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hello everyone


i am have issues with my 1975 yamaha rs 125.

I found the bike stored in a friends rental accommodation a while back and chatted to the previous tennants about it. It had been standing in the same spot for approx. 15years untouched and very unloved. This didn't sit rightly with me, so when the owner told my mate that the bike would never run again AND that i could have it for mahala (free) i just jumped at the oppurtunity to breathe new smoking life into it. I set about on a mission to restore the thing (which i soon realised was a fools errand) due to the rust and general bad state of the bike. After much pondering i decided that i would customise the little guy, which i did.

This was all a little while back, the customisation is now sort of complete, but i am having issues with the little guy. Unfortuantely when i got the thing it was in a few pieces and i cannot seem to find any information on the bike whatsoever. This waffle now leads me to my point.

I have never, and still do not know the correct timing setting for the bike. This cost me a piston earlier in the build, but that could also be due to the fact that i forgot to tighten up the points properly one time:Blackeye:. I have managed to find the specs on the rs100 engine which were 1.8mm BTDC which i used to set this bike. the issue i am having now is that the bike will run lekker (fantastically) on full throttle at top speed for a little while, maybe 10secs, and then it breaks down into a coughing and spluttering fit of missfire and general bad behavior. as soon as i release the throttle and gas it again it will run sweet for another 10 secs and then same again. oh, i should mention that i designed and built and expansion chamber for the little guy, but to the best of my memory it has always has this problem in some way shape or form.

SO

in my mind the most likely issue is timing, but there are a few other unknowns in the equation due to nothing being set before i got it or due to me fiddling around, these are: oil flow from the pump, mixture (i opened up the main jet a touch) carb float level and centrifugal advance on rotor arm.

Being a much older 2 ticker, it runs points and condenser so it has 2 timing adjustments (one on the base plate above the stator and one on the points itself


rs125.jpg
 

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Start with the basics.

Ensure fuel cap is venting. Petcock and screen are clean (and tank). Is petcock flowing fuel adequately? Carb spottless? vent tubes open? Float level set? Fuel flowing into carb bowl fine(fast enough)?

Clean all electrical connections, add dielectric grease to them. Make sure ALL grounds are clean bare metal, including coil mount. If plug boot is removeable trim 1/4" off wire, install boot with di-grease. Ohm coil see if it's within specs. Make sure point gap is dead on, and they are clean and flat contacts. Have you ever replaced points and condenser? Spring might be weak and you're getting bounce.

Next try backing timing off. .1-.2mm, maybe even more then sneak back up on it. How does plug look with plug chop/read? Maybe try larger main. Disableing advance might tell you if you have too much timing. or maybe mech is allowing to much advance/ too quickly. Try a new condenser and/or coil.
Can you tell is it misfire or detonation happening? Has the spark plug ever been loose when you went to check things?
What octain fuel you using?

And the last thing you want to hear is it might be the design of your pipe.
 

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On my bike I can feel an extra vibration when the timing is too advanced. So just play with timing settings till you feel the extra vibration and then retard it till you don't feel that.
 

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also on a regular (not fine tipped) plug there should be a clean ring next to the end of the center electrode about .5mm in length. More than that indicates too much spark advance.
 
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