How to set Timing on Rotax 185

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Postby rayjb60 » Wed Aug 11, 2004 2:47 pm

Some people are asking How to set Timing on Rotax 185's so I thought I would
document it here so everyone can add their Tips to my description, if they have a better way.


Steps by the numbers.

1) remove flywheel from rear of engine.

2) place several alignment marks on the ignition housing plate and engine casing,
so you can go back to your previous timing if required.

3) Remove the sparkplug, and hook up a vernier scale or micrometer to the top of the engine so that it touches the top of the piston at top dead center.

4) Attach a continuity tester of some kind, either the light kind, or use a multimeter on the resistance setting, between the points. The idea is to detect
when the points open and close.

5) Set your points to the recommended gap of 0.15" with a feeler gauge.

6) Loosen the ~3 screws that hold the ignition plate, so that you can rotate it.

7) Rotate the engine shaft in the direction of prop....clockwise I believe from the front, and when the piston is 0.16" lower than its top dead center position, your points should open and fire the ignition(I incorrectly said "close" before).
Rotate the ignition plate accordingly to make this happen and then tighten down.

8) Reassemble everything and your done.

Feel free to add comments on this procedure in case you know a better way, or have some helpfull tips.
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Postby Guest » Fri Sep 10, 2004 5:11 pm

2 additional questions. 1. Is the point gap important..? I had to set mine wider to accomplish step 7. 2. Regarding step 7., I thought I read somewhere that the points should 'break' at .16 before TDC...you mention 'close' at .16 before TDC..? Also, shouldn't the prop be backed off from the right direction to get .16 before..? Thx
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Postby rayjb60 » Tue Sep 14, 2004 3:18 pm

As far as I know, the point gap is not important, but the "break" is.

My terminoligy is incorrect when I refer to "points should close" I like the term "break" better.

The spark should fire at .16" before top dead center.

So now that your done hows your engine running now?

I notice mine ran smoother...less vibration at all seetings, especially just above idle.
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Postby Guest » Tue Sep 14, 2004 5:45 pm

Ray what RPM's are your engines turning up ?
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Postby Guest » Wed Sep 15, 2004 7:30 am

Thx So, the spark fires right at the break of contact..or .16" BTDC..? And lastly, at what distance BTDC would your spark fire before the piston made it to TDC... .17"...18"....19"....20..?
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Postby Guest » Wed Sep 15, 2004 8:54 am

According to the manual .13" BTDC is the mark.
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Postby rayjb60 » Thu Sep 16, 2004 2:48 pm

Just to clarify, the spark fires when the piston is 0.13"(according to manual) before the top of its stroke travel (BTDC).

Have not measured my rpms, but it sure sounds sweet and smooth when they are at full speed.

I had both flintstone brakes straining on the ground practically lifting the entire plane to hold it in place while sitting in the seat at full power.

Those engines are powerfull and sing sweetly together....Lovin it!!!!!!!
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Postby Guest » Thu Sep 16, 2004 5:04 pm

Ray - Ignore my ignorance. .13" BTDC is a distance. Do you mean, with points breaking at .16" BTDC, another .03" and the spark fires..? I'm trying to understand this timing thing logically and need to know when the spark is created. At the break of the points, a little after the break of the points, or when the break of the points reaches a gap of .14 inch..?
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Postby Guest » Sun Sep 19, 2004 9:10 am

The points should contact and FIRE somewhere between .13-.17" BTDC. 16"-17" BTDC seems to be the best setting.
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Postby rayjb60 » Tue Sep 21, 2004 3:00 pm

That measurement .13-.17 inches is the distance from the top of the piston, to the top of its travel.

You set a gauge in the spark plug hole and zero it, when the piston is at the top of its travel......then you bring it down .15 of an inch and then you adjust the points so that they close (contacts touch).

You use a continuity tester of some sort to determine exactly when the contacts touch, on the points.

Do be sure that the piston is in the BTDC position....I believe the roation of the prop is clock wise when looking at the front of the engine.

If you time it to the After Top Dead Center, your engine will run very poorly if at all.

If you still dont understand then I will draw a picture and paste it in the post, that should help clear it up.

I think your confusion lays in what that measurement is..... It is NOT the points gap, it is a measurement from the top of the piston to TDC through the spark plug hole.
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