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  1. #1
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    Overheating at 5000 rpm or over hard too idle sounds sick mercury 150

    Hey Guys im having some trouble with my merc 150 it's a 1985 . Rebuilt in 1998.. I haven't been unable too use it in the past 3 years.. Took it out the other day and it ran horrible at idle speeds...perfect at top end....i took carbs off and rebuilt them reinstalled no effect...smooth top end...crappy idle ..bad hole shot... have too stay on the throttle too keep it running sounds like only hitting on a few cylinders...Any idea what is causing the overheating>? new water pump also...and removed thermostats no effect....

  2. #2
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    A marine mechanic friend of mine tells of an engine like this, drove him nuts for days. He finally decided to pull the carbs and check the reeds and found the skeletal head of a dead mouse stuck in one of the reeds.

    Did you look at the reeds while you had the carbs off?

    Have you checked the base mechanical things, like compression?

    John

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    yes they where in good condition....i was wondering if there could be a prob with the timing advancer retarding the timing not enough buring idel and too much hot spark during wot...cause it was running fine b4 it set up for 3 yrs and never overheated before... i messed with it so much....it's getting on my nerves....compression is 125 120 122 120 115 120.... any suggestions? has a new stator too btw...stock.. i changed the plugs too....still idles like crap... oh and when u first fire it up sometimes it will backfire..b4 it fires up...

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    The thumb sized filter might be plugged up. I had quite a bit of trouble with a little used boat I bought last year. It didn't overheat, but cut out at high speed. Changed the filter, all good for a few days. Cut open filter and couldn't see anything. Tested the thing with a fuel pressure gauge and confirmed that the filter was the culprit. Cleaned out the fuel tank and put in a Racor suction line filter. End of story.

    Mine acted kinda funky, just out of the shop, when I first bought it. Went through it and found all adjustments were to the wrong engine spec. Carburetor floats were all wrong and different from each other. Same mechanic did $400 damage to the LU trying to replace shaft seals.

    Put the t-stats back in, they're needed to properly ballance the cooling.

    You might have a leaking head gasket blowing a bubble in the top of the block. Is the pee stream steady or does it sputter? (after you put in the t-stats. If possible, look at it at WOT.

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    ya checked the filter...i have quicksilver water seperator new on it also... with the thermostats in it doesn't pee much at all at idle but with them out lots of pressure coming out....full stream coming out at wot....i'll pull the heads off and check...u might have something there...might have leaking head gasket....i'll see what it looks like...any other suggestions i welcome ... any for sure ) i got too get this thing running right

  6. #6
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    The pee stream only shows some of what goes through the thermostats. There are other cooling paths, so you wouldn't expect a real strong stream at idle, just steady.

    If it sputters at speed, it could indicate exhaust getting into the water jacket.

    It's well worth your while to do a formal link n sync. Just be sure you have the specs and procedure for the right engine. When I looked at mine, the idle timing was fast, the carb pickup occured way late, at way too fast timing, and the WOT timing was 4* slow. It's a whole nuther motor now. It appeared to be set up to a different motor spec.

    If it's a Frankenmerc, you may have to think a little harder about the link and timing setup.

    hope it helps
    John
    Last edited by j_martin; 04-28-2007 at 09:50 AM. Reason: ebonics removal

  7. #7
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    Retarded timing wil do exactly what you're describing. Might want to check it...
    '89 Hydrostream Vegas XT, '90 Merc 2.4 Bridgeport PCU EFI
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  8. #8
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    im gonna check that out tomorrow i was looking at the timing advancer...and i have a few wires that have dry rotted.. i have one from a 200 merc will it work on a 150? or should i just pull it off and set the time at 24 or 26 degrees? anything else i should do? thanks for the replies...

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    If you're talking about the idle stabilizer/advance module, you'll not find it listed at mercurypartsexpress for your engine. I think that's because most of them find a home in the chit can.

    I'd pull it off, set your timing by the book, If you took off an advance module, you'll have to set your max at 23* or so, book might say 18 or 20. Go through all the linkage adjustments in sequence. A mistake there can really cause goofy symptoms. Check your compression and run the right gas for the compression. Mine's at 145 psi and I run 92 octane, no alcohol.

    If you're running strong at 5000 RPM, and that's all she'll do, you are probably a little bit overpropped. That can heat it up. Merc's like to spin.

    The fuel filter thing I mentioned above will most likely show up at WOT as a fade off (leaning) or cut out. don't run it there, ever.

    hope it helps
    John

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    ok i'll pull it off today and set the timing max at 23 and check out a few other things hopefully get this thing back too how it use too run.....thanks so much for ur info... oh and it will turn the rpm's 5800-6000 rpms....at wot trimmed ...just overheats on me... then if ya let off on it back down too round 4000 rpm it will cool back down and u can go at it again for a short distance......i'll check a few things out today and get back with ya....thanks again for the help....

  11. #11
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    Have you checked the location of your fuel vent? I had a bass boat and the fuel vent was located on the starboard side just above the top of the transom. Water was creeping in and I never imagined this was the problem. Had the motor checked out and it passed with flying colors. It ran great until it eventually filled up with water again Motor wouldn’t idle worth a damn but ran great on top end. Moved the vent and the problem ceased. Just a thought, Naiscoot

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    check while engine is ideling if you have vaccuum leak spray all gasgets with carb cleaner if idle raises yot might have a leaking gasket from sitting they dry up that would cause a lot of problems and if it's lean up top it will run hot

    i had simalar problem and it was reed block gasket dryed up from sitting

  13. #13
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    I'd do what the captain says asap. Running lean on top could get pretty costly, as in burned pistons.

    hope it helps
    John

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    hey guys....checked the timing was way way off and lobing up and down.. on idle i removed the timimg idle stabilizer...and reset the timing fixed my rough idle...and made top end better.. I did check round the reed gaskets... seems no leaks... i still have the over heating problems at over 5000 rpm's but if u back it down too bout 4000 rpms it will cool back down and u can go again....any suggestions? maybe a headgasket problem?..

  15. #15
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    Sounds like T-stats not opening or your water bump vains? I know you changed the water pump but thats what mine did today and all the advice I recieved geared toward water and blockage? Mines a 225 promax and did exactly the same thing idle was good pressure good temps good at mid speeds like 3500-4000 she would get hot 180. I would back off and she would cool right down. i was told still have to check it out that some bits of the water pumps vains may have broken off and are restricting the flow requied for higher rpm. Could be that the bits get pushed up to the t-stats and blocks them enough that they arnt opening all the way? Just what Im going to check out. James

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