make sure your low speed carb jet has the little plastic bearing inside. If it doesn’t, the motor will do exactly what you are saying. These usually are either broken down or completely missing from these motors. Also make sure the larger plastic bearing (the one you can see which the needle initially threads into) isn’t cracked or just plain missing.

These two pieces are absolutely critical to make these carburetors work right. If either are missing, cracked, wallowed out, or worn, the motor will not run right no matter what you do. Low speed needle should be backed out approximately 7/8" to 1 full turn when everything is right.

Also check the leaf plates. I have dealt with several of these motors where the leaf plates are worn, and/or the manifold plate is wallowed from high use. These plates have to be 100% correct and seated FLAT on the intake manifold or NOTHING you do with the carb will make any noticeable pattern of sense. The motor will run erratically at all ranges.

So my advice after having chased that very same problem on many of these motors:

1. insure your link and sync is CORRECT
2. insure the low speed jet has both plastic bearings
3. insure float height is correct and all jets are open and clear (might want to use a good known carb for a little trial and error to speed things up)
4. if 1-3 doesn’t fix things, obviously check you fuel pump screen and tank pickup.
5. if #4 is OK, shine a flashlight down the intake manifold (carb removed). You can see the top leaf plates. If you see ANY gap in those plates, remove the powerhead, intake manifold, and inspect. You can see my website where I actually show a 15hp OMC with this very problem (will post more pics this winter…have even more from motors from this season).
6. make sure your cylinder head is torqued down to the correct ft-lbs. I had a motor where it was actually LOOSE (from another mechanic), and the motor would not run much above 1/4 throttle (immediate stall, only ran with 1/2 choke, and WOULD NOT IDLE DOWN LOW).

And my final, and maybe most important piece of advice…USE AN ELECTRIC DRILL GUN TO START THE MOTOR VIA THE FLYWHEEL NUT. (eye protection in place, or course) Yanking on that damn manual starter will accelerate your blood going to a boil!!!!!!!