Please note this thread has been inactive for 90 days. For the best results, please start a new thread.
Results 1 to 16 of 16
  1. #1
    Cadet
    Join Date
    Oct 2007
    Posts
    11

    Default Mercury 90 HP 2-Stroke Rough Idle and accelaration

    Hello everyone, I'm looking for some help on my Mercury outboard. Its a year 2000 2-stroke, 90 hp mounted on a 2000, fiberglass starcraft 1700 bowrider. I bought this set up used and do not know all to much about its maintenace history, but it does appear very clean and un-messed around with.

    Anyways, the problem is that the idle is very rough and when I go to accelerate for the first time after leaving the dock it gians RPM and then stalls quickly and thens comes back and runs fine. Or it will just have a very hard time accelerating and then after a couple seconds smooths out and runs great above 1200 rpm.

    Each time I attempt to fix the problem it gets slightly better then progressivly gets worse again. My first attempt was to bring it to a local outboard service tech and have it gone over when I first bought the boat. He changed the gear oil, adjusted the carbs, and de-carbed the engine. When I got it back it ran slightly better than before but then got worse again after a couple weeks.

    So then I adjusted the carbs and the idle smoothed out slightly again, and then got worse again. So then today I noticed that the primer bulb would not stay hard. So i changed the fuel line and bulb and now it stays hard but the motor does not run any better.

    I'm proficient in automotive repairs but this is the first outboard I've owned.
    So any ideas on were to start with this problem would be appriciated. Thanks for the help,

    Collin

  2. #2
    Cadet
    Join Date
    Oct 2007
    Posts
    11

    Default Re: Mercury 90 HP 2-Stroke Rough Idle and accelaration

    Just an update, today I bought a fuel pump rebuild kit and rebuilt the fuel pump. No change in how its acting. I was moving at about 1200 RPM's and then I put the throttle all the way down and the engine cut out for about 1 second and then accelerated.

    Any help is appriciated, Thanks!

  3. #3
    Vice Admiral
    Join Date
    Apr 2007
    Location
    Wright City, MO
    Posts
    5,426

    Default Re: Mercury 90 HP 2-Stroke Rough Idle and accelaration

    Idle mixture on 2-strokes have to be a bit fatter than 4-strokes to allow just enough 'richness' for acceleration.

    Adjust each carb to best idle + 1/8 to 1/4 turn, ideally, adjust out on the water a blade width at a time each, just until any throttle hesitation is gone.

  4. #4
    Petty Officer 1st Class
    Join Date
    Dec 2005
    Location
    South Africa
    Posts
    347

    Default Re: Mercury 90 HP 2-Stroke Rough Idle and accelaration

    If that doesn't help you might have float problems. Your float level might be too low or too high. Also, have you done a decarb (FAq section) and cleaned the carbs?
    If you decide to rebuild the carbs be sure to blow each and every tiny passage with compressed air.

  5. #5
    Cadet
    Join Date
    Oct 2007
    Posts
    11

    Default Re: Mercury 90 HP 2-Stroke Rough Idle and accelaration

    I don't believe that the carbs have been cleaned. But the engine has been de-carbed. How is the float level adjusted on this motor?

    Also, today when I was taking off the fuel pump I dropped one of the bolts to the pump in the bottom cowl part and had to remove the air intake to get access to it and when I took it off there was a little pool of gas/oil in there and it appeared to be leaking mostly out of the bottom carb. What would this tell me??

    Thanks,

    Collin

  6. #6
    Petty Officer 1st Class
    Join Date
    Dec 2005
    Location
    South Africa
    Posts
    347

    Default Re: Mercury 90 HP 2-Stroke Rough Idle and accelaration

    I have no idea if your needle levers look the same as mine, so I can't help with the float adjustment.
    I'm also not familiar with the air intake system, but at a guess that's just fuel/oil dripping from the carbs when you trailer the engine. They all dump a bit of fuel because the needles can't seat at that angle.

