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  1. #1
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    1988 Mercury 200 blackmax WOT low rpm

    Hey everyone -I posted about this in October but didn't get to many replies on the topic and I still chasing the issue...

    I'm trying to find some good advice on diagnosing a problem with my 1988 200 mercury black max. I'm not getting full RPM at WOT - I know this motor should be capable of 5800-6000 rpm on my 21' Seebold eagle - I bought it without a water test - wow stupid I know. It starts an idles perfectly but will not go over 4400 rpm without trimming out the motor - still only gets 5000 trimmed high. I have a very common chopper prop for this size boat 14X24" with correct height and low water pickup. Others agree it should not be over propped - besides I'm off by way more than 200-400 rpm. I have completely gone thru the fuel system:

    New poly tank
    New fuel line all clamped with no air leaks
    New water separater and inline filter under the cowl
    Fuel pressure tested at idle 4.5 psi
    Carbs cleaned/inspected found no issues/float height 1/16th below the gasket surface when inverted as per the Seloc manual
    Checked compression - 125psi even all 6 cylinders
    BU8H plugs

    Nothing has helped the issue and I'm now wondering if its a high speed miss or timing issue? I talked to some local mechanics and to be honest they sounded like they knew less than I did so I refuse to pay for their hourly wage when I can swap electronics myself. I checked basic connections on the switch boxes/rectifier/coils - I cleaned them but they seemed ok. I tested coil resistance they were in spec @850-875 k ohms. My plugs looked new but I replaced anyway - never any fouling or water than I can see. The motor is not running thermostats which I don't think would cause this issue...I've also read that this motor uses an advance module that only gives more timing above 5600 rpm which I cannot achieve. Supposedly this can be removed and you can set the timing statically to max WOT timing.
    I guess I'm looking for a magical diagnoses from someone who really knows Mercury 2.4's since I can't find any around me! I would hate to buy a stator and switchboxes to find a coil defective...I need a good place to start ruling out the fuel system - I never pulled the intake but the reeds all looked fine when I pulled the carbs.
    Any advice is extremely appreciated.

    -Tom

  2. #2
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    i am new at outbord repair but in last couple months i have been learning how they work by having my 200 in pieces looking for speed. but i would try running in water at wide open throttle if it wont go past 4400 rpms i would try pushing key in to activate choke if it is wanting more fuel it should breifly increase rpms if it does i would ckeck fuel pump.but all fuel problems i have had run great and just nose over and quit same as tank going empty. a dead cly sometimes is hard to hear other than a little shutter under load.the best way to check is to pull all spark plugs and get a spark checker from any parts store. adjust gap on tester to about 1/2 inch hook plug wire on one end and the other end to a clean ground have someone turn key and when motor is cranking over look for strong purple spark.check all 6 clys one at a time if one has no spark or weaker orange spark you will have a clue where to start.also while plugs are still out put spark tester on number 1 cly open throttle all the way hook timing light to #1 plug wire spin motor and check timing it should be about 20 degrees btdc.
    Last edited by davemvegas; 11-23-2011 at 09:36 PM.

  3. #3
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    Step 1 is to remove the timing box and set timing to about 23* @ WOT and check the compression to make sure you dont have a bad cylinder. Step2 do what dave just said but also get a dva meter and check the stator for peak voltage. Really sounds like its dropping a cylinder or either its way lean or way rich so check the size of the main jets in the carb if everything else checks out.

  4. #4
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    Carbs have stock jetting for wh46's -.056/.080/.082 and im at sealevel. Trying to find Eva adapter- still not entirely clear what the DVA does-freezes the peak voltage reading? How can I see if the stator drops voltage using a device that ...

  5. #5
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    my carbs are marked wmh on 1991 merc 200hp. idle air jets in front are .034 and main jets in bowl are.066 it would turn 28 chopper 6600 rpms on 20ft vegas and as far as i know it was a unmodified motor. so what i am wondering is what is the difference between wh46 and wmh carbs that would cause such a big change in jet size on same size and hp motor.

  6. #6
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    I was wondering why the wh46's seem to be so far off from most other V6 carbs too. Per the seloc manual carb specs it's stock though.. I just reread most post and realized that I left out some troubleshooting I already did... I did verify spark on all six cylinders but only on land not at wide open throttle - I really can't see myself running the boat on li sound hanging off the back with a timing light- maybe on a calm lake but thats not where I'm running... Way to heavy a chop for that! Can anyone explain in a little more detail how/what the DVA does diffently than the multimeter? Also my voltage at the battery is high- 15.5volts! Jay from jsre said thats way to high - others have said mercs run high like that normally... I think it's high but not sure if that means the rectifier or voltage regulator of do I change both?

