring the factory OEM needle to perfection... 



Get a copy of Phil Irvingâ€™s book â€œtuning for speedâ€ its the standard bible of engine tuning...



At the end of the day inlet manifold pressure, exhaust header temperature and oil pressure, an attentive ear and backside is all you need to monitor a motor; everything else is ancillary and a bonus!

budyerly@msn.com

Joined: 05 Oct 2019
Posts: 286
Location: Florida USA

Posted: Tue Sep 21, 2021 8:30 pm Post subject: Carb Balancing

No doubt about it:



Today we are spoiled by the neat electronic stuff that makes our lives easier.  



I've had dual Webers and dual Hollies and frankly, I find the Bings to be very simple, but they wear and adjust differently than venturi carbs.  If the Rotax engine runs smooth so that you can easily read the ignition modules at all RPMs the Bings last forever.  If the engine is always shaking, the carbs will be trashed soon.  Balancing the carbs is straight out of the Rotax manual and now videos.  There should be no excuse for how to do a carb balance by the book, but that feel and sound is most important however, that ability some just never get as there are too many things going on in an engine.   I don't think its skill as much as research, observation and patience to determine a root cause of a problem.  You great mechanics study engines and tinker and never give up no matter how hot, miserable or hungry until you understand and then solve the problem.  The old Thomas Edison approach, just keep on trying.  Of course, as I get older, just remembering what I did is slipping away.



I wish we just had a simple SDS fuel injection with a simple pot lever arm for a mixture control and a knock sensor.  Ah to dream.  



Best Regards,

Bud Yerly



--

Area-51

Joined: 03 May 2021
Posts: 391

Posted: Wed Sep 22, 2021 6:58 pm Post subject: Re: Carb Balancing

Yip...



If we build it the people will come!!! ðŸ™‚ðŸ™‚ðŸ‘

europabill

Joined: 11 Nov 2021
Posts: 4
Location: Loganville, Georgia USA

Posted: Sat Nov 20, 2021 7:36 am Post subject: Re: Carb Balancing

I had a similar issue with the engine running rough between 2500 and 3500. 

After researching on the web I found someone with the same problem. 

What he found was there are 3 ports that are drilled in each carb and the

outside plugged  with  steel balls and the epoxy applied over them to seal any leaks.  With time the epoxy cracks allowing air to leak in.  I resealed on both

carbs and the roughness went away.



Bill

N51EU

Mallard

Joined: 22 Oct 2019
Posts: 49
Location: Ireland

Posted: Sat Nov 20, 2021 7:48 am Post subject: Re: Carb Balancing

Thanks for that information Bill. I'll keep that on file for future reference.



In my case, I consulted the Rotax Forum & saw advice from an A & P mechanic who stated that even though Rotax advise to balance the Carbs at idle rpm, in his experience he found that balancing the Carbs at 3500rpm gave a better result.

So I tried it & it seems to have solved the problem. 



Regards

James

_________________
Seat of my pants

Area-51

Joined: 03 May 2021
Posts: 391

Posted: Sat Nov 20, 2021 1:28 pm Post subject: Re: Carb Balancing

Yes, that would work well as a diagnostic; it can be taken further by removing the cold air intake and with a flat screw driver lift each carbi piston about 1/4â€™ at a time and note the rpm drop; a larger rpm drop indicates a richer mixture on that carburettor, a lesser drop indicates a leaner mixture on that carburettor (on the bing units any rpm difference may indicate a worn main needle or jet, or a manifold air leak)...same diagnostic test can be done at idle too... make sure the air balancing has been done first and both damper pots have the correct oil level

Gilles

Joined: 16 May 2020
Posts: 33

Posted: Sun Nov 28, 2021 12:51 pm Post subject: Carb Balancing

Le 20/11/2021 Ã  22:28, Area-51 a Ã©critÂ :

 	  Quote: 	
		   



 Yes, that would work well as a diagnostic; it can be taken further by removing the cold air intake and with a flat screw driver lift each carbi piston about 1/4Ã¢â‚¬â„¢ at a time and note the rpm drop; a larger rpm drop indicates a richer mixture on that carburettor, a lesser drop indicates a leaner mixture on that carburettor (on the bing units any rpm difference may indicate a worn main needle or jet, or a manifold air leak)...same diagnostic test can be done at idle too... make sure the air balancing has been done first and both damper pots have the correct oil level


	




Hello Area-51,



Just out of curiosity, what does this mention of damper pots oil level 

refers to?



In my experience that was a feature of SU carbs, not Bing. Or am I 

missing something?



Thanks,

-- 

Best regards

Gilles

http://contrails.free.fr

http://lapierre.skunkworks.free.fr

_________________
Best regards,
Gilles
http://contrails.free.fr

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