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90HP Mercury Outboard (not Mariner) Running at Half Power?

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ilarson007

New member
Hello all. I am posting because we are having issues with my parent's boat. It is a 2000 Starcraft that has a 90hp Mercury Outboard. The engine is a three cylinder, two stroke, and is card's (probably fairly standard in the marine industry, but I only have experience working with cars.

So the first time my dad took it out this season, it started misfiring. And the first thing we thought was bad gas. So it had half a tank, and we filled it up with 91 octane (it requires 89), and took it out, and it will only go like 12mph. If my dad goes out himself, he can get it up to like 30, because it planes out, but it's still not at full power.

It seems like it is firing only every other or every three times at idle.

Today, we pulled the plugs out, and they looked fine, and we are back out at the lake, and it still only goes 11/12. No power. We put seafoam in and filled up again from a half a tank with 93 octane this time.

So, what are the next steps? Nothing changed over winter as far as the carburetor or throttle cable or whatever. I'm just not sure what to look at next.
 
its a 2 cycle so check compression and make sure the fire will jump 7/16 inch to start...after doing that i would run it and use an inductive timing light to determine if it is a fire or fuel problem...you could still have a fuel problem if the ethanol gas set up over the winter..you could try a 6 gallon tank with new gas in it...another way to use sea foam is to get the motor running..then stop it and disconnect the gas line and start the motor with the gas line down in the can of seafoam...let it run til the seafoam kills it...let it set overnight and hook the gas line back up...get the motor running(it will smoke) and run it til all the seafoam is gone...your basic decision is it a fuel or fire problem....
if the motor set up over the winter wit untreated gas in the carbs then they will probably have to be pulled to clean properly...
 
papyson said:
its a 2 cycle so check compression and make sure the fire will jump 7/16 inch to start...after doing that i would run it and use an inductive timing light to determine if it is a fire or fuel problem...you could still have a fuel problem if the ethanol gas set up over the winter..you could try a 6 gallon tank with new gas in it...another way to use sea foam is to get the motor running..then stop it and disconnect the gas line and start the motor with the gas line down in the can of seafoam...let it run til the seafoam kills it...let it set overnight and hook the gas line back up...get the motor running(it will smoke) and run it til all the seafoam is gone...your basic decision is it a fuel or fire problem....
if the motor set up over the winter wit untreated gas in the carbs then they will probably have to be pulled to clean properly...
Click to expand...

Well, he did use stabil, the green stuff, but it was a different stabil then he usually uses. Also, he said when he winterizes it, he runs all the gas out of the carbs.
 
the green stuff is for marine use i think..at twice the price as i remember...i have gone to the treatment and cleaner sold by merc..its expensive also but the way i figure with the cost of the motor and the cost of using a boat at the end of the year it dont cost that much percentage wise..i am also running a computer controlled motor so in effect have to have anything fixed by merc...and that aint cheap so try to prevent it...
 
putting a higher octane gas in an outboard that calls for regular dont buy us anything as far as i am concerned...in fact the flash point on premium is lower than premium...so the timing is different..if he dont have a water separator installed i would install one...especially with ethanol...west marine handles them for 30 bucks or so including the clamps after you cut the hose to install it..ethanol makes water plus other junk that the filter will catch...if he has another external in line filter take it out..
 
We just took the flame arrestor off and there was has in the bottom of it. Not sure if that is normal for this motor or not. This engine requires 89, not regular. My thinking was out some higher octane in if the gas went bad to try and offset the bad stuff.
I belive this is a computer controlled controlled motor. At least it has a computer mixing the oil. Model number 1090412YD
 
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