Author Topic: Ignition timing on BS 90 sport  (Read 1833 times)

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Offline grundlegrabber

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Ignition timing on BS 90 sport
« on: July 04, 2011, 05:03:41 PM »
Need a little advice on setting the timing on my 1969 90 sport. I bought this bike last year and after a complete fuel system overhaul, and undoing some improper repairs made by the previous owner, it runs pretty decent. I think it could be tuned a little better, though, and I'd like to make sure the timing is set correctly.
The crappy manual I have says to line up the TDC mark on the flywheel with the one on the cover. There seems to be no mark whatsoever on my flywheel, only see the one on the cover. Also says to connect an ohmmeter to ground and to the black wire coming from the magneto. I have done this, but am not seeing any obvious change in the resistance when the points open.
 I have a dial indicator that I would like to use to get an accurate TDC measurement, so the lack of timing mark on the flywheel is not much of a concern. I previously used this to set the timing on my Yamaha DS7 twin, and got very good results. On that engine the spec that was given was 1.8mm BTDC I believe. Manual does not give a spec for this, only tells you in degrees, which I have no way to measure accurately. It seems to me by eyeing it up that the points are opening way too early, but perhaps I am missing something? Don't want to run this if the timing is too far advanced, for risk of burning it up. Anyone have experience with this?

Offline OldSwartout

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Re: Ignition timing on BS 90 sport
« Reply #1 on: July 05, 2011, 02:05:13 PM »
Here's a chart of crank rotation  vs. stroke for timing purposes.  It works for 90, 100, 175 & 200 (all same stroke).

               Piston            Piston
Crank      Travel          Travel
Degrees     MM             Inches
18          1.343          0.053
19          1.495          0.059
20          1.654          0.065
21          1.820          0.072
22          1.995          0.079
23          2.176          0.086
24          2.365          0.093
Karl Swartout
Mooresville, IN
BS175 Roadracer

Offline reed

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Re: Ignition timing on BS 90 sport
« Reply #2 on: July 05, 2011, 11:24:32 PM »
On my Bridgestone 90 trail  i have a fire mark and a TDC mark on my flywheel it fires before TDC.
So try this ,get a strobe warm the engine and check your marks on the flywheel you might pick
Up the marks with the strobe light etc. On my strobe it uses a 12 volt battery so it gets a good
Light and if you are setting the points use a brass feeler gauge (Non magnetic).



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