A brand new BF20D stalls when opening throttle;
idle is running fine, by very slowly opening throttle the engine rpm increase.

if I open the throttle normally the engine stalls and stops.
A replacement with a new carburetor did not solve the problem.

The timing is exactly conform the description.

So any suggestions to solve this problem?

Sounds like you covered the two most obvious potential problems. You said this motor was new.

How new?

The accelerator pump is supposed to shoot a little extra fuel into the chamber when you accelerate. You can see the piston rod on the outside of the carburator on the side. In Part # 14 Honda Outboard Parts 2007 and Later BF20DK0 LRTA OUTBOARD ENGINE, JPN, VIN# BAMJ-1400001 CARBURETOR (AUTO) Diagram

is the rod that you can see coming through the little rubber bellows. You should be able to see the rod go down when you advance the throttle. In fact, you can take your finger and push it down and the engine should accelerate in speed...then the speed will come back to where it was.

It is highly unlikely that a new carb would have the same trouble as the old one.
Although, it could happen.

Have you checked your fuel for water?

Also, check to be sure that the oil level is below the full mark. If these motors have too much oil, they will bogg down.

So some things to check/do...

1. Check your fuel...better yet...try another tank of fuel.
2. Check your oil level...be sure the engine is vertical when you do.
3. Make sure the motor is not tilted down too far on the boat. This can cause acceleration and running issues.
4. Make sure the motor exhaust reliefs are well above the water level.
5. Make sure it has the right prop on it for the size boat that you have. If you have a fairly large boat and just using it as a kicker, you probably need a high thrust prop....about a 7 pitch. These usually come with a 10 pitch prop unless it is a high thrust model. If you have a small fishing boat (dinghy), the 10 pitch is probably pretty close to the right one.
6. Has it ever accelerated normally? If not....take it back to the dealer to get it right.

Mike

Thanks Mike for all your tips
But;;;
it appeared that the relation air-fuel mixing was not okay
closing the air intake for 3/4 solved my problem.

the real cause is a too small dosing of fuel when accelerating;
I have no dimensions of tubes and boreholes and no drawings on that of this carburator;
May be a dealer canhelp me with details?
I am based in Haiti and the dealer knows less than normal and that is nicely said. So here I can not get answers or technical specifications/drawings

who knows about this?