1967brutus

Mind you: for lubrication it is not strictly necessary. Any decent sythetic oil will do the job.
The reason for castor is its tendency to create teflon-like deposits on the hottest parts, which will be the running surface of the piston and liner. This will basically "replace" the pistonring that these engines do not have.

mk13

Yes I understand for the effect of the castor oil like a little piston ring. Thank you!

The seal on the solenoid, it's aim to separate the inlet from the outlet of the valve, or just avoid leaking from the top of the valve?

I try a lot of things this morning, change again hose, change tank location, change position of the valve (outlet up, outlet down,...), test all the fuel line to search any leak, but nothing form me solve the bubbles problem.
I noticed when the outlet of the housing of the valve is down (toward the ground) and hose came up to the carb, I have no more micro bubbles, but every ~10s a big bubbles, like if micro bubbles agglomerate in one big bubbles.
And always no bubbles from the hose between the tank and the valve.
I'm thinking there is a micro leak but where? May be if I worn the seal during the assembly of the valve with the housing?
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1967brutus

What happens if you install the valve with the outlet to carb pointing upwards?

The microbubbles form due to cavitation, the valve opens and closes and this creates tiny shockwaves in the fuel. If the fuel has too much volatile components, this creates microbubbles.
Indeed, these can agglomerate into larger bubbles.

The O-ring seal only seals the outside of the solenoid, it is near impossible that this O-ring is leaking, but did you push the solenoid in all the way? There is a square ridge on the solenoid, and it is imperative that this ridge is in full contact with the metal of the housing..
Did you mount the solenoid with a thin smear of vaseline?

What you can do, this is a bit of a crude method, is to mix a bit of fuel, and leave the container open for half an hour (to let the most volatile components evaporate. Then test that fuel. The engine might be a bit harder to start, but should run, and chances are, this will cure the vapour issues.
Normally however, the volatile content of gasoline is tailored to the climate and the season (wintertime, gasoline is more volatile than summertime) and I have not even seen this issue in tropical climates (one of my own planes is located in Indonesia, and functions there on the local fuel exactly the same as it did in wintertime the Netherlands, did not even need to adjust the curves...)

Which exact kind of fuel are you using? E10, 98 or Alkylate fuel?

mk13

If I put the valve with the outlet up, there are continuously micro bubbles that go into the pipe and finally agglomerate into big bubbles.

Yes, I put the solenoid in the housing (contact between the square edge and the housing).
I mounted the solenoid with oil (10w40 for car engine).

I agree with your explanation about the shock wave created when the valve turns, but is it surprising that I am one of the only guys to have this problem?

I use the fuel of my lawn mower, 95E10 + 10% synthetic 2-stroke oil (I think this fuel is 3 months old)