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Yamaha 40 hp 2 stroke not running as great as I remember from last year

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  • Yamaha 40 hp 2 stroke not running as great as I remember from last year

    Ugh. I had the whole thing typed out, and with one stroke of a key, the sucker's gone. Here goes again:

    1984 Yamaha 40 ELN 2 stroke -- bought it last year, hadn't run for nearly 15 years. Took the carbs off, rebuilt, all new fuel lines inside the engine housing, new impeller, new fuel filter and housing, new fuel line and bulb from tank to engine, drain and fill the lower unit, etc. Ran great last year, nearly 15 mph which, on a pontoon, is not terrible for this setup.

    This year, I'm getting closer to 10 mph.

    Pulled the carbs, cleaned (not much in them, but some), put back together, went on the water, no change. Brought to my 82 year old buddy who is something else when it comes to engines... he tested the coils, and one was bad. Ordered online, replaced, took to the water, maybe a hair better, but still not getting higher rpms like I remember last year (no tachometer -- this pontoon has no frills -- lucky to have a steering wheel.) and kind of stumbles a little bit in the lower rpms. In fact, for the first time ever yesterday, it started and died. Just had to work to keep it running a little until it warmed up.

    I noticed yesterday that the fitting on the gas tank/line doesnt fit as tight as I think it should - seems a little loose? Primer bulb doesn't seem to get good and firm like I remembered last year -- it's a new fuel line and bulb last year, but... I suppose it could go bad, huh?

    As I said before, this pontoon had no frills -- not even power tilt. This spring, I started parting out an old OMC drive, and took the new trim/tilt pump and a cylinder, and modified it to work on this thing... Works well -- but I'm wondering if part of the hydraulic cylinder sticking below (maybe an inch or so) the engine pod would cause enough turbulence that the prop isn't getting good clean water??? But wouldn't that raise the rpms instead of lowering them? And, that wouldn't make a difference in the lower rpms anyway...

    Any other ideas?

    Thanks guys,


  • #2
    Re: Yamaha 40 hp 2 stroke not running as great as I remember from last year

    You need to hook up a tach to monitor RPMs ,it is the only way to see what motor is doing, mph means nothing to us.
    If you do not have the proper prop and hitting the proper WOT RPM you can hurt the motor.

    Check compression on all cylinders and post what you get

    Might read this and check to see if fuel supply is OK http://forums.iboats.com/showthread.php?t=283363

    check fuel pump for leaks
    and use timing light to see if timing is in spec
    The light can also show if you have spark on all cylinders if you move it to the different plug wires


    • #3
      Sign up today
      Re: Yamaha 40 hp 2 stroke not running as great as I remember from last year

      As I stated prior, I had some issues with posting the problem the first time -- it appears that I left out some information on the second posting.

      Last years speed was 15 mph, this year's is around 10 mph (gps verified both times)... so, while 'mph means nothing to us'... it DOES indicate that something's different.

      Prop never changed either.

      Still looking for a tachometer solution --- I have 2 multimeters, and neither measure frequency. Guess I get to buy a third.

      It appears as though the fuel supply is fine, according to the thread posted above, and I don't see any leaks anywhere.