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  1. #1
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    sometimes low rpm trouble

    hey folks. this is my first post here. I recently bought a 1976 Checkmate with a 1985 Mercury 115hp inline 6. Since buying it, I've rebuilt the fuel pump, replaced some fuel lines and clamps, replaced stator, powerpacks, rectifier, control cables, and some wires. Changed the plugs, LU oil and re-lubed everything with and without a zerk. Set the timing and adjusted the idle mixture screws to about 1 1/4 turns out. Got it going and had it out on the lake. I love it!

    But here's the problem: After, and only after, I've run at WOT or close to it and throttle back, it wants to stall. Starts right up again and then if I open up the throttle, it limits to about 2300 rpm. Sounds smooth but no oomph. Then, if I rev it a few times, it will sometimes "catch" and go right up to full power, get on plane, and be good. Does this some of the time, not all of the time.

    Control Box is an old Merc Commander style and the "throttle only" button is sticky but works if you mash it.

    Any ideas what could cause this?

    Thanks.

    Diane
    Ithaca, NY

  2. #2
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    May 2002
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    My 1st guess would be too lean

    Try opening each carb srew 1/4 turn (if it has adjustable idle mixture). My 1975 115 did and it was picky about being lean at idle.

  3. #3
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    Iwould replace all the gas lines and if it has a pimer bulb I would replace that and if that don't work put a Holley red fuel pump on it and block off the manual one.I f you do put a electric red Holley fuel pump on you don't need the primer bulb

  4. #4
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    Took boat out again

    Thanks for the replies so far.

    ok, i had the boat out today for about a 10 mile run. Lots of fun but the problem persists. _I really don't think it's the control box. _Starts up with not much trouble...throttled some and choked. _Runs pretty smooth in low idle when I'm starting up. _About 850rpm. _Then I bring it up to speed: _gets up to about 45mph at full throttle.... could run all day. _When I throttle down to below about 2600rpm most of the time it will slowly lose rpm until eventually it will die unless I give it more throttle. _In other words.. I'll throttle down to about 2500rpm, leave the lever alone, and the motor will keep slowing down over about 30 secs until it dies. _It only does this at the lower speed. _At that point it's hard to start and runs a bit rough. _Then if I throttle up over 2600rpm, I'm good for miles. _When I come down again, same problem.
    _
    Feels like maybe a fuel problem? _I dunno. Clueless. _ _
    _
    Diane

  5. #5
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    midrange maybe

    i'm really new at working on this motor but am trying to think my way through this one. my carbs are side bowl back draft. could the back draft system/jets have something to do with this problem?

    or... are there supposed to be some reducers in fuel lines that might be missing?

  6. #6
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    low speed timing

    low speed timing is set at 5deg BTDC. High speed is at 21deg BTDC at cranking speed.

    The other thing that is odd that might be related is that I can't quite get the idle speed low enough. With the screw backed out all the way, still can't get below about 1000rpm in neutral, 850 in forward.

  7. #7
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    newest version of motor behavior

    I made sure I had no fuel line leaks, made sure timing was set. Disconnected the idle stabilizer and the fuel enhancer. Removed the little brass jets from the float chambers. Banged on the side of the float chambers. So there is improvement but still weirdness.

    Took the boat out today to see how the last round of repairs/mods worked. Motor started up in neutral with choke when I turned the key.. needed no throttle. That is after sitting for three days. Great improvement. No stalling. Also great improvement. Went right up onto plane.. to about 5300rpm no problem. The problem is with the mid-range but it's not quite how I described it before. Here goes....

    If I throttle up to about 1700 and then leave it alone, the motor will ON ITS OWN increase throttle to about 2700rpm and then stay there until I move the lever myself. If I'm above 2700rpm and then bring it down to 2700rpm, the motor will ON ITS OWN throttle down to about 1700rpm and then stay there unless I move it by hand.

    So what the heck is that about?

  8. #8
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    It sounds like to me that it is leaning out at mid range your on the right track when you mentioned the backdraft circuit what you need to do is put the backdraft jets back in the bowl covers and remove the backdraft hose were it connects to the black nipple and take a BB and push the BB up into the tube and reconnect hose do this on each carb then you need to remove the high speed jet whitch is located behind the brass plug in front at bottom then go up 2 jet sizes problably has 74ths need to go up to a 76ths This sounds like a lot but it will more than likely solve your problem And also remove the idle stabilizer and throw it as far as you can because it can wack out the timing at wot and smoke a pistion also if you say you can not get engine to idle down could be bia circuit in switch boxes and also check the linkages between each carb and make sure thre sync right all need to be closed at idle and need to open at the same time Sorry to be so long winded Iv'e been doing this for 21 years and I cut my teeth on these engines Hope all this helps OPTI

  9. #9
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    Thumbs up I agree with OPTI,

    On pluggin The back draft hoses. We used to do that on every one that came into the shop. You should check that for sure. Some of the later motors came from mercury that way, If I'm not mistaken

    Also, Talon made some good points on something binding.

    Is this site great or what?!!

    Good luck, Brian
    Last edited by shadowman; 06-23-2004 at 10:44 PM.

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