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Rejetting a 914 during Winter?
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darinh

Joined: 20 Apr 2006
Posts: 327
Location: Utah

Posted: Fri Jan 02, 2009 10:40 am Post subject: Rejetting a 914 during Winter?

Does anyone rejet their 914 (or 912) during winter?  Our summer temps at altitude (6000' to 8000') are usually in the range of 60 to 80 F, but in the winter drop to the 20's to around 0 at times.  I have noticed ever since the thermometer started heading south of the freezing line, my engine won't hit my max TO rpm of 5700 and it seems to run a bit rough at more than 75% throttle setting.  I know that Rotax jets these things for 60F and 1000' msl which is now where near what I fly at.  



My stock jetting (main) is 158 on the left carb and 156 on the right.  Looking at a Bing jetting correction chart for altitude and temp. I am coming up with recommended jets of 150 and and 148 respectively for 0 to 32F and 6000'.



Can any of you engine gurus tell me if this may be the source of my slightly diminished engine performance?  Also, one last thing that makes me think the engine is running a bit rich is that if I land at a small strip and shut down right after landing, it will sometimes backfire on me.  If I have to taxi to parking, the rundown gets all the extra gasoline fumes out and the backfire does not occur.  Seems to me that this says it is a bit on the rich side.  Any help or suggestion is appreciated.

_________________
Darin Hawkes
Series 7
914 Turbo
Kaysville, Utah

lucien

Joined: 03 Jun 2007
Posts: 721
Location: santa fe, NM

Posted: Fri Jan 02, 2009 5:46 pm Post subject: Re: Rejetting a 914 during Winter?

 	  darinh wrote: 	
		  Does anyone rejet their 914 (or 912) during winter?  Our summer temps at altitude (6000' to 8000') are usually in the range of 60 to 80 F, but in the winter drop to the 20's to around 0 at times.  I have noticed ever since the thermometer started heading south of the freezing line, my engine won't hit my max TO rpm of 5700 and it seems to run a bit rough at more than 75% throttle setting.  I know that Rotax jets these things for 60F and 1000' msl which is now where near what I fly at.  



My stock jetting (main) is 158 on the left carb and 156 on the right.  Looking at a Bing jetting correction chart for altitude and temp. I am coming up with recommended jets of 150 and and 148 respectively for 0 to 32F and 6000'.



Can any of you engine gurus tell me if this may be the source of my slightly diminished engine performance?  Also, one last thing that makes me think the engine is running a bit rich is that if I land at a small strip and shut down right after landing, it will sometimes backfire on me.  If I have to taxi to parking, the rundown gets all the extra gasoline fumes out and the backfire does not occur.  Seems to me that this says it is a bit on the rich side.  Any help or suggestion is appreciated. 	




My plane has the 912ULS on it and flies regularly up to close to 10,000' without any apparent enrichening of the mixture. It runs as smoothly as it did at sea level, I think the EGT's may be between 50 and 100F lower but that's about all.



The carburettors should take care of it as they're altitude compensating.



One thing that will make them run a little rich is if the little vent tubes on the sides of the carbs aren't routed to the intake. I.e. if you don't run the airbox and they're not connected to anything. I ran mine to fittings pressed into the backs of my air cleaners. 



Before I did that, the carbs did run a little rich at full throttle, but nothing that would cause rough running and etc.....



Doesn't sound normal to me...



LS

_________________
LS
Titan II SS

lucien

Joined: 03 Jun 2007
Posts: 721
Location: santa fe, NM

Posted: Fri Jan 02, 2009 5:50 pm Post subject: Re: Rejetting a 914 during Winter?

Oh, I should actually address your question . No, I havn't found the need to do any rejetting with the change of seasons. In fact, in winter the motor puts out very slightly more power than during the summer. The EGT's barely change at all that I can see.



LS

_________________
LS
Titan II SS

darinh

Joined: 20 Apr 2006
Posts: 327
Location: Utah

Posted: Fri Jan 02, 2009 9:57 pm Post subject: Re: Rejetting a 914 during Winter?

I think I may have found the problem...the cable on my waste gate for the turbo had stretched a tad bit and needed readjustment.  Now if the damn weather would cooperate long enough for me to fly it I could test it out.

_________________
Darin Hawkes
Series 7
914 Turbo
Kaysville, Utah

Roger Lee

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

Posted: Wed Jan 07, 2009 5:57 pm Post subject: Re: Rejetting a 914 during Winter?

Don't worry about the jetting. Re-pitch the prop. You will loose 100-125 engine rpm due to the colder denser winter air verses the warmer summer air with the prop. We do it in Tucson all the time. The summer temps are 90 F + and the winter mornings are 32-40F. You will run better. Take out approximately 1/2 degree to pick up another 100 rpm.

