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Fun Questions


Roger Lee

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At what rpm does the starter spin at?

 

At what approximate rpm does the ignition modules switch from starting ??? degrees BTDC to ??? degrees ATDC. (3 answers here)

 

What is the degree setting for start up with the soft start module with a new style flywheel?

 

 

 

Hard extra credit question.

 

What happens to owners that don't use cool outside air for the engine intake and only use air filters mounted on the carbs inside the cowl using hot engine air which is like running carb heat 100% of the time and using 100LL all the time?

 

 

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Hard extra credit question.

 

What happens to owners that don't use cool outside air for the engine intake and only use air filters mounted on the carbs inside the cowl using hot engine air which is like running carb heat 100% of the time and using 100LL all the time?

 

Other than the obvious engine power loss, I have not heard of long term problems with this.  This is how the RV-12 is set up.

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My Sky Arrow takes a middle ground.

 

It uses a an oiled foam filter attached to the airbox - think K&N.

 

Though it draws air from inside the cowling, it's at the very front of the cowling, so the air reaching it is essentially at the outside air temp.

 

15166916066_2410503a19_c.jpg

 

Seems to work well enough.

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Hard extra credit question.

 

What happens to owners that don't use cool outside air for the engine intake and only use air filters mounted on the carbs inside the cowl using hot engine air which is like running carb heat 100% of the time and using 100LL all the time?

They never get carb icing.

Their engines run rich..........but why should this setup be like using 100LL all the time?

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"It's running rich, so increased lead deposits."

 

Bingo!

 

Nice one Andy.

Give that man a Lollipop. I'll buy lunch next time we meet.

 

Running with the air filters in the engine compartment and only using the hot air inside the cowl is the exact same as running your carb heat all the time and this makes the engine run rich. If you use 100LL all the time it will cause much larger lead deposits. It will clog the starter sprag clutch and it will need to be replaced. Parts alone are around $450 plus pulling the engine and the whole back end of the engine down. The gearbox overload clutch will gum and it will need to be removed, disassembled and cleaned. This usually happens around the 800 hr.+/- mark.

Running like this isn't a big issue if using 91 Oct., but you are still running rich which gets worse as altitude increases. 
 
Most MFG's use this alternate air source setup due to lack or space in the engine compartment and or cost.
What they don't know is the long term affect because these guys never go to class or listen to Rotax. I know this from a little inside info.
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Is it possible to lean the 912 ULS somewhat to account for this configuration?  I have an RV-12 ELSA and burn Autgas 91 (with @ 8% ethanol) @ 90% of the time.

 

I think other than at idle (which is set with the idle mixture screw) the mixture is set by the needle jets.  You'd have to get custom jets to make a change, and then you might go lean and detonate at low altitudes if you ever went there.  There is a manual "HACMan" unit to make changes, but many have said the danger of using them is not worth it.

 

Somebody correct me on this if I'm misunderstanding.

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There's also a clip on the metering pin where it meets the slide, with 4 (I think) notches.

 

I think lowering the pin makes the entire midrange leaner, raising it makes it richer.

 

When the slide is all the way up, the needle is out of the picture and the main jet determines the mixture.

 

I think. Let me find a good summary to link to.

 

Here: http://avsport.org/acft/Rotax/bing64.pdf

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Eddie got that right. Related: at high elevations, people benefit from raising the clip to needle position 2 (up from 3), but as a result they cannot fly to lower elevations.

 

In his link, on the last page, the needle is #4, and the clip is #14. #15 is a tiny o-ring that dampens vibrations, it snugs up against the clip.

 

There are other ways of changing the mixture too, but disclaimer: none of this is endorsed by rotax and I'm not responsible for your experimentation!!!

 

Adjusting float level: this has a light to moderate effect on mixture across throttle ranges. This is because it requires more vacuum to draw fuel on lower float levels, and less vacuum on high levels.

 

Changing the idle jet adjusts mixture in low throttle.

 

Changing the needle jet adjusts mixture in the midrange RPM.

 

Changing the main jet adjusts mixture in high throttle.

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Roger,

 

Do you have any pictures of pistons in various states (normal, "leaded" , other) that you could share?  It would be interesting to see some of the differences in the running conditions...

