Runs with choke out, not in. Already tried a lot of things

I have a 4-stroke outboard Honda from 1984 which works great when the choke is pulled out (including proper idle and the full range of different rpm), but when I push the choke in, the motor gets very unstable in terms of rpm and can't hold idle, it "suffocates" at lower rpm, unless I pull the choke back out.


Other things to note:
- I carefully cleaned the carburetor
- I cleaned the spark plugs with thin sandpaper
- The tank is made of plastic
- The o-ring at the fuel intake is OK, no leak
- I haven't used the boat for a couple of months, so maybe the gas has lost its octanes
- I haven't changed anything to the various adjustment screws, I just tried different adjustments for the needle at the bottom of the carburetor (fully screwed in, 1, 2 turns out), which doesn't solve anything


Thing is, this motor runs well when the choke is out, so, that already excludes a lot of possibilities

Thanks for your hints

You don't say which motor.

These carbs are difficult to clean.
You need to soak them in carb cleaner and then use something like a fine wire to clean the passages.
I use a cutting torch tip cleaner.

Sorry about that.
The model is B100L

Ok, I'm still going with carb needing cleaning.
They are finicky.

I agree with alumarine. Honda carbs are finicky when it comes to cleaning them.
But you need to be cautious about the type of solvent you use to clean them.
Hondadude says DO NOT USE carb spray but to use brake kleen so as not to damage the plastics and coatings. He is in Ohio.

I'm in California and I use products labeled carb spray and have had no problems. But I'm positive carb spray that is legal in California is much milder than carb spray in Ohio. I can't even find Berryman's carb soak kits anymore but you absolutely DO NOT WANT to soak a Honda carb in that stuff! It will come out really clean but about a third of it will be melted.

You should have a BC carb. Stamped on the carb will be two letters. Yours should be stamped BC. And with BC carbs there are a couple of things you can try before taking it off and cleaning again.

First, these carbs have two passages on the top that are open to atmosphere. This is to keep ambient pressure in the float chamber to help "push" the fuel through the jets. Keeping them clear also prevents a vacuum occurring in the float chamber. Locate those two passages and flush them out as best you can.

At the opening of the carburetor "throat", near the choke plate, is the opening of a horizontal passage called the slow air jet. The passage is about 1/8" or so in diameter. You can put the spray straw from a can of cleaner in that "tube" and blast away in there.

On top of the carb is a sort of plug with a slot head. Use a slot tipped screwdriver and remove that plug.The "plug" is actually the slow jet and receives air from the passage mentioned above. In the cavity that is exposed with the slow jet removed is the junction where the slow, transition and high speed circuits all converge. Minerals and dirt can accumulate in there. Blast away in there and then put the jet back in and try running her. If no difference then the carb should come off and some other "techniques" will need to be put into play.

One other thing I should mention is that this symptom could also be caused by the supply line sucking air or the tank vent being partially plugged or not open completely. The supply hose can get a pin hole in it and suck air. This starves the carb of fuel and necessitates pulling out the choke to keep her going. Another place to look is at the fuel line quick disconnect. Sometimes they don't engage and lock properly. Sometimes they are just worn out. The pin that pushes on the inlet valve gets ground down due to vibration. This shortens the pin so that it doesn't completely open the valve and, again, the engine starves for fuel.

Good luck.

Thanks for the very detailed answers.
As a follow-up, it was indeed the carb.
I had only cleaned the most accessible part, but I left it to a pro to clean the jets and it solved the problem.
At least I learnt a lot.
Thanks again.

Ok, thanks for the update. Glad to know you got it going.