  7. #7
    Supreme Mariner Silvertip's Avatar
    Join Date
    Sep 2003
    Location
    Midwest
    Posts
    25,770

    Default Re: Mercury 90 HP 2-Stroke Rough Idle and accelaration

    You said you are do car repairs -- outboard floats are adjusted the same way you adjust float level in a car. Bend the tang. Some 75 and 90 HP triples have an inherent stumble as you describe but it should idle relatively smooth if the mixture screws are set properly. These are fairly big displacment engines and do dance a little at idle. Have you changed the plugs?

  8. #8
    Fleet Admiral Texasmark's Avatar
    Join Date
    Dec 2005
    Location
    N. Texas, USA
    Posts
    9,407

    Default Re: Mercury 90 HP 2-Stroke Rough Idle and accelaration

    I have the same engine in '02 version. I had a similar problem and opening the low speed jets (only ones adjustable) helped. I don't remember if I opened 1/8 or 1/4 turn but that was all it took.

    The other thing I noticed about the engine is this. I used to launch (alone), and park the boat at a pier with the engine idling to warm it up.

    I'd park the truck, jump in, move out to the no wake marker and hammer down. Usually It would misfire and hesitate slightly and then pull on out. I changed plug type which helped, but what helped most was to shut it off while I was parking the truck. Seems I load up the plugs with all that cold engine idling.

    Course I have a big load on a cold engine as it is on a bass boat with 3 big batteries and 20 gallon fuel tank immediately forward of the transom. Additionally I am running a 13 1/2 x 24P SS prop which is a big load outta da hole.

    So doing the latter and holding the rpms down on initial plane, allowing for the engine to warm up seems to work best. The colder the weather the more significant the problem.

    Once it gets warmed up for the day, no more problems.

    HTH

    Mark

  9. #9
    Cadet
    Join Date
    Oct 2007
    Posts
    11

    Default Re: Mercury 90 HP 2-Stroke Rough Idle and accelaration

    The plugs were replaced when it was serviced.

    I've never adjusted jets on a carb, that sounds like a good possibility, because I've tried just about everything else. SO how would I go about adjusting the carb jets?


    Thanks for all the help everyone,


    Collin

  10. #10
    Fleet Admiral Texasmark's Avatar
    Join Date
    Dec 2005
    Location
    N. Texas, USA
    Posts
    9,407

    Default Re: Mercury 90 HP 2-Stroke Rough Idle and accelaration

    To really know (how to adjust the jets) you need a benchmark. You need to take the boat out and get the feel of it in the areas mentioned.

    Then you have to get the engine cold which would suggest you have to go out a second day with the jets (all 3) opened 1/8 turn. See what difference there is. If it seems to be a bit huskier, great. If it doesn't stumble, leave it be.

    If it still does do another eighth turn.

    If no change, something else is your problem.

    To access the jets, Pop the hood and look at the port side of the carbs just in front of the mounting flange. Each has a slotted, knurled set screw with a spring behind it. Open is ccw.

    Mark

  11. #11

    Default Re: Mercury 90 HP 2-Stroke Rough Idle and accelaration

    try some premium fuel and some seafoam fuel additive

  12. #12
    Petty Officer 2nd Class
    Join Date
    Jan 2009
    Location
    overseas Bulgaria
    Posts
    147

    Default Re: Mercury 90 HP 2-Stroke Rough Idle and accelaration

    I think Mercury is a pain in the ***, I had one that I repeired all winter, used for an hour or two and repair the rest of the season, I changed it and now boat all season, even changed my job for a much less paid one with a longer summer vacation.

  13. #13
    Petty Officer 1st Class
    Join Date
    Aug 2010
    Location
    Long Island N.Y
    Posts
    265

    Default Re: Mercury 90 HP 2-Stroke Rough Idle and accelaration

    Quote Originally Posted by Texasmark View Post
    I have the same engine in '02 version. I had a similar problem and opening the low speed jets (only ones adjustable) helped. I don't remember if I opened 1/8 or 1/4 turn but that was all it took.

