Rotax 912 ULS cold start

H. Paul Shuch is a Light Sport Repairman with Maintenance ratings for airplanes, gliders, weight shift control, and powered parachutes, as well as an independent Rotax Maintenance Technician at the Heavy Maintenance level. He holds a PhD in Air Transportation Engineering from the University of California, and serves as Director of Maintenance for AvSport of Lock Haven.

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Warmi
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Rotax 912 ULS cold start

Postby Warmi » Sun May 12, 2019 12:44 pm

My wife recorded a video of me starting a Rotax 912 ULS with a choke ..

https://youtu.be/QBZ2IF68KIM

I really don't like the violent shudder for a second or two when it starts - it is there anything that can be done to make it smoother ( of course, it does start much better when it is warm ) ?
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Re: Rotax 912 ULS cold start

Postby rcpilot » Sun May 12, 2019 12:54 pm

When was the last time the carbs were serviced? Mine used to be like that but since I had my carbs rebuilt at Lockwood it starts up much quicker and smoother when I have to use choke. I'm curious to see how it will be in Winter.

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Re: Rotax 912 ULS cold start

Postby Warmi » Sun May 12, 2019 1:03 pm

I am at 350 hours , the 5 years rubber was done in late 2017 and the carbs were looked at as part of annual a month ago - how do you tell if it is time to have them rebuild ?
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Re: Rotax 912 ULS cold start

Postby Wm.Ince » Sun May 12, 2019 3:00 pm

Warmi wrote:. . . . the 5 years rubber was done in late 2017 and the carbs were looked at as part of annual a month ago - how do you tell if it is time to have them rebuilt?

“Looked at?”
Just exactly . . what does that mean?

Did they check carb balance?
If not within spec, I hope they rebalanced the Bing carbs.
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Re: Rotax 912 ULS cold start

Postby Warmi » Sun May 12, 2019 3:07 pm

Yeah, carbs are balanced - I do it myself with the carb mate when I feel things are getting rough.
Looked at means , they opened the carbs and inspected them for debris etc ... everything looked good I was told.

The engine does run smooth- it is just the cold startup that makes me worried - pretty violent imho.
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Re: Rotax 912 ULS cold start

Postby Wm.Ince » Sun May 12, 2019 3:23 pm

Warmi wrote:Yeah, carbs are balanced - I do it myself with the carb mate when I feel things are getting rough.
Looked at means , they opened the carbs and inspected them for debris etc ... everything looked good I was told.
The engine does run smooth- it is just the cold startup that makes me worried - pretty violent imho.

Cold starts . . . . with choke lever fully aft . . . are the choke arms on both carbs fully closed?
During the cold start attempt, is your throttle lever fully aft?

My starts, on my 912 ULS, are instant.
When it starts up, the engine comes alive, almost with a jolt . . . then, after backing off choke and slowly feeding in throttle, engine smooths right out. That’s kinda’ normal.
I do not have the “soft start” feature installed, which reduces that jolt . . somewhat.
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Re: Rotax 912 ULS cold start

Postby FastEddieB » Sun May 12, 2019 5:28 pm

The “choke” - not really a choke - performs best with the throttle completely closed. Even though some checklists have “Throttle - Open 1/4” or similar.

Just a thought.
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Re: Rotax 912 ULS cold start

Postby Warmi » Sun May 12, 2019 5:42 pm

Yeah, I agree - for choke only starts I have the throttle closed fully.


Here is similar start from inside ... does this seem like a normal start without undue hardship on the engine at 50 F ?
I have my idle set at around 1500-1600 or so ... I wonder if it is too low and it is causing harder starts than necessary .. or I am just being paranoid :D

https://youtu.be/j-_3tsNE_Tg
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Re: Rotax 912 ULS cold start

Postby dstclair » Sun May 12, 2019 6:02 pm

Warmi - you are way too low for a Sting. 1800-1900 is perfect.
dave

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Re: Rotax 912 ULS cold start

Postby Warmi » Sun May 12, 2019 6:18 pm

dstclair wrote:Warmi - you are way too low for a Sting. 1800-1900 is perfect.


Yeah, it may be, although after getting down to my hangar with throttle fully at idle , just before I begin the shutdown process, I settles at around 1600 and it is actually pretty smooth - once I shut off one mag ,and if I wait too long, it will start hammering a bit ... but generally I never let it idle below 2000 during warmup so that doesn’t seem to be a problem - just the startup.

I will try with higher idle at 1800 and see what happens.
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Re: Rotax 912 ULS cold start

Postby Sling 2 Pilot » Sun May 12, 2019 7:07 pm

1600’ Gear Box No Likey...

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Re: Rotax 912 ULS cold start

Postby Wm.Ince » Sun May 12, 2019 10:01 pm

dstclair wrote:Warmi - you are way too low for a Sting. 1800-1900 is perfect.
Sling 2 Pilot wrote:1600’ Gear Box No Likey...

Concur.
1800 RPM is much better.
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Re: Rotax 912 ULS cold start

Postby ShawnM » Mon May 13, 2019 6:34 am

I have the soft start ignition modules so I can't really compare my starts to yours. Like Bill, my plane starts instantly every time. I also agree that your idle is too low. Mine is set for 1750-1800 and settles in this range somewhere. I never idle it below 2000 once started for the sake of the gearbox. Well, let me rephrase that, during my runup I check that the plane will go to idle without dying after I do my mag check. Then it's back to 2000+ RPM.

My start procedure is exactly as Bill explained. As soon as the engine comes alive I slowly roll off the "choke" while adding in a little power at the same time to keep the idle between 2000 and 2200 after it starts. During this dance on the choke and throttle levers my soft start modules kick off after about 5 seconds and the rpm increases a little after that.

I also agree that if you have the choke (fuel enrichment circuit) on during a cold start the throttle needs to be completely closed or it defeats the purpose of the "choke". None of this "crack the throttle 1/4" nonsense regardless of what your POH says. Ask any Rotax certified mechanic. :mrgreen:

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Re: Rotax 912 ULS cold start

Postby Warmi » Mon May 13, 2019 7:26 am

I have the soft start ignition modules and they do work , there is marked RPM jump after about 2 seconds - which is where it gets smooth, it is the first 2 seconds where the engine is jumpy and rough.

When it is 65+ F or so , the engine pretty much starts right away - the issue only occurs when it gets below 60-55 F.

Shawn:
Would it be possible for you to record the startup procedure on your plane when it is colder outside ( which by Florida standards probably means 60+ degrees :-) ) ?
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Re: Rotax 912 ULS cold start

Postby rcpilot » Mon May 13, 2019 8:43 am

Warmi wrote:I am at 350 hours , the 5 years rubber was done in late 2017 and the carbs were looked at as part of annual a month ago - how do you tell if it is time to have them rebuild ?

The carbs are supposed to be "rebuilt" every 5 years also. Replace the gaskets, O rings, etc. and check the weight of the floats and replace them if needed.


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