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  1. #1

    Default 93 Mercury 75hp Loads Up

    Hey guys, I have been struggling with this for a while. I finally narrowed it down to the bottom cylinder, it seems to be loading up with carbon and oil? Oh and I have a 3 cylinder. I am hoping it's just a bad plug, but they are all new. I know that doesn't mean that it's good. So, I'm going to start there. Just wanted to see if anyone else has had a similar problem and could maybe provide a solution?? Any help is appreciated....

  2. #2
    Join Date
    Sep 2010
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    21,965

    Default Re: 93 Mercury 75hp Loads Up

    The trouble shooting starts with a compression test.----Post your numbers----Then check for strong spark ( able to jump a 3/8" gap ) on each lead.---If those are good then look for fuel issues.----Could be a ruptured fuel pump diaphragm.

  3. #3

    Default Re: 93 Mercury 75hp Loads Up

    My compression is 130lbs across all 3 cylinders. I've had the entire fuel system apart and cleaned as well as replaced all fuel lines and I checked the diaphragm in the fuel pump during all that too, it was in good shape. I checked the spark as you stated and it has beautiful bright blue spark on all 3 cylinders. (side note) I have replaced all the electrical components, coils, wires, plugs, stator, trigger, power pack. I've cleaned all 3 carbs. I couldn't figure this out, but I finally narrowed it down to the bottom cylinder giving me the issue. It just seems like it's being given too much oil or something? It idles fine in the water, but once you put it at WOT, it bogs down and won't go past say about 3k RPMs? Mercury's are known for stator problems usually being the culprit of this type of problem from my experience, but this one has got me stumped....

  4. #4
    Join Date
    Sep 2010
    Posts
    21,965

    Default Re: 93 Mercury 75hp Loads Up

    You have spent a lot of time and beer tokens I would say.----The bottom cylinder can not get too much oil by itself.----The could be issues with oil recirculation system.-----Or perhaps cylinder sleeve position. ---Or perhaps the bottom crankshaft seal.----Trouble shooting required on your part.----I would remove exhaust cover and have a look in there at pistons / rings.----You have replaced water pump impeller I hope.

  5. #5

    Default Re: 93 Mercury 75hp Loads Up

    Yeah, definitely spent lots of time, money and beer tokens on this motor! I have not done anything as far as checking internal components yet, but I suppose that's my next step. Just for giggles, I'm going to replace the spark plug first just to eliminate that since it's a simple, cheap thing to do? I have had bad plugs right out of the box before, so I'm hoping (fingers crossed) it might just be a bad plug!

    Not to question your knowledge on this, but, would the rings be bad and still give me good compression? I know there are oil rings and compression rings, but I would think that if they were any type of an issue, it would show on a compression test right?

    To answer your question, I have not done the impeller yet. It still pumps like a champ for now and I don't want to do any PM to it until I can get this misfire type of issue resolved first. The tilt and trim system leaks like a styrofoam cup full of gasoline lol, but again, I don't see the point of dumping anymore time or money into it until I know it runs solid if you know what I mean. I greatly appreciate your input here!!!

  6. #6
    Join Date
    Sep 2010
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    Default Re: 93 Mercury 75hp Loads Up

    Is this a 4 stroke ?-----There are no ---" oil rings " ---if this is a 2 stroke motor !----Nothing has seemed to solve this for you.----Just $30 or so for gaskets if you remove the exhaust cover.
    Last edited by racerone; 05-13-2021 at 02:12 PM.

  7. #7

    Default Re: 93 Mercury 75hp Loads Up

    If it won't go past 3k rpm that usually means the high speed winding is bad. If it a 2 stroke that's the reason why there's oil in the cylinder. its not burning. Im stunned you haven't changed the plug. Should be first step if cyl. isn't firing

  8. #8

    Default Re: 93 Mercury 75hp Loads Up

    Quote Originally Posted by racerone View Post
    Is this a 4 stroke ?-----There are no ---" oil rings " ---if this is a 2 stroke motor !----Nothing has seemed to solve this for you.----Just $30 or so for gaskets if you remove the exhaust cover.
    It is a 2 stroke. I've only completely torn down a 2 stroke engine once before and that was many moons ago! lol
    If that plug doesn't solve the issue......off comes the exhaust cover!!

