OT: 1973 2 stroke dirt bike

JOLTRON

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Join Date
Aug 2006
Location
MI
Posts
692
You guys have helped me threw so many other issues in life I figured I would throw this one out there. I have been given a old 2 stroke enduro bike that was torn apart by someone that knows less about bikes than me. Not a good thing :) Anyways of the side of the came is a spark advancer plate with points that need to be set and the position of the plate controls the timing.

Anyways the cam was removed and not marked as to what position it was it.

So the question is:
If the cylinder is at top dead center how do I know what position the cam is supposed to be in?

Any help would be great, I'm having a hard time finding a rebuild guide or manual for this old bike.

-Joel
 
Yeppers SL 125, I have posetd the question already at the first link there bikez.com but I don't think that site gets much traffic. I tried a 2 stroke forum but they didn't like my gmail address for some reason.

Oh well, I guess I will have to guess at the cam possition. If the piston is at top dead center then I only have 360 degrees of choices :)
 
Well if I understand correctly, you're talking about the ignition points cam? You can "guess" a whole lot closer than 360 degrees.

The spark is created when the points open and the field created by the primary side of the coil collapses across the secondary side of the coil.

The relative position of the piston to top-dead-center is how the "timing" is defined.

Every small engine I've ever worked on had a "timing" setting of somewhere between 3 degrees before top-dead-center and 15 degrees BTDC. So you can at least get THAT close by observing the exact point at which the points crack open relative to the piston.

There should be some sort of scale or degree marks and pointer on the flywheel that you can use a timing light on.

If you can get it RUNNING, you can then experiment with the timing to see where it performs the best. Hopefully by THEN you will have heard something from the other forum.

You might try an Ebay search for "motorcycle owners manual". You won't likely find the exact ONE, but you may get some names of collectors, enthusiasts, or Ebay stores that trade in them.

Stationmaster
 
Thanks for the advice I will look into that today. I'll check on Ebay and see what I can dig up. I wouldn't have thought of that my self. Thanks.
 
Stationmaster's 3° to 15° sounds a bit too little advance for me, my guess would be more like 20° - 25°.

Five minutes of googling turned up the fact that the points gap should be 14 thou but unfortunately in that time I couldn't find a figure for the advance. Browsing the remaining 8900 hits which turn up when you enter "Honda SL125 timing" in Google, will almost certainly provide the answer, though! :)

Once you've got a basic figure, there's a good description [URL="http://oldmanhonda.com/MC/Rtiming.html[/URL"]]here which explains nicely how to do the final tweaking of the timing using a strobe light.

Good luck!
 
Last edited:
Just in case you wanted to know...

I was a Mercury/Yamaha marine mechanic for years...

Most of the CDI and earlier systems (Contact-point Ignitions) fire at BOTH of their respected TDC and BDC positions.

So, with that information in hand, use it wisely....
 
After talking to a few people and reading all these post's I'm coming to realize that the 2 stroke bike my friend gave me is actually a 4 stroke. I found a repair manual on ebay and ordered it since I think im to the poing of being over my head so I may tinker with her a bit more but nothing crazy till it shows up. Thanks for all the help and advice so far!📚
 
Just tossing this out there...

My 76 Yamaha RD400C 2 cylinder, 2 stroke street bike uses 2 sets of points, and the timing is set with a dial indicator installed in the spark plug hole. The spec is in mm before TDC.

Your SL (that's the one with the 2 pipes up high on one side if I remember correctly) may have a similar arrangement.

BTW, when you said 2-stroke, I was confident you could put it back together even if it was in a box. But a 4-stroke has a lot more small parts to misplace.

Good luck with the rebuild.
 
That particular engine is fun. It's the first motorcycle engine that I ever worked on.

The biggest problem with it is that the timing chain has a tendency to war out the gear and jump off. The good thing is it's not a interference engine so the valves should never get bent.

IIRC you pull the lower cover off of the gear housing and line up the marks on the main gear with the marks on the cam gear. Set the points to 0.014 and off you go.
 

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