mk13

Yes you have right, with oil in gasoline the bubbles are significantly smaller (but there is lot of small bubbles).

I use a synthetic oil for 2 stroke from my supermarket (I can't remember the brand).

Yes I have for the moment the open curve from Guillaume. I will try with the solenoid electrically disconnected to set the needle a full throttle and after set the solenoid curve.

I will check the exhaust pressure line tomorrow if I find some leak, and will do the test of disconnect exhaust line from the muffler at full throttle.

​​​​​​Thanks a lot for all your way of investigation!
I hope I will find the root cause in the next days 🤞

1967brutus

Good luck testing, I am pretty sure we;ll find a solution, and this is exactly the reason why we should do this in the open discussion, not via non-public private messaging.

Let the learning continue!

1967brutus

Something completely unrelated to your problem, just sprung to mind: You are basically, of all people involved, running the smallest engine yet in this entire saga.
I have converted an OS FP10, and when doing so, I pulled the head and noticed that these small OS engines have a rather thin cylinder head. For the FP 10 this resulted in a sparkplug that protruded way too deep into the combustion chamber.
Please check if this is the case with your .25 engines, and if so, add an extra washer under the plug. The threaded base of the plug should be, as a minimum, flush with the ceiling of the combustion chamber, or even slightly recessed, since the electrodes will be pretty close to the piston top otherwise.

It has nothing to do with the bubbles, but a protruding spark plug will negatively affect the scavenging process of these tiny 2-strokes.

Jim.Thompson

That makes sense, I should have guessed. Thanks.