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Mercruiser 140 - stalls @ acceleration

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  • Mercruiser 140 - stalls @ acceleration


    I've got a 1987 Mercruiser 140 with the Mercarb. I recently purchased this, and have been learning a lot about boats (love this forum). Had it on the lake last week, and it would bog on acceleration from time to time and would not drop below 3k rpms after shifting back into neutral. I spent some time on it this week rebuilding the carb (idle rough as well..) and making what I thought was a minor adjustment to one of the cables to fix the de-acceleration issue..

    So - took it to the lake today, started ok, idled around 1100rpms..didn't bother me too much, figured I could adjust that now that I was on the lake. Once I was clear of the marina, I gave it some throttle and it just died. Started it, gave it some throttle..and it died again! Here's what I did while I was on the lake to troubleshoot:

    Disconnected the carb throttle cable so I could manually throttle it up w/o putting it in gear. Would immediately die when I pushed the throttle lever. I verified that the rod that connects that to the accelerator pump was in place, and I had a strong stream of gas coming from the (venturi cluster?) when I pressed the throttle lever. After fiddling with things for a while, I figured it was better to take it home. I now have the carb in pieces again, and plan on putting it back together tomorrow & try again. I'm pretty sure I had all the measurements correct on the float & whatever throttle linkage there is.

    So..If this doesn't work , where else should I look? I did pump some gas into a jar and let it settle - no water. Bowl appeared to always be full of gas. It would die immediately, so surge or anything. Sometimes a backfire would happen, or a backfire when I started it up again.

    Thanks! I'll post the results of any suggestions after I try them.

  • #2
    Re: Mercruiser 140 - stalls @ acceleration

    Sounds to me like you don't have the Fuel/Air mixture set properly... when you attempt to throttle it up... it's sucking in too much Air and choking out. when starting.. this would also cause a backfire when the fuel pump kicks in...


    • #3
      Re: Mercruiser 140 - stalls @ acceleration

      Check for water in the fuel.Jerry

      Buy American made Cars and Trucks!!!Towing insurance for a boat,a GREAT thing.
      ​Tools to make my life easier: Battery tester, die grinder, temp.gun, compression tester, DVA meter.

      In Scenic , Beautiful San Sebastian by the sea Florida

      88 Bayliner 21CC 2001 Mercury 225 HP EFI


      • #4
        Re: Mercruiser 140 - stalls @ acceleration

        I spent some time on it this week rebuilding the carb (idle rough as well..)

        If you Know what you're doing,+ have full access to All the proper Tools,....
        Rebuilding an old Carb is a 50/ 50 proposition,.... At Best.....

        It sounds like you need a New Carb....


        • #5
          Re: Mercruiser 140 - stalls @ acceleration

          Originally posted by helqud
          started ok, idled around 1100rpms
          that idle is way to high... should be about half that.
          Don't mess with the carb just yet.
          First double check your base timing, if it's wrong fix it, then try it.
          Vacuum leaks will cause high idles as well, get a spray bottle of soapy water or some 6 in 1 foaming oil and spray around the carb base gasked, all round the intake gaskets etc and look/listen for a leak.
          and with any boat that won't hold a proper idle, do a compression test.
          When compression gets low idle is the first thing to go.
          Again... i'm a poet and i dont even know it.
          just because you found it that way does not mean it is supposed to be that way.


          • #6
            Re: Mercruiser 140 - stalls @ acceleration

            Well, I already had the carb broken down before I posted this. I'm going to finish cleaning it and put it back together today. It's in pretty good shape..looks like someone did a recent overhaul (new float, needles, etc). Once it's on, I'll check for leaks. I don't have a timing light handy, so I won't be able to check that until later this week.

            Thanks for the suggestions, I'll try them all and post back (carb leaks, timing, compression).


            • #7
              Re: Mercruiser 140 - stalls @ acceleration

              I believe idle should be 650-700 in gear. Going from memory .

              Did you clean the filter at the fitting going into the carb. How about the other filters. Fuel pickup in the tank. And the anti-syphon valve, take it off and blow thru it maybe it's sticking.

              Back to the carb, you soak it overnight. I use acetone. Blow it dry. That kit you have it the correct one for the carb? Accelerator pump fit tight. Ball bearing you take it out, and put a new on in there? A few real tiny holes in the carb when you blow it dry make sure there ain't nothing left clogging them. If not use a can of carb cleaner with a straw.

