Remi Guerner
Joined: 14 Dec 2010
Posts: 284
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 Posted: Mon Dec 27, 2010 9:34 am Post subject: Rotax new flywheel hub for 912 series |
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Hi all,
This is the copy of the message I just posted on the Rotax forum:
Rotax has recently introduced a new fly wheel hub with lengthened trigger cams which change the ignition timing during starting. This is described in SI-912-020 R4, section 74-00-00, page 1 and 4. With the new hub, ignition timing during the starting phase will be 3 degrees after TDC instead of 4 degrees before TDC. I am very interested in doing this mod on my 912ULS as it should prevent any possibility of kicking back.
However I am puzzled by the dimensions shown on the drawing on page 4. The drawing says the old cams are 22 mm long and the new ones are 29 mm long. I measured the length of the cams on my 6 year old engine: they are 30 mm long! I also did the maths: the radius of the flywheel cams is 78 mm, so 1 degree rotation moves the cam edges 1.36 mm. From 4 to 26 degrees, that is 22 degrees difference, the length should be 1.36x22=29.95. So i believe my hub is good and the Rotax drawing is wrong.
I have 2 questions:
- to anyone who has access to the back of his engine: could you please remove the 3 screws which hold the plastic cover and measure the developed length of the cams.
- to any one who have the new flywheel hub: please do the same. According to the SI, engines fitted with the new flywheel hub start with the following serial numbers:
912S: from S/N 4,923,847
912ULS and ULSFR: from 6,775,360
Waiting for your answers
Regards
Remi Guerner
F-FGKL
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