Rotax rebuild

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Postby Shannon » Thu Jan 13, 2005 2:04 pm

Guest @ Jan 13 2005, 02:12 PM wrote: Sorry, I'll try again. Does the spark from the points fire exactly when the points split (.16" BTDC, 0.0 point gap) or when the point gap reaches its optimal gap (.15" point gap, and guessing 0.0 TDC) ..? If when first split, does it take the spark a while to get to the plug, hence the .16 BTDC setting..?


Nope, the points touch and fire occurs at the BTDC moment. Yes it does seem odd that the piston has not yet reached TDC but that's how engines work. In the split nanosecond before the piston reaches its highest point the compressed fuel and air mixture is ignited.
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Postby JPXman » Thu Jan 13, 2005 2:17 pm

yes think of this like pushing your child on a swing.

You always start to push when they're coming towards you so that you have your force fully applied by the time they start going away from you.

If you waited until after that point, you would waste energy chasing after them before you imparted your force.

...get too efficient though and you'll launch your kid out of the playground :)
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Postby Guest » Fri Jan 14, 2005 8:31 am

Now you have me really confused, Shannon. (When I rotate the prop in it's proper running direction) my points break at the .16" BDTC and are touching all the way up to that point...
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Postby Guest » Sat Jan 15, 2005 8:45 am

Ok, following your directions. Clockwise from front (which I believe is opposite of engine running direction) my points close as travel reaches .16 BTDC. So, if the engine was running, my points would break at as the piston comes to .16 BTDC and be fully open at 0 TDC.
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Postby Guest » Sat Jan 15, 2005 9:39 am

Yes that's right. The points will be "open" but slowly "closing" as the piston is reaching BTDC location. The points will remain closed approx. ~ 90 degress or a little less of crank rotation after contact is made. After this point the lobe on crank will start opening the points back up for the next firing sequence.
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Postby Guest » Sat Jan 15, 2005 4:48 pm

I know this isn't rocket science but clock-counter clock, opening-closing, which direction is hard to follow in emails. And I know the lobe cannot be moved on the crank, so I can't really screw anything up. I don't even think you could make the points open or close or whatever at .16 ATDC. I understand the dial guage, buzz box, etc. So, bottom line, when the rotation is in the proper direction of the engine running, my points 'open' right upon .16 BTDC, they don't close then. Therefore, my points will fire (assumably) as the points break open, not when they close...
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Postby Guest » Mon Jan 17, 2005 12:54 pm

Thx. New topic but still rotax. I must have gotten a 185 that was a mark-3. The 4 studs that hold the spacer and the pull starter were too small. So I had them dirlled and tapped. Now the studs I screw in them wobble a little even when all the way in a good 5/8". Can I use some metal fill, dumbo, or whatever to fill this..? The wobble stops after tightening but I think there is still some space in there that should be filled. Or, should I sart over with a larger hole (that obsorbs all of the old threads) and then re-tap them.
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