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@pdlouche
Small update on the GN125 saga. I believe that I have found and remedied the cause of the running problems.
Recap: from cold would start on choke then drop shortly and sputter and could stall out unless dropped to half choke or less, and even warm was idling bad under 2k, was lazy off the throttle, occasionally stalled out when idling or (engine) braking to a stop at a junction, seemed like a lag in throttle response, and most recently 50/50 refused to start up after parking when warm.
Seemed like a lean/rich issue but spark looked okay. Battery health fine. LED lights so limited load on electrics. Maybe a dirty carb so cleaned it yet again. Could have been tight valve clearances so not enough opening when hot, but when checked they are bang on spec. Was also suggested could be poor compression and possible rings need replacing. Do not want that.
Rings made me thing o-ring. O-rings, the donor carb. Donor carb with wrong sized jets and needle had a plastic o-ring type cap on main jet needle. Mine never had.
So since ownership in November I have never managed to work out that the issue was a bastard o-ring missing from the main jet needle has been absent. Part #32 in this diagram. I assumed that it was complete, since it was running but running poorly.
Took the grey one off the other needle shown in photo (correct needle is lower one missing it).
Result:
Choke start nice fast 3-3.5k rpm steady and no drop after a minute or so. Dropping to half choke a clear change in revs, and steady idling at 1.5k-ish when warm. 30-odd miles in very warm British weather and there is a positive response from the throttle, it seems to have remedied the lag and laziness, and the bike starts up when hot. Might be jinxing it, but hopefully that’s the issue sorted. Could probably get it idling under 1.5k at maybe 1-1.2k. Phew.
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