40 HP Mariner too lean??

Now that I got this boat registered and wet I have another problem. Its a 14'6'' aluminum with a 1983 40HP Mariner 6E9-S-102886.m. It boggs down from 1/2 throttle and up. It must be leaning out because if I have a buddy sit at the back and give er a little choke its fine. @ full throttle it needs about half choke, then it rips right along. I cleaned and checked the tank, pick-up screen, fuel pump, filter, carbs (High and low jets , main nozzle, needle and seats, idle system, and checked float levels which aren't adjustable anyway). Primer bulb works fine but makes no difference while its acting up. Compression was 120 p.s.i. on both cylinders (cold). It idles great, starts great, and runs great up to half throttle. I looked at the reads and they look like new. The motor looks like new as well as the carbs, jets everything and has always been stored inside. I'm no 2 stroke mechanic but it is so simple its just gotta work after checking everythig. I must be overlooking something. I can't run it like this much even for testing.. worrying about burning up a piston. Any ideas would be muchly appreciated.

 

engine

my neighbor had a similiar problem. It turned out to be the fuel hose connection. At higher power settings it was allow air to be sucked which deminished the fuel flow. The motor sat for a year or so, took about a week of soaking and poking with a small wire to get all the passages/jets open in the carb so that might be another place to look.