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

imap8ntr(at)cox.net
Guest

Posted: Wed Jan 07, 2009 6:36 pm Post subject: Rejetting a 914 during Winter?

Why are we flattening the prop pitch?  Are we doing this for better climb or 

are we doing this for mixture control with the Bing altitude compensating 

carburetor having a needle to control the mixture rather than a mixture 

control?  I thought flattening the prop will make it run leaner and we want 

to richen up things a bit for the winter?  I am confused about something.



Ivan

---

darinh

Joined: 20 Apr 2006
Posts: 327
Location: Utah

Posted: Wed Jan 07, 2009 8:03 pm Post subject: Re: Rejetting a 914 during Winter?

Roger,



Thanks for the post but I have an Airmasters CS prop so that is not the issue.  It is not a problem with load on the prop, rather when it hits the top 10% of throttle, it runs a bit rough as though it is jetted wrong.



My understanding is that the main jet controls the top end and the engine is smooth as silk through the the range from idle to approximately 90% throttle then is runs rough so this is why I was thinking it may have something to do with the main jetting.

_________________
Darin Hawkes
Series 7
914 Turbo
Kaysville, Utah

rholder(at)avnet.co.uk
Guest

Posted: Thu Jan 08, 2009 2:28 am Post subject: Rejetting a 914 during Winter?

darinh wrote:



 	  Quote: 	
		   Thanks for the post but I have an Airmasters CS prop so

 that is not the issue.  It is not a problem with load

 on the prop, rather when it hits the top 10% of

 throttle, it runs a bit rough as though it is jetted

 wrong.


	


 	  Quote: 	
		   My understanding is that the main jet controls the top

 end and the engine is smooth as silk through the the

 range from idle to approximately 90% throttle then is

 runs rough so this is why I was thinking it may have

 something to do with the main jetting.


	


 	  Quote: 	
		   -------- Darin Hawkes Series 7 914 Turbo Kaysville,

 Utah


	


Rotaxes are designed to run with the jets that come with

them. I am sure Rotax would say that there is never any

need to change the jetting for winter, or high altitude.



One possibility for a missfire at high power is that the

fuel flow is not sufficient. Have you checked your fuel

flows and your filters ? Does it run better with both fuel

pumps on ?



Richard Holder

Thom Riddle

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

Posted: Thu Jan 08, 2009 6:08 am Post subject: Re: Rejetting a 914 during Winter?

What do your EGTs telling you?

_________________
Thom Riddle
Buffalo, NY (9G0)

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

rampil

Joined: 04 May 2007
Posts: 870

Posted: Thu Jan 08, 2009 6:12 am Post subject: Re: Rejetting a 914 during Winter?

Lucian and Ivan,



Happy New Year.

Just a note on the Bings:



There is no evidence that these carbs do ANY altitude compensation.

Not anywhere on the web that I have found. Certainly not in my

flight testing, not in the Diamond Katana DV-20 manual both of

which show that the fuel flow actually increases with altitude.



You might also note that Rotax has ceased claiming altitude compensation

in all their more recent literature.



If you want to respond, please enclose actual data, not hand waving.

I put up the data from the DV-20 and my own tests several months ago.



I have also spoken to Rotax and Kodiak technical people and they do not

deny these observations.  Ah well, flying at 8000' will cost you

more fuel than at 4000'.  Maybe the wind will be better

_________________
Ira N224XS

Roger Lee

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

Posted: Thu Jan 08, 2009 6:24 am Post subject: Re: Rejetting a 914 during Winter?

You haven't mentioned making sure the carbs have been synced lately? Even if they have been and it was before the really cold temps I would do it again to eliminate the possibility. It should not be a jetting issue. We need to look at differential diagnoses.

_________________
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: Thu Jan 08, 2009 6:32 am Post subject: Re: Rejetting a 914 during Winter?

Hi Lucian,



They have to be pressure compensating because there is no direct cable attached to the carb slide and the holes that are drilled in the carbs allow for pressure differential on top of the carb to be different from the carb throat and the pressure differential is what moves the slide up and down. With altitude and throttle position changes come pressure changes.



I like the word pressure over altitude.

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

Thom Riddle

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

Posted: Thu Jan 08, 2009 6:56 am Post subject: Re: Rejetting a 914 during Winter?

Rampil,



A Katana DA-20A (80 hp Rotax 912 A or F, 80 hp not 100 hp) flight manual can be found at



http://www.diamondaircraft.com/library/16/10/DA20-A1-AFM-inc-Rev17.pdf



In this document the charts, tables and data indicate a constant fuel consumption rate for a given % power with no indication of variation with density altitude.

