1985 15hp Evinrude, cant figure out hesitation when trying to accelerate.

Been having this problem for several years and cannot figure it out.
1985 Evinrude 15hp E15RCOB
Motor starts and idles fine and can put around all day. I can get past the hesitation on acceleration by pulling the choke momentarily while turning the throttle. If I don’t do this it will stumble and finally die. Back to neutral and it starts on 1 pull. Acts like it is low on fuel in the bowl. It only does this under load, not while out of the water running on muffs.
Went thru the carb a couple of years ago without any change. Replaced the fuel pump last year and no change.
Today I pulled the boat and tested in the driveway.
110 PSI on both cylinders.
Checked fuel pump and runs at 1.5-2 PSI at Idle. 2.5 at higher RPMs.
Fresh gas 91 octane non ox mixed at 100 to 1.
Made sure idle is smooth with idle mixture adj on front.
Would not exhibit symptoms in forward or reverse in the driveway.
Put back in the lake and stumbles almost every time after returning to idle.
Itdoes this whether accelerating slowly or fast.
I work on a lot of motors and equipment and cannot figure this one out.
HELP ! What am I missing?

Thanks,

Well 100:1 was a blunder.----They marketed the concept for a couple of years.----There was a service bulletin to go back to 50:1 on them.-----Open the low speed mixture needle 1/8 turn at a time and see if stumble goes away.---Also check that throttle plate is not opening too early.

Thanks for the quick reply. Is there an adjustment screw for the throttle plate? I will go try the opening the idle micture a bit more right now.