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2002 75HP 3 cyl Mercury Outboard bogs down at full throttle

N

Nicculus

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Hello all! New to this forum and just bought a used McKee Craft bowrider. My issue, to start, was the boat only went trolling speed when full throttle. Found there was an issue with the fuel line leaking at the coupler that couples the fuel line from the gas hand pump to the short fuel line going into engine. I fixed that. I noticed the air/fuel screws on carburetor (this has a carb tree. 3 stacked) were possibly out of whack. One screw was 1 1/2 turns from seating, one was 14 turns from seating and the other was like 5 turns from seating. I got those in sync while using muffs at home. I was ready to put it in the water to adjust them with a load on the prop but before I left, I fired the engine up on the muffs. I noticed gas slowly leaking out the open hole on the carb, just to the left of the air/fuel screw (see attached pic). Just to test, I turned all air/fuel screws on the carb clockwise and lightly seated them. I thought the engine would choke out but it didn't?!? It may have been running higher RPMs when all 3 were closed. So, why didn't the engine choke out with all 3 air/fuel screws closed? Also, why is gas trickling out of the open hole next to the air/fuel screw? Thank you for helping
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Since nobody hit on this one I'll take a shot. That may be a mounting hole to support a screw or stud for holding an external part. I have been running these carbs in 3 and 4 cyl Merc 2 strokers in this hp range for over 15 years and never had fuel come out of that hole.

Since you are having other problems and your jets were so messed up in their setting, tells me that the PO was having fuel problems.

My suggestion is to get 3 carb kits for your engine and mark your carb positions...123 as the high speed jets are 123 position sensitive..usually on these engines; 1 is the top one. Upon removal, and removing the top plate during your cleaning, inspect THAT hole on all 3 carbs and see if there is any difference.

On cleaning, check for small black particles of rubber indicating that your engine internal fuel lines need replacing. If you find any, ensure that you use a good cleaner and when finished, blow out all parts with a good dose of 100 psi air.....I found a mysterious problem once with the air.....had a tiny piece of black rubber hose was stuck down in the pit, at the bottom of the bowl ,where the high speed jet is located...was intermittently blocking it.

Then get a new engine fuel filter and fuel pump kit. If you haven't changed your squeeze bulb and associated "engine to gas tank" hose assembly throw that in the to-do list. Cut your old fuel filter open and if you find any tan chips in the filter it's from the fuel line with the bulb, and any black is from internal engine fuel line degradation.

Manual says to start with all 3 carb low speed jets 1½ turns open from a 2 finger soft closed position. Then get on the water and evenly, and a tad at a time, open up the jets until you can firewall it in the hole shot and the engine will take it.

If this doesn't help anything it surely doesn't hurt anything and probably well time for the upgrade.
 
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Thank you very much for the answer! With a little searching, it seems as if that hole is an air vent hole. Not sure why gas was coming out of it. This morning, I dropped some seafoam in the gas tank and hit the air intake on all 3 carbs with a full can of carb cleaner, while bumping the RPMs up a smidge from idle (on land with muffs). I didn't see gas leak out of that hole today. The engine sounded great today. About to put it in the water to adjust the carbs while in water. If this doesn't work, the carb tree is coming off and I will get the carb kits, like you mentioned. New boat owner so I have a lot to learn. Thanks again.
 
Nicculus said:
Thank you very much for the answer! With a little searching, it seems as if that hole is an air vent hole. Not sure why gas was coming out of it. This morning, I dropped some seafoam in the gas tank and hit the air intake on all 3 carbs with a full can of carb cleaner, while bumping the RPMs up a smidge from idle (on land with muffs). I didn't see gas leak out of that hole today. The engine sounded great today. About to put it in the water to adjust the carbs while in water. If this doesn't work, the carb tree is coming off and I will get the carb kits, like you mentioned. New boat owner so I have a lot to learn. Thanks again.
Click to expand...