  9. #9

    Default Re: 93 Mercury 75hp Loads Up

    Quote Originally Posted by Gavin27 View Post
    If it won't go past 3k rpm that usually means the high speed winding is bad. If it a 2 stroke that's the reason why there's oil in the cylinder. its not burning. Im stunned you haven't changed the plug. Should be first step if cyl. isn't firing
    Well, literally EVERYTHING was all brand new! I replaced it all at the same time when I got the boat after noticing it had the misfire symptom. I had issues with the old stator so I replaced it with the same type that came off, but it was a newer style that had a stupid adapter with it (red stator). After it continued to have the same symptoms even after the new stator, I replaced EVERYTHING for piece of mind. STILL had the problem!! Then, I took that crappy red stator off and bought the mercury original and tried it, only to find out that the bottom cylinder wasn't firing consistently under a load at WOT. Spark plug is gonna be replaced tomorrow and I'll let you guys know how it goes!! (again, fingers crossed!!)

  10. #10

    Default Re: 93 Mercury 75hp Loads Up

    So, spark plug did not seem to solve the issue 100%. I put it in the water on the trailer, started it and revved it up and it seemed a ton better! So I took it off the trailer and tried it in gear at WOT and still has no guts. I just don't get it!! If it were something to do with oil fouling the plug, the new plug should have made it run like a top until it loaded up again I would think??? At the least. I'm ready to unbolt this thing in the middle of the lake and use it for an anchor!!! Any thoughts besides checking inside the exhaust housing??? Simple checks I might have overlooked???

  11. #11

    Default Re: 93 Mercury 75hp Loads Up

    Does it rev right up in the driveway? does this only happen under load? If it only happens under load its a classic symptom of a dirty carb. I know you said you cleaned them already but give that bottom one another go over. When you put it in the water was there that shine trail of gas on the top of water? Maybe your bottom carb float is not working correctly and flooding? Would explain why that bottom spark plug was dirty.

    to narrow it down to that carb, pinch off the fuel line into the bottom carb with vice grips. Use a can of quick start and spray it into the carb when you rev it up under load. If it takes off, you found your issue. Fuel delivery to bottom carb. Replace the internals of that carb or just replace the entire carburetor

  12. #12

    Default Re: 93 Mercury 75hp Loads Up

    Man, I am almost glad to hear you say that! I was rattling my brain all night trying to figure out what it could be and that was about the only simple explanation I could come up with, was dirty carbs or cracked/broken reed valves. I'm gonna pull them off today and reinspect the reeds and pull apart the carbs and clean them up again.

  13. #13

    Default Re: 93 Mercury 75hp Loads Up

    Well, I rechecked the carbs again and reeds. Nothing found to be suspect, so I found a sure fire way to fix the problem! I pulled the motor off my boat and bought a 115 Yamaha, since I couldn't find any Merc's around for a good deal unfortunately. I'm going to sell this thing to a mechanic who can show it the love I wasn't able to. I appreciate all you guys for your input and opinions!! This is what a boating community is all about, tossing a line when someone needs a tow!! LOL

  14. #14
    Join Date
    Sep 2010
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    Default Re: 93 Mercury 75hp Loads Up

    If you can solve it with money you do not have any problems.

  15. #15

    Default Re: 93 Mercury 75hp Loads Up

    Quote Originally Posted by racerone View Post
    If you can solve it with money you do not have any problems.
    Oh, I don't have any money! I'm as broke as a joke! But, thankfully I have good skills at bartering....

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