Are you getting spark on all cylinders? If so, is it adequate spark? You can test this with a Spark Gap Tester. Those little xenon gas jobbies will light up with just about anything. But only a good strong spark will jump a 7/16ths gap. I'm guessing one of your coils is shot.

 

if it was spark problem you'd probably be getting misfire issues especially under load

the fuel priming bulb should relax a bit once the motor starts sucking fuel thru the line

integrated carb fuel pump I believe?

I am guessing a clogged carb or the diaphragm in the fuel pump has a small hole or tear in it and your fuel pressure is close to ok but volume or flow is low. Also check the fuel lines at the motor, they can get clogged too.

rebuild kit is like $60 http://www.maxrules.com/fixmercurycarbkits.php#9169

2 things usually cause your symptoms: old gas mixed with oil that percolates when it gets hot or doesn't stay mixed properly causing fuel feed vapor problems OR fuel pressure

also check for cracked fittings, the line to motor connection for example........they get cracked sometimes or the little brass/chrome ball check valve malfunctions or gets clogged on the hose connector

The last possibility and I have no idea what your carb(s) look like but many have adjustable mixtures and your high speed setting might be out of whack, probably not but it is worth looking at.Sometimes the fiber washers that provide friction to keep those things in one place dry up and quit working. Your intermittent power issue makes me think its your pump or a clogged high speed circuit in the carb.

 

I'm going with coil as well. Easy first.

 

take your tank line off ,look inside the fuel line, see if the liner has failed those gray hoses have a clear liner inside, been a problem, if it has you need to clean the fuel system, lines carbs pump .it just sounds like your not getting enough fuel.

 

I'll second bounty hunters suggestion as the 1st place to start.
Mine started doing the same. Would start instantly and idle all day. But would fall in its face at WOT. Bulb would stay hard. I replaced fuel line ass'y from portable tank to mtr.
Went back out and it ran better but still had issues at WOT.
Went home and started looking at the old fuel line and noticed the clear liner had been coming apart. Ended up having very small pieces of the liner wedged in the fitting on the mtr where the fuel line plugs in. Removed the small line from that fitting going to filter, cleaned out fitting and fuel line. Also removed and cleaned fuel filter on mtr. Unscrewing the filter canister, I noticed very small pieces of the liner in the filter canister trapped by the screen.
Thoroughly cleaned everything out and thankfully it ran like a champ. Didn't need to get into the carb.

One other thing that's easy to check if you have a portable fuel tank that can cause the same thing...check the fuel tank vent to make sure it's open and not partially clogged and check the filter inside the tank and make sure it's clean. Most likely since your bulb is not collapsing, these two are not the problem but doesn't hurt to check them.

 

Thanks guys, before I start throwing parts and money I really appreciate the suggestions, keep em coming!!

 

Darty carb and/or bad coil.

 

Try some Seafoam. Will clean carbs and valves. Cheap, easy, a 91 probably could use it.
Edit: no valves on a two stroke but it still helps.

 

Check the spark & compression first. The compression readings should be within 10 % of each other.If the motor has good idle qualitythe compression should be fine, but verify that with a guage to be certain. Spark should jump a 1/2" gap.....bright blue in color, consistent, & strong enough to hear it snap. An inline spark indicator is also handy to ensure that the ignition system isn't breaking down at higher RPMs. It sure sounds like you've either got a cylinder that is laying down or a fuel delivery issue. Once you have ensured that the ignition is firing properly (while cranking) you can also try doing a cylinder drop test when the engine is bogging down. In your case, a single carburetor is feeding both cylinders so a fuel delivery problem could also be present. You can remove the airbox/silencer from the front of the carb & restrict the airflow by partially blocking the carburetor air inlet with one or two fingers. Do this gradually so you can tell if it makes a difference. This should be done with the motor running at the RPM where it has started to bog down. If the RPM picks up you'll know that a fuel restriction issue is the problem. Mike

 

My in laws ran into this same problem years ago on their 50 horse evinrude. Took it to a marina and found the carb gummed up . Cleaned it and it ran like new. Seafoam may do the trick for you.

 

i also think a fuel flow may be the problem, if it was runnin good before and suddenly started acting bad back blow your fuel lines , and try a different tank supply if you can. other wise you can check the high and low speed voltage off your coils.. for that you would have to have or borrow a GOOD fluke meter. fuel is a good place to start, when you get into the electronics go carefully in diagnosing , because between the stator, triggers,coils,wires,plugs, it can be costly trying to put parts on by trial and error.