115 Mercury stalls mid throttle

I have a 1985 115 mercury that has been worked on twice by a mechanic with no luck. In the driveway in runs great in gear and out. At the lake it revs in neutral but put in gear I only get about 1800 rpm and dies if I give it too much. Fuel pump diaphram?

I would start with a carb clean and fuel line check/replacement. Fuel lines could be collapsing and restricting flow.

Put motor in 250gal tote, finally got it to fail like on the lake. Pulled fuel line successfully pushed air. Put in can 50:1 and seafoam with no change. Working up the line, I have diaphram ordered. Hope to put it on Sunday.

Does the fuel primer bulb build adequate pressure? Have you tried squeezing the fuel bulb while the problem occurs? If fuel system checks out all okay, then there may be a problem with spark interruption. Happened on my 1986 115, I almost got caught in a fast moving severe thunderstorm. There is a limiter built into the spark control unit, keeping rpm's under about 3000. I replaced all components with CDI equipment. Motor has been perfect now for over 10 years. Be sure to avoid any ethanol in your fuel.

Yes I tried pumping bulb also. Doesn't seem to be from the tank to the diagram. It should be in Friday so I can put on Sunday. But you are right it seems more electrical to me. On muffs it runs perfect but stuff it in water and it will only give me 1800. This is my first boat I will look up cdi components but if you could expand a bit on that it would be great. Also were is the spark arrestor? Guess I need a book for this motor. I'll try to find one somewhere.

Check for spark.----If it is an inline 6 cylinder then 1 switch box may have failed.----Running on 3 cylinders I think.----And a 6 cylinder will sound great on a hose with 3 cylinders firing.-----Some simple trouble shooting finds the problem Quickly !

good call. the mechanic just replaced all 6 but will double check before run I it after replacing the diaphragm. thank you

update: new fuel line and bulb, new diaphragm, new fuel and perfect mix all with no change. so today checked timing.... it is set according to the book (4-6 deg. BTDC) but the initial pick up according to the sticker on the motor says 6 deg. ATDC. I adjusted according to the sticker and smoothed out the idle. pulled fuel line and pushed full throttle to check max spark at cranking speed looking for 21 deg. and the adjustment went to far out so would go into idle because linkages were bumping. I decided to double check pickup and it was missing bad so stopped engine and water wiped all plug and blew out chamber with air hose. restarted in rpms acting crazy timing could be check because high idle, smoke started boiling from lower.... forgot to turn water back on! i think i had it figured out timing was off. now waiting for new pump over holiday weekend. QUESTION:

QUESTION: Do the plugs need to be removed and the wires grounded to get proper max spark timing setting?