CT_Matt   look at the pictures on page # 5 of the attached document

= = = = = = = =

 

running with air filters breathing warm (cowling) air makes the engine running too rich for the carburators calibration coming from the factory [ main jets ] so  installing smaller # main jets would correct that .   but best is having the engine breathing cold (ram)air.

 

I remember a customer who had the airbox on his ULS but the single filter on the airbox was still ''under the cowlling''.. I had him relocated the filter and installing a Naca scoop...so it was now breathing  outside air...result: .the engine gained 300 rpm...so we had to get more pitch on the prop to have the same 'wanted ' 5600~ rpm WOT

912 maintenance article Conair.pdf

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Thanks Jacques! 

 

That is helpful but I am hoping to see some actual pictures of pistons since you can see them without taking the engine apart (through the spark plug holes).  This would be more of a sanity check to ensure the engine is "healthy" and doesn't exhibit an unusual amount of buildup that might require a tear down for further inspection.  Any easy to do anytime you change plugs.

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Eddie got that right. Related: at high elevations, people benefit from raising the clip to needle position 2 (up from 3), but as a result they cannot fly to lower elevations.

 

 

Corey, 

 

There is nothing to prevent you from flying to Death Valley at the leaner needle setting.

 

If you are worried about detonation you can climb at WOT and bypass the extra leaning.

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Corey, with the needle set to position 2, what is the altitude to start worrying?

This is one of those unknowns. It changes from engine to engine. The rotax class I took said around 6-7,000.

 

Corey, 

 

There is nothing to prevent you from flying to Death Valley at the leaner needle setting.

 

If you are worried about detonation you can climb at WOT and bypass the extra leaning.

This is true, but try landing like that!

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At what rpm does the starter spin at?  


6000+


 


At what approximate rpm does the ignition modules switch from starting ??? degrees BTDC to ??? degrees ATDC. (3 answers here)


850 rpm from 4 BTDC to 26 BTDC


 


What is the degree setting for start up with the soft start module with a new style flywheel?


3 ATDC if you also have the new style flywheel


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This is one of those unknowns. It changes from engine to engine. The rotax class I took said around 6-7,000.

 

 

This is true, but try landing like that!

 

There's a CT around here based near sea-level but crosses the Sierra at 14,000' and he uses the leaner needle setting.  Landing is done with a retarded throttle and not under load.  Seems to work fine.

 

Personally I don't see the point unless you want to fly at high altitude and low power settings.  It would make more sense to me to use a leaner main jet.

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There's a CT around here based near sea-level but crosses the Sierra at 14,000' and he uses the leaner needle setting.  Landing is done with a retarded throttle and not under load.  Seems to work fine.

 

Personally I don't see the point unless you want to fly at high altitude and low power settings.  It would make more sense to me to use a leaner main jet.

 

There's a point where the manifold air pressure is so low, that WOT doesn't open the slide all the way, and thus does not ever go full rich. The slide is actuated by vacuum above the piston and piston diaphragm, and the higher the Delta P to atmospheric pressure below the diaphragm, the more the slide opens. There is absolutely no mechanical linkage between the throttle arm and the slide. I have been interested in figuring out a way to observe the slide operation on a bench. I might try a powerful vacuum cleaner and put a gauge and a bypass gate on it... I DO have a spare carb here in front of me!

 

Anyways, as for our sea-level friend: as long as his systems function correctly, this isn't a problem. But, when I set things, I'm always thinking of "what if some deity hates me?"

 

EDIT: By vacuum, I am referring to vacuum created above the piston and diaphragm, NOT engine vacuum! Added red text to indicate clarification.

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Hi Corey,

 

"The slide is actuated by vacuum"

 

This isn't quite right. We just got through going over this in 8 days of class. It is actuated by pressure differential. As air passes through the carb it creates a venturi which cause a pressure drop. The piston (slide if you want to call it that) has two holes (#19) in it that go up through and into the carb dome (#23). The carb has a large air hole on the air filter side of the throat at the top (#2). This atmospheric pressure enters a chamber there (#1). The atmospheric pressure is higher than the lower air pressure in the domb. This lifts the piston up against the spring force pushing it down. So what lifts the piston is pressure differential under the diaphragm verse what's on top.

 

The low pressure is in space 23 and the atmospheric pressure is in space 1. The holes in the piston are 19 that go up into the dome. 2 is where the atmospheric pressure enters out by the air filter.

 

bmw-om-carb-cv.jpg

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