My YS120 engine does not idle well. The RPM will remain high (4000) for a few seconds then drop to 2500 for a couple seconds and jump back up. It does this cycle a few times then dies.

air leak-gaskets needle o-ring etc


gary

Lightly wet sand the contact surface of the valve cover, sometimes it has a high spot that cause the leaking. Otherwise, check for hair line splits on all fuel & vent lines.

Adrian

If its a 120AC the o rings on the air chamber need replaced about every 200 flights.


Jim

I'd like to add that it runs ok with the glow plug battery attached

i'm sure you changed the glow plug? ys#4 os -f

gary