              Now to the setup after its on the boat. If it has 2 screw, all the way in gently with both and out about 2 turns to start with. Than start it and adjust.
              " The Chief Has spokin "

              1986 Citation 19' 140 merc motor transplanted Sept. 2008. Stringer drive.
              1976 Arrowglass couger, 76 Evindrude 70 hp.
              1970 Aluminum jon boat 3 hp air cooled sears out board

              Be good, be happy, treat people as you would want them to treat you, for tomorrow is promised to no man.


              • #8
                Re: Mercruiser 140 - stalls @ acceleration

                if you need it, I have the merc manual to rebuild the carb "the factory way" in pdf form. pm me and i'll email it to you. sounds like the mix or an accelerator pump issue. and as stated, 1100 is too high for idle.

                good luck


                • #9
                  Re: Mercruiser 140 - stalls @ acceleration

                  I have the factory manual in pdf form thanks to a previous post. I put the carb back together, taking my time to make sure I did it right(tm). Starts with the first touch now, but the idle is still pretty high. I touched the throttle a few times in the driveway for a few short seconds (muffs on..but still), and no backfiring etc. This was after adjusting the idle mixture.

                  Next on the list is to check the timing, hopefully later this evening. Thanks for all the advice so far!


                  • #10
                    Re: Mercruiser 140 - stalls @ acceleration

                    commit this to memory dude...... it's always timing before fuel.....
                    everyone else that reads this.... commit it..... whenever you think you have a fuel problem... take the 2 minutes it take and buy the cheapest dang timing light you can can and just check it....
                    idle's high... check the timing
                    idle mixture screws do nothing... check the timing
                    bogs on acceleration.... check the timing
                    incorrect timing can make an engine appear to have fuel issues!!!
                    just because you found it that way does not mean it is supposed to be that way.


                    • #11
                      Re: Mercruiser 140 - stalls @ acceleration

                      I agreee. Check timing first. I was having a similar problem, wouldnt stall but had a bad hesitation off idle. I rebuild the carb, which needed it anyway, and it fixed the off idle hesitation (acclerator pump) but still had poor acceleration. Put 2 more degrees timing into it and completely changed it. Smoother all around, and no more hesitation, pulled hard all through throttle.

                      I was about ready to trade off this 3.0 for a bigger engine, but after getting it fixed up, I am pretty happy with it now. And its easy on gas.


                      • #12
                        Re: Mercruiser 140 - stalls @ acceleration

                        doesn't take much does it
                        just because you found it that way does not mean it is supposed to be that way.


                        • #13
                          Re: Mercruiser 140 - stalls @ acceleration

                          Ok, the timing was way way way off. I've adjusted it to 6deg BTDC, but it's now idling way high (2k rpms!). The idle adjustment screw is backed all the way out, so the throttle valves are completely closed! Here is a question that will really show my ignorance - what determines idle speed in that situation? Is my float set to high?


                          • #14
                            Re: Mercruiser 140 - stalls @ acceleration

                            Well, I'm a genius! A closer look at the rod that connects the throttle lever with the accelerator pump lever fixed the problem. It was keeping the throttle lever from closing all the way, and hence the higher RPMs. I adjusted it per the manual (thought I did the first time! Gotta be more careful). Re-did the timing, and it idles now around 1300 rpms..still way to fast. I should say 1300 rpms is where it idles smoothly. If it is below that, no amount of adjustments on the idle mixture screw do anything useful. I did take it out on the lake last night, and the acceleration stalling/backfiring problems are solved.

                            So at this point: New carb? I sprayed some soapy water around the base of the carb while it was running, didn't see any leaks. Worth buying a rebuild kit & poking around a salvage yard, or just break down and buy one on the internet? Any recommended sites for a mercarb replacement?


                            • #15
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                              Re: Mercruiser 140 - stalls @ acceleration

                              You are describing two classic symptoms those of a vaccum leak and those of mixed up plug wires.

                              With a vaccum leak, at some point in the RPM range the air/fuel mixture ratio becomes correct and the idle smooths out but you won't have much power since you are running extremely lean. A smooth, correct idle of 450-600 rpm is impossible.

                              With mixed up plug wires, again at some magic point the firing sequence will overlap and the miss will smooth out. Add a little juice to it and get spark advance and everything good goes bad.

                              For vaccum leaks do what was suggested above only forget about looking for something, just listen. If you spray and the idle changes you have a leak.

                              For plug wires, just locate the firing order on the engine or in the manual and make sure that the plug wires are on the correct plugs according to the firing order and their location on the distributor cap. Start with # 1 and continue through the order.

                              Hope this helps.