i have put new plugs, new bulb, new hoses, and it still runs really rough and falls on its face right when it plains out. any chance on of the carbs needs to be adjusted? i thought it was sucking air from somewhere but can’t find it. any advice would help. thanks. 
fuel line… fuel pump….???
If it gives rpm’s then maybe the prop hub is spinning (kinda like a shear pin but lets you limp home)
You could check the resistance of the primary and secondary side of the ingintion coils. Using an ohm meter check from the plug cap to ground and compare to other three. Then also check the input side of the ignition coil to ground and compare to other three. This is not a perfect way of checking but close…
Does this motor still have points? If so I would replace… Isn’t there also some type of ignition advance system? Might need to bring into a trusted shop…
i have replaced the fuel line and primer bulb. still nothing. it has fire on all cylinders and 125 lbs. of compression on all cylinders. going to try adjusting the carbs while running. if that doesn’t work probably take it to a shop. 
test the spark then rebuild carbs and replace all fuel lines on the head too. 1978! Might need a tune up every 32 years or so.
lol. yea i guess its probably time for a check up. the motor is 2 years older than i am. when i get the bill i am going to get 
Take your motor and run it.
When it “falls flat on it’s face”, shut it down, pull the cover and pull a plug.
Is the plug dry?
I know you replaced lines, bulbs, plugs, etc.
However, have you checked your fuel filter/screen?
Sounds like there is fuel until you wind it up at full throttle, then you are coming short of fuel when you get there. Sounds like it can’t get the fuel to keep it going.
If it was running fine earlier this year I wouldn’t try adjusting the carbs. Something changed, just need to find out what that was (maybe dirt in the carb, cleaning them would be the most I would do). Does it shut down completely, any misfire or backfire, keep idling. Does it run ok at low RPM’s? I just figured out an issue with my kicker, it idled great and started to miss as rpm’s increased…it was a loose ground wire, found after several carb cleanings and a few other checks. Good luck and let us know what else works or doesn’t. Where are you located, maybe someone could from here could help you out, it’s always nice to have a fresh set of eyes on it.
I don’t mean to confuse with all the other suggestions, but What shape is your fuel in?
How old is it and have you tested for water contamination?
gary, i will try that. even just running it with ear muffs on. it just doesn’t have the throttle response that it once had. this problem happened all of a sudden. ran like a top all year this year and last year. was on the water all day when it happened.
onestout, it starts and runs kinda rough. then when i start going it plains out and right when it plains out it chokes out and dies. i will try adjusting the carbs and see if one of the adjustment screws rattled loose.
herb, how do you test for water contamination in the gas?
oh, i keep my boat by desoto.
disconnect the fuel line at the engine and using the bulb pump some fuel into a clear glass and see if it seperates.
I agree with Gary, look for things line fuel filter, pump, dirt in the fuel, etc.
thanks everyone for the help. i wanna learn how to work on boats so i know what to look for if i have trouble. 
Lots of good advice here. My advice is to haul that thing down to Starks and have them tune it up. They will have you good to go in no time and they are a good shop. Usually they will let you watch them work on it if you set an appointment. Why waste good fishing weeks trying to figure out an issue when you can get someone who knows old mercs as good as anyone do it for you.
Reason why I mention the fuel “screen”, is that I had an old Merc that acted the exact same way. At the time, didn’t know there was a fuel screen.
When I opened it up, it was 90% clogged.
Cleaned it up, and worked like a charm.
Another way is to have a 2nd person squeeze the primer ball when this is happening to see if that fixes it, which again will prove fuel starvation.
i have checked the fuel screen. (bottom left side of motor) and it was clean. unless there are more than one of these screen traps.
sounds like a stuck float… I wouldnt try to adjust the carbs. They dont just go out of adjustment, something goes wrong. Bad gas, water in gas, sruck float, or bad needle. Is it a built in fuel tank in the boat? If so, where is the air vent? I had an older thundercraft with a 115 merc and it was on the outside of the hull and after a rough day on LOW it died from water intake through the vent. had to move it into the inside of the boat
the boat is a 93 bluefin spectrum. and the tank is built in. i don’t know where the air vent is. never seen it before.
can you get to the drains on bowls on the carbs?? Its the single nut on the center of the bowl. see if you can crack them loose and drain the bowl into a jar and look for water in the fuel. and like posted before take the fuel line off and pump fuel via the bulb into a big jar and look for water. if you can drain the bowls and borrow a portable can with fresh gas to run on that will tell you if your inboard tank is ok so you can isolate that out. my guess is water in the tank/carbs or stuck float/needle in the carb. What kind of fuel do you run? It pays to be picky with your fuel
started it up with earmuffs on and it was running rough. loosened the gas cap and sure as sh*t it was running like a top in no time. thanks everyone. just need to clean the vent out. thanks everyone you all helped alot. 
It’s always nice when it’s something that is easy to fix yourself and doesn’t cost a lot (relative to other things it could have been). Good job finding it.
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