    The other thing I noticed about the engine is this. I used to launch (alone), and park the boat at a pier with the engine idling to warm it up.

    I'd park the truck, jump in, move out to the no wake marker and hammer down. Usually It would misfire and hesitate slightly and then pull on out. I changed plug type which helped, but what helped most was to shut it off while I was parking the truck. Seems I load up the plugs with all that cold engine idling.

    Course I have a big load on a cold engine as it is on a bass boat with 3 big batteries and 20 gallon fuel tank immediately forward of the transom. Additionally I am running a 13 1/2 x 24P SS prop which is a big load outta da hole.

    So doing the latter and holding the rpms down on initial plane, allowing for the engine to warm up seems to work best. The colder the weather the more significant the problem.

    Once it gets warmed up for the day, no more problems.

    HTH

    Mark

    How much does the boat wigh and what is the top speed with that large pitch prop?

  14. #14
    Petty Officer 1st Class
    Join Date
    Aug 2010
    Location
    Long Island N.Y
    Posts
    265

    Default Re: Mercury 90 HP 2-Stroke Rough Idle and accelaration

    Change the diafram in your fuel pump
    Uniqe aluminum boat rebuild technics and My 191 cc Sea Nymph Striper Center Console Aluminum Boat Rebuildhttp://forums.iboats.com/boat-restoration-building-hull-repair/aluminum-boat-rebuild-methods-191seanymphstriper-434575.html


    My Brain Never stops! My radical idea im thinking of doing! Taking (2) starcraft islanders and converting them into (1) 36ft Center console
    http://forums.iboats.com/boat-restoration-building-hull-repair/starcraft-cc-rebuild-idea-convert-2-26-starcraft-islanders-into-1-36ft-cc-439225.html



  15. #15
    Petty Officer 1st Class
    Join Date
    Aug 2010
    Location
    Long Island N.Y
    Posts
    265

    Default Re: Mercury 90 HP 2-Stroke Rough Idle and accelaration

    Ok listen. Put the boat in the water on your trailer with all trailer tie downs still locked down at the transom and bow of the trailer locked. Back her into the water so the motor when tilted down is fully submerged and able to pump water. Or if you keep it docked just tie up real good. Make sure your parking break on the truck is on good! But in reverse the thrust is less so you wont have a issue!


    Start the boat. Rev it up and get her in reverse at 1700 rpm. With your air breather off of all the carbs close each butterfly one by one with your hand by sticking your finger in the carb and close each one and see if it bogs down on each cyl while the boat is under load at 1700 rpm in reverse.


    They each should bog down the motor!


    If you close one butterfly and dont here a bog or takes longer then others to get boged thenn that cyl is either getting to much fuel so the cyl is flooded or barly sparking and working less causing your ruff idle.


    Now this can mean a couple of things. If the fuel pump is conected to the cyl that takes longer to bog its the fuel pump. because whats happening is the diafram in the fuel pump is warn punctered etc.

    And what will happen is that cyl will draw alot of fuel flooding it causing the ruff idle and low spark bc of the flooded plugs.

    Just because the fuel pump is shot does not mean that the other cyl arent getting fuel and wont get fuel.


    It just means 1 cyl is getting to much fuel....


    Or that cyl that takes longer to bog or dosent bog at all may have a bad plug or failed plug, or a bad coil, or a bad wire that goes from the coil to the plug.


    Now these are the quickest easyiest ways to fix and try to solve the issue. Do these techmecs before you start ripping the carbs off...


    """"(Also when the fuel pump is removed from the engine, start the engine with all hoses disconnected etc just as you disconnected it. And give some throttle and let it come down in idle and put in nuetral. Now if it is the pump she should idle in nuetral because now the 1 cyl is not getting extra fuel and the fuel left in the carbs will even out and should idle for about 2 min to 5 min,,,,,)""""""

    """""(Now if she dosent idle without the pump in then its not the pump!)""""