_________________
Thom Riddle
Buffalo, NY (9G0)

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

Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Thu Jan 08, 2009 7:04 am Post subject: Rejetting a 914 during Winter?

First of all the 912 is a four stroke engine so the pitch of the prop won't

affect to any great degree the mixture of the engine.  Thinning the prop

will allow it to pass through the air easier so you will be able to develop

take off horsepower.  You don't want to thin it too much because the extra

oxygen in the air will also allow your engine to develop more horsepower

while of course burning more fuel.



Noel



--

_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats

larry(at)macsmachine.com
Guest

Posted: Thu Jan 08, 2009 7:14 am Post subject: Rejetting a 914 during Winter?

Hi Thom,

The EGTs should provide exhaust gas temps that indicate the lean or 

richness of the mixture at various rpm

and altitude.  The Bings compensate for lower pressure at altitude and 

my EGTs also reflect that as well.

The EGT is a very reliable diagnostic and I wouldn't be without it.



Larry McFarland 601HDS at www.macsmachine.com



Thom Riddle wrote:

 	  Quote: 	
		   



 What do your EGTs telling you?



 --------

 Thom Riddle

 N1208P RANS S6S, Tailwheel, 912UL

 N197BG FS1/447

Thom Riddle

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

Posted: Thu Jan 08, 2009 7:27 am Post subject: Re: Rejetting a 914 during Winter?

Larry,



Yes, I know and that is why I was asking the original poster what his EGTs were telling him about rich/lean condition.



http://riddletr.googlepages.com/a%26pmechanix

_________________
Thom Riddle
Buffalo, NY (9G0)

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

rampil

Joined: 04 May 2007
Posts: 870

Posted: Thu Jan 08, 2009 8:08 am Post subject: Re: Rejetting a 914 during Winter?

Hi Thom,



Happy New Year.



We went through this several months ago. You posted a 912/DA20

POH and I posted the DV20912S. 

http://forums.matronics.com/download.php?id=8745      for your convenience  



That plus Eric, Kerry, and Phil have all agreed with my data from my

tests.  Sorry.  Just install a floscan, read it and weep.

_________________
Ira N224XS

Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Thu Jan 08, 2009 8:09 am Post subject: Rejetting a 914 during Winter?

A problem, well known to racers, is the octane rating of the fuel...  if the

octane rating is below spec, then the engine will run very rough at high

rpm.  This can actually be very damaging to the engine.  I'd check with a

Rotax shop before trying any of the octane boosters available and in ther

meantime only buy high octane fuel from high volume gas stations.  They have

the freshest fuel.



Another slight possibility is you are over revving your engine.  You may

also want to check the calibration of your tachometer.  I haven't heard any

discussions on the reliability of the 912 tach. but there is always a first

time.



Noel

Rotaxes are designed to run with the jets that come with

them. I am sure Rotax would say that there is never any

need to change the jetting for winter, or high altitude.



One possibility for a missfire at high power is that the

fuel flow is not sufficient. Have you checked your fuel

flows and your filters ? Does it run better with both fuel

pumps on ?



Richard Holder

_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats

Thom Riddle

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

Posted: Thu Jan 08, 2009 8:17 am Post subject: Re: Rejetting a 914 during Winter?

Rampil,

I didn't say you were incorrect, just showing different documentation from the same source, Diamond Aircraft. I don't have a fuel flow meter so I can't do what you suggest. I wish I did have one and may add one some day. 



I have not noticed any meaningful difference in EGTs from sea level density altitude to 12,000' DA on our 912UL as long as I keep the engine above 4500 rpm. I know this does not say anything about actual fuel flow but it does say that the automatic mixture adjustment with changes in DA is working well, which is my first priority.

_________________
Thom Riddle
Buffalo, NY (9G0)

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

darinh

Joined: 20 Apr 2006
Posts: 327
Location: Utah

Posted: Thu Jan 08, 2009 9:16 am Post subject: Re: Rejetting a 914 during Winter?

Wow, a whole bunch of post since last night!  First of all, the issue I am having really has nothing to do with calibration of my tach since the engine is running rough at full throttle...if it was smooth and still not generating the RPM it should, I would suspect the tach but this is not the issue.



I use only Premium fuel from a station that has fairly high traffic and the fuel is 100% ethanol free.  Having said this, I don't think any station has a huge demand for premium fuel due to the prices and most cars are fine with the mid-grade stuff.



Anyone know why Rotax would jet the two carbs differently?



Roger,



I have synched them again a couple times.  I think I mentioned this in a earlier post but I am only getting the misfire or "bump" on the left carb.  I will PM you with additional information.

_________________
Darin Hawkes
Series 7
914 Turbo
Kaysville, Utah

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