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  1. #1
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    200 merc 2.4 jetting

    As per my earlier post, I have a mid - late 80s Reman 200 merc on a 150 mariner 150 mid. 1.87 gear lower. It had the top starboard piston melted with a hole in the center. Got it split, and ready to rebuild, but cannot find a reason it blew. I expected to find plugged jets, stuck float.. something, yet I found nothing. I'm going to replace water pump as a precaution. I have the wh-39 carbs. Are these the proper jets?

    Any ideas as to why she blew? Other 5 cylinders look great. Perhaps old fuel and too far advanced timing?




  2. #2
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    I'm guessing seriously lean since pistons 3 and 5 look too clean also. That burnt #1 is dry and shows signs of melted aluminum from pre-ignition /overheating. Do you have a pic of the plug coming from that hole? I don't see signs of oil related failure since the walls look ok. I've seen sparkplugs get so hot on motocross engines that they glowed red and a holed piston was often the result. Hopefully there is no cylinder, crankcase, or crank damage. Gordon

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    The other cylinders/pistons look good. Nice brownish black from carbon and very well oiled. I dont have the plug, as the motor was blown when I got it, and had no plugs

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    Timing would affect all 6 cylinders the same or should. That piston got very hot dead center of it's crown so air/fuel ratio, plug heat range or both made combustion temps enough to melt aluminum.

  5. #5
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    double firing switchbox may be the culprit
    death is certain; life is not


  6. Likes Dave Strong liked this post
  7. #6
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    All good ideas - how to test if its double firing?

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    Timing light. You can mark-up the flywheel with white or florescent paint in the timing sequence. The light will show if firing two cylinders at once or the wrong one out of time. I've used that to find a bad CDI on an OMC, but not had the problem on a Merc.

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    I found a link to illustrate -

    https://www.youtube.com/watch?v=D-hBeGdE6oE

    Gordon

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    Anyone know what the wot timing should be?

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    Your problem was caused by low-level detonation,in my opinion.
    Detonation and preignitioned defined

    I see folks refer to detonation and preignition often,and even now and then someone suggests predetonation as the reason for certain engine failures.btw,predetonation isn't a condition that has taken place...sort of like a preheated oven,it's either heated or it's not,you've either suffered detonation or you havent.
    We will start with the simple one and get it out of the way,preignition.This is also slightly misnamed,it would be more correct to be named pre-planned-ignition.Preingition refers to the ignition of the compressed fuel/air in the cylinder before the spark takes place that was meant to begin the combustion process.This is usually caused by a glowing piece of carbon or spark plug tip,and can be brought on by overheat,wrong plugs,or excessive carbon buildup.In certain rare cases it can also be caused by the hot exhaust from a nearby cylinder igniting the fuel/air mix as the ports are closing on the subject cylinder,this is usually encountered on highly modified high rpm motors.enough of preignition,and onto the common real problem,detonation.
    Detonation,by definition,is simply a condition in which the piston is forced to compress an already expanding (burning)fuel/air charge in the cylinder,resulting in higher temperatures/pressures than normal,and the resulting shock and damage is termed "detonation damage".This damage can be insidious and slow,in mild detonation cases,or immediate and extensive in heavy detonation cases.
    Detonation damage is usually most apparent in severe cases as melted and seized pistons,in mild or low-level cases the constant bouncing of needle bearings can cause chatter marks(the chipping away of surface metal) on the crankshaft journals,as well as the wrist pins and rod bearing surfaces.Piston ring land breakage is another common result of low-level detonation.While most folks understand the severe cases,the low-level damage is often attributed ot other causes.So...
    To understand detonation,we have to examine its causes,and there are plenty of them.A motor is designed and tuned to capture the force created by burning fuel/air and put it to work,turning a propeller through a series of rods,cranks,shafts and gears.In order to maximize the useful available power,we must light the fire at precisely the right to get a full burn without developing excessively high pressures.Many factors come into play,but the main ones are compression,fuel octane,timing advance,engine temperature,and operating rpm.Realize this...probably the most important things for the boat owner to understand,is the fuel/air burns at a constant speed although the motor changes speed.Because of this we have systems to advance spark timing to assure that at high rpms the engine won't outrun the burn time of the fuel/air mix.The other factors that play heavily in this equation are fuel octane and engine rpm.Rpm is simply the speed of rotation...revolutions per minute,and octane is a relative speed of burn,the higher the octane the slower and more consistent the burn,or"push".
    Realize that lowering engine rpm effectively increases spark timing,as it causes the piston to get to top dead center later,allowing time for the combustion pressure to grow,setting the scene for detonation damage.
    This trouble can be avoided by using fresh fuel of adequate octane,maintaining a clean fuel system,doing proper routine maintainence,and propping to achieve full recommended rpm.Low level detonation is a lot like high blood pressure,over time it breaks things down.
    Hope this helps
    By the way,although it did not cause your immediate problem,the 082 mains should be feeding cyls 2 and 5,080 on the rest.Remember the port side of the carbs feed the starboard cylinders,and vice-versa.Also,realize that even you can see a nice hole through the jet,it is no assurance there isn't a couple thou of varnish in it,making it undersize.Clean them with a torch tip or jet cleaner,Chris

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  13. #11
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    I agree wit chris. detonation. ,maybe to big of prop and timin to high. Timin affects all 6. But a merc switchbox sucks , u can have 4 or 5 cyl on max timin and have 1 or 2 cyl with 2 or 3 more degrees and then u get ur pretty hole in center. Also those 39s. did u look up stock jetting? 56 idles seems quite lean for stock.

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  15. #12
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    I looked a little and found that should b stock jets for the 39s. 2 and 5 need the 82s ,rest 80s. 1 of the 82s are n wrong spot

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  17. #13
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    thank you guys so much. Ive thought it was caused by an overheat/timing/detonation problem, and that makes sense to me. I got the boat just a few weeks ago, but the previous owner told me it blew not long after the installed the powerhead. I just called him up and asked if they drained out the gas tank before running the new motor, and they said no, they just added oil to the mix in the tank already, which at that point, was several months old.

    looking at the back of the motor, is it..

    2 1
    4 3
    6 5

    I soaked the carbs in cleaner for 24 hours, and used a wire to clean out all the small orifices and holes, and compresses air to blow everything out.

    thanks everyone!
    Last edited by JoshKeller; 11-02-2019 at 09:52 AM.

  18. #14
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    Quote Originally Posted by Merc 2.5 View Post
    I agree wit chris. detonation. ,maybe to big of prop and timin to high. Timin affects all 6. But a merc switchbox sucks , u can have 4 or 5 cyl on max timin and have 1 or 2 cyl with 2 or 3 more degrees and then u get ur pretty hole in center. Also those 39s. did u look up stock jetting? 56 idles seems quite lean for stock.

    would i be well off to replace them both with aftermarket as precaution, or just drop the timing a few degrees? Im ok with losing a few rpm vs having the motor go kaboom again.

  19. #15
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    U must need index flywheel and use timin light to figure out what all cyls r runnin timin wise ( variances n cyls) but yes turn timin down never hurts. Big props and low rpms is a killer

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