If you got out the SF and the engine responded favorably, go to a www search engine and look up the word DECARB. Do the decarb and clean or better yet install new plugs. Then go out for your water tests and after you get SOME improvement and have given the SF time to continue to clean out the carbon in your engine, redo your carb on the water setup. Glad to hear you are moving forward and impressed that SF was part of it. I have been using it in my OB engines, every fill up, since 2004.....having found out about it. Use it in all my engines, large and small, 2S, 4S and diesel.
 
Yup, put a second can of SF in today (two full cans in 21 gallons of ethanol free gas). Got out on the water and it was a tiny bit better but still bogged at full throttle. I was also able to hook up my buddy's gas can to it and it ran the same, bogged at full throttle. So, I know it isn't the fuel, lines, or bulb. I am getting 3 new spark plugs tomorrow. Other than that, I am not sure what to do short of pulling the 3 carbs off. I was hoping to exhaust all avenues before I take that on. Any more troubleshooting tactics?
 
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I bought a used boat last year with a 2002 115 HP, same series just 4 cyl as your engine. After spending a lot of money on this and that trying to get a feel for what to expect from that engine and boat combination, and what performance I'm getting, in retrospect, seems that the problem was a clogged fuel filter primarily and afterwards a fuel pump diaphragm that I feel that had gotten stiff and wasn't pumping as it should......just the finishing touches on low fuel flow. The fuel filter was full of tan chips which I had seen in my 2002 90 HP 3 cyl when I had it. I found them in the fuel line connector at the engine input (removable connection on that boat) and they had stopped up the line at the inlet connector. Thought it was the hose deteriorating but after replacing the hose and bulb, I sliced the gray, slick (silicon???) hose open and it was slick inside. I was running unleaded in that engine with SF.

Knowing the condition of your sparkplugs is always a good idea. I didn't respond to this exact post on another site that I frequent.
 
Thank you very much for taking the time to reply. That was a lot of great information. I am just going to go ahead and change the fuel filter and check the fuel line coming into the motor for trash. Attaching a pic. Is the 3rd item the inlet connector? I'll report back!

Thanks again!
Nick
 

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I don't know. My 90 was purchased with the boat were used and the PO had the fuel line permanently attached to the engine. What you see are male and female connectors used on older Mercury's for sure. Just don't know how long Merc. kept using them and on what engines. If you buy one buy male and female if in doubt. Fuel line for the 90 is 5/16 minimum per spec.
 
Changed spark plugs. I haven't received the fuel filter yet but I took it out on water and it is still bogged. I just went ahead and ordered 3 gasket kits and am going to tackle the carbs.....never done it but it seems fairly "easy" with my motor.
 
I ended up taking the carbs off and cleaning them. There was some gunk and trash in them but not crazy. The old gaskets looked rough and that might have been the issue. Swapped all gaskets. The carbs are brand new again. About to take it for a water test next chance I get. I am hoping this resolved the issue of bogging down when full throttle which I end up at a trolling speed.
 
If the carb had black particles in them replace the engine fuel lines and diaphragm. If tannish/brown replace the line from boat to engine and replace diaphragm also
 
I am happy to report the boat went full speed with fully throttled!!!!! It was the carbs. Most likely, the gaskets were dilapidated and allowing air in. The old fuel filter only had 1 black spec in it. The previous owner may have swapped it before I bought it, though. Felt good to be going full speed! This is my first boat and I have never messed with carbs on anything so I am ecstatic. This motor runs like new again and I got a hell of a deal on it. Thank you to everyone, especially Texas.
 
Glad you got it running and on never having done the carbs before, my motto is "nothing ventured, nothing gained............or lost". 42 years ago I transplanted from a city boy to a country bumpkin. That motto has been my guiding light and is still on the plus side. The other thing I learned was that what's behind you helps you to have the courage to deal with what's ahead.

On what your PO did to a piece of equipment, I had the same wonder on my current boat. Not too bad considering.
 
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