Rotax B box question

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Flivverflyer

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Newbie alert to start out here-- I have only 15 hours on my new-to-me Sky Raider. I have the 2.58:1 B box on my Rotax 503, with a Powerfin 66" two-blade ground-adjustable prop. I understand the PSRU box has a centrifugal clutch inside. Is it the RK400C clutch? I don't understand why my prop doesn't freewheel with the engine shut down. In fact, it feels like I can pull the prop through the compression cycle to reorient the blade position. How does this work? I'm used to Lycomings and Continentals, so this is new to me.
 

challenger_II

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The Rotax "B" box does not have a centrifugal clutch. It has a "dog" clutch inside the box, to absorb the impulses from the engine. The dog clutch does not disengage, therefore your propeller not free-wheeling.
 

Flivverflyer

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Ah, okay, that explains it then! So the dog clutch is like a direct-drive gear system but with a bit of play between the drive dog gear and the driven dog gear. This is why I can move my blades back and forth a bit before feeling resistance, yes? So, theoretically (not that I'd want to), you can hand prop start a Rotax with the B box? Just wondering in case anyone tries to turn the standing prop while the ignition switch is still on.
 

TFF

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Don’t guarantee it won’t start with the CDI just in the premise of safety. A friend was turning through a 912 and a P lead was broken and the thing burst to life. The would not want to try to start it, but if it’s right, it will.
 

TrikeTrash

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Just thinking out loud. Since the voltage produced by the lighting coil is dependent on rpm, I wonder if a wiring bypass switch at the ignition box with say a 28 volt lithium battery or the like would allow for a much easier starting engine. Start it on battery and after it starts running, flip it back...
 

challenger_II

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Neat idea, much like the old "Shower of Sparks" ignition system back in the Dark Old Days.
However, if I read the schematics correctly, the CDI on a Rotax two-stroke is receiving A/C current from the lighting coil.
 

TrikeTrash

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Agreed that it's AC on the way in but it has to be rectified at some point, otherwise the polarity of the sparkplug itself would be AC and I don't think that's the case. Also, I'm thinking that the lighting coil may also need to have a load (big resister?) of some kind while it's switched out. To many questions and not enough answers, for sure. But it would be nice not to have to yank so much - I ain't getting any younger!
 

Flivverflyer

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Now that we are on the subject, I've been doing a whole lotta pulling lately before I get airplane noise to happen. Is it because of the hot weather? Do I need to prime a whole bunch, or would I just be flooding a good engine if I did? I'm starting to consider an electric starter conversion for my Rotax!
 

Flivverflyer

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I start with five good squishy pumps, then pull at least 60-70 pulls (good workout!). The primer is on the instrument panel, so maybe a couple of feet to the carbs?
 

Flivverflyer

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So, from your experience, is it way easy to flood the Rotax with a bit too much priming? Or is it pretty tolerant of some healthy squirts?

I got a nice backfire once on my Cessna 150 from over-priming (dead of winter), and started a fire around the nose wheel! Apparently, the excess fuel was dribbling out the exhaust pipes. I was able to put it out without any damage, fortunately, but it made me respect the primer on that engine.
 

challenger_II

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Small Continentals prime just above the carb flange, and over-priming wets-up the air box. A lean condition will always backfire through the intake, so it is a recipe for a cowl fire. Don't ask me how I know... :)

As you are running fuel through the crank case of a Rotax, they are a bit more tolerant of over-priming. You will have to experiment to see just where the too-lean prime, and the too-rich prime are. I keep my primer very close to the cab (8", there abouts), and one to two shots gets her in the mood to start on the first, or second, pull.
 

Tuneturkey

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I start with five good squishy pumps, then pull at least 60-70 pulls (good workout!). The primer is on the instrument panel, so maybe a couple of feet to the carbs?
If your primer piping is clear plastic you should be able to see the fuel in the pipe. That fuel remains in the tubing after you prime, so the fuel doesn't have to travel a long distance to get to get to the carb. I prime 3 squirts on my 503 (dual Bing carbs) and it starts 2nd pull.
 

Tuneturkey

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The rectification would happen in the CDI module. If you introduced DC current, you may smoke the module.
The lighting generator circuitry is separate from the pulse coils for CDI. The lighting system generates an A/C voltage of varying frequency. consequently, it has to be rectified as with any a/c power supply, and that is done with the regulator. The regulator rectifies the a/c, filters it, and prooduces an regulated output DC voltage of 14.4 volts. However, the full output only occurs at high RPM. Your best bet is to use a 12 volt battery as your source with the output from the regulator recharging the battery. A small battery works well for DC supply to a radio, USB outlet, fuel tank level device, etc. You can run an A/C device directly on the output of the lighting circuit, but the voltage will vary depending on engine RPM. Panel lights is a common use.
 

TrikeTrash

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It's my bad for calling the voltage source, the lighting coil - just lumped the whole coil thingy together :> Still think the idea has merit though. Read somewhere that the minimum voltage required for the Rotax to start making sparks is 16 volts. The 28 volt reference came from seeing batteries available used to drive portable drills and such. It would be easy enough to keep a couple charged for easier starting. Might need to bump up the voltage up to 36 volts, add a converter to convert the DC to AC, a bypass switch between the Rotax voltage source and battery and then even the most anemic pull would fire up the engine...
 
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