  7. #7
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    Static tests with a multimeter will only show short, open, ground, or high resistance. DVA readings are more "real world", and show actual voltage being produced. DVA can also be tested on high and low side, so no reason to risk life and limb running wide open hanging over the back of the boat.

    The charging issue may, or may not be related, but 15.5 does not give me any great concern.

    If oil injection is already removed, trash those idiot timing modules, they're nothing but a problem, and have ruined many good engines. Run timing and synchronization procedure, set max timing to 22 degrees. My bet is that will solve it.
    Living in the Freedom provided by Bud Conner and his fellow warriors.
    R.I.P. my Heathen Brother






  8. #8
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    Thanks for DVA explanation. Yes the oil injection has been previously removed by an old owner -supposedly the the motor was rebuilt within the last 10 years so maybe it was done then. Many things have been modified on this engine which makes it harder for me to know what the stock setup should look like... The original airbox is removed and there are velocity stacks mounted instead - from what I have seen the advance module/ idle stabilizer is usually mounted on the air box. I found drawings in seloc manual of different modules and it seems to be on the port side of block behind the flywheel. My problem is that ive seen voltage regulators on eBay that look exactly like it so can't identify which module it is-'it may already be removed... My rectifier is on the side of the motor as normal- then there is a small square box with only two wires attached to lugs (red/white) dead center on top of the block behind the flywheel. Then the mystery module is next to it- about the size of an iPhone- lol. It also never had a flywheel cover so I bought one but I need to calibrate it and need to get a dial caliper to find true tdc. Could the timing hold it back by 1000 rpm? I guess if it was really off.... Thanks again for the reply- if anyone has pictures what this motor should look like or the actual rectifier/voltage regulator/idle stabilizer please share!

  9. #9
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    Shoot some pics of that thing so we can see what's there.

    Yes, low timing could cost 1000 rpm.
    Living in the Freedom provided by Bud Conner and his fellow warriors.
    R.I.P. my Heathen Brother






  10. #10
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    Will do this week - I'm not home today. Is timing the motor without it running acceptable and accurate ? I wonder the advance module was removed and never properly timed.... That would be nice!

  11. #11
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    Quote Originally Posted by speedman79 View Post
    Is timing the motor without it running acceptable and accurate ?
    Yep..
    Living in the Freedom provided by Bud Conner and his fellow warriors.
    R.I.P. my Heathen Brother






  12. #12
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    Just had the same issue with my 2.5 efi. Lost some r's at the top. Pulled all the black boxes off and now it runs great. Connect the white w/black stripe wires from the switchboxes together. Check out thttp://www.biggerhammer.net/mercury/ the timing process is detailed here by jay smith. Good info on lots of things mercury...

    83 V-King, 96 Mariner, 200 hp ff block 2.5 w/a 28p choppa
    We gotta clean this liberal mess up, VOTE TRUMP TO MAGA!
    Rebuild thread:
    http://www.screamandfly.com/showthre...-it&highlight=
    http://www.screamandfly.com/showthre...cs.&highlight=
    Videos

  13. #13
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    Ok, I identified the advance module (93772a 2 -high speed control assembly) on top of my port side of the block. Should I check timing with it connected first then compare with it disconnected to check for malfunction? I am waiting on a DVA and borrowing a dial indicator for the pointer alignment to TDC. Any tips on finding true TDC I've never done it on a 2 stroke outboard - on my old Bronco I was able to ballpark TDC on cyl 1 and just dropped in the distributor, this needs more accuracy since the flywheel cover/pointer is not stock to this motor. Also what is the .462 mark for if .150 is 25 degrees?
    XstreamVking can you elaborate on your post "Pulled all the black boxes off and now it runs great. Connect the white w/black stripe wires from the switchboxes together." I wasn't sure what needs to be done if I remove the module beside retiming...

  14. #14
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  15. #15
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    Thank you - I have seen Jays method and will definitely use it once I'm clear on how to properly disconnect the black box module. I have a feeling my motor may have had a separate idle stabilizer that was removed - all I have is a high speed module. Anyone aware of any mods that are needed when removing the blackboxes besides re timing. Any jumpers needed?

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