    Now if you change the difram in the fuel pump and you change the coil, plug, wire and you still have a issue with that cyl then take all the wires off the plugs and do a complete comp test and be sure you have good even comp. Anything within 15 psi of each other is still considered good.

    So if its 115 120 110 105 its still good but will give you a little uneven ness but not enough to sound like a uneven idle or be an unevian idle.


    So if you check compression and its all good..... Then take that carb yjay bogged slowly or not at all off and rebuild it and put it back on.... Then it should be fixed..... You may find debri etc or signs that moisture may have bean in it etc.

    Let the carb soak excluding the float in (SEA FOAM) (a product to clean fuel systems) for about 3 hours as long as all plastic pieces are off yje carb. Then rebuild the carb!


    Those are the only things that can cause the un even idle....

    You now at this point after doing this have checked and repaired spark,fuel,air, carb all the things you need to make her run... If you don't know how it works you wont be able to fix it!


    Unless your stater is no good which if it is that would be most unlikly and it wouldnt even start at all. Or a elec short which is all uncommon


    Follow these steps and trust me youll find the issue...


    Dont start drilling your carb etc etc ...... Just not a good idea with debri shavings etc..could cause more problems.. ( No offence to Daviet at all) (I Just wouldnt do it at all personally!)


    Get a very small hook pick and a small strait pick and get it out... And again No offence to daviets advice at all but I just wouldnt do that personally! Daviet may be a very experienced mechanic for all we know and he may have a very steady hand!

    You can do whatever or however you would like to get it out! But just be carfull if your drilling


    And tell me what happens...


    Also your phone number was edited out and you cant post your number directly on the thread. You can click on my screen name and send me a P.M (personal Message) with your number and i willl walk u threw it again!


    Hope my advice and daviets advice helped!
    Uniqe aluminum boat rebuild technics and My 191 cc Sea Nymph Striper Center Console Aluminum Boat Rebuildhttp://forums.iboats.com/showthread.php?t=434575


    My Brain Never stops! My radical idea im thinking of doing! Taking (2) starcraft islanders and converting them into (1) 36ft Center console
    http://forums.iboats.com/showthread.php?t=439225



  16. #16
    Vice Admiral
    Join Date
    Apr 2007
    Location
    Wright City, MO
    Posts
    5,426

    Default Re: Mercury 90 HP 2-Stroke Rough Idle and accelaration

    Go here

    http://forums.iboats.com/showpost.ph...11&postcount=3

    Download the service manual for your motor.

    Remember the password 'ribforum' as you will need it to unpack the compressed file.

    Read the section on the Fuel System, twice.

    The manual very clearly spells out the proceedure to take off the carbs, clean them and put it all back together.

    Then follow the directions to 'Link' the carb throttles and 'Sync' the throttle to the timing.

    Once that is finished then you can adjust the idle mixture, the directions are well spelled out in the book.
    Last edited by CharlieB; September 13th, 2010 at 09:21 AM. Reason: Spelling

Similar Threads

  1. 2004 Johnsom 25 HP 4 stroke rough idle
    By SplishSplash in forum Johnson & Evinrude Outboards
    Replies: 4
    Last Post: July 30th, 2010, 07:01 PM
  2. suzuki 02 140 four stroke rough cold idle
    By tunaman1 in forum Yamaha & Suzuki Outboards
    Replies: 3
    Last Post: March 16th, 2010, 04:28 PM
  3. Yamaha Four-stroke 225 runs rough at idle
    By topperbt in forum Yamaha & Suzuki Outboards
    Replies: 12
    Last Post: June 9th, 2009, 07:10 PM
  4. 4 stroke yammi/rough idle
    By gwseafarer in forum Yamaha & Suzuki Outboards
    Replies: 9
    Last Post: September 6th, 2006, 11:55 PM
  5. Merc 90 hp 4 stroke - rough idle no power
    By ebubeck in forum Mercury & Mariner Outboards
    Replies: 12
    Last Post: November 24th, 2003, 03:48 AM
  1. iboats Forum Directory - Over 100,000 forum posts organized by topic