40 HP Mercury 4 stroke

Discussion in 'Bubba's Outboards' started by blackwaterkatz, Mar 12, 2007.

  1. blackwaterkatz

    blackwaterkatz Active Member

    Messages:
    3,659
    State:
    Andrews, SC
    We use a couple of 4 stroke 40's at work. One of them has become hard to crank after it warms up. It will start just fine cold, idles and runs great at speed, and will run all day long as long as you don't shut it down. After it is run a while and warms up it seems to develop vapor lock or something when we shut it down. We can remove the fuel filter on the engine and drain the fuel out, then reinstall and it will usually fire right up and run just fine...until we shut it down again. We have been leaving it running all day long to avoid problems with cranking again. This is a very busy time for us and we can't spare it to go to the shop right now, if possible to avoid it. There is also a water separator filter inline, which is changed regularly.
    Any ideas?
     
  2. blackwaterkatz

    blackwaterkatz Active Member

    Messages:
    3,659
    State:
    Andrews, SC
    What, no takers? Surely someone has an idea. :big_smile:
     

  3. jtrew

    jtrew New Member

    Messages:
    4,404
    State:
    Little Rock, AR
    Have you tried unplugging the gas line to shut the motor off? Pretty inconvenient, but if it works, you might be better off shutting down that way till you find the underlying problem.
     
  4. Thommo

    Thommo New Member

    Messages:
    19
    State:
    Tasmania, Australia
    One possibility is that your air filter is dirty. I had a similar problem some years ago with a 2 stroke. Had me beat; would run all day but impossible to start when hot. Ater doing all the obviouswith filters, carbies and lines I cleaned the sponge air filter in kero and then rinsed it with petrol and it workd fine.
     
  5. blackwaterkatz

    blackwaterkatz Active Member

    Messages:
    3,659
    State:
    Andrews, SC
    Thanks, John, and welcome to the BOC. I'll check on the filter deal. I've never had much dealings with 4 strokes, but I'll see what the manual says. Also will try unplugging the gas hose some time -- when I'm near the landing. :big_smile:
     
  6. Bobpaul

    Bobpaul New Member

    Messages:
    3,039
    State:
    Supply NC
    Tommy, hot 4 strokes don't like to start. Do you know that the water pump is in good shape? I'll check further today on the problem.

    John, was that 2 stroke on an outboard, or some other unit that you operate on land, as in a chainsaw? Not only the air filter restricting the intake, but there's a screen in the exhaust on some 2 strokes that restrict exhaust and give a similar problem when clogged with carbon.
     
  7. blackwaterkatz

    blackwaterkatz Active Member

    Messages:
    3,659
    State:
    Andrews, SC
    Bob, the motor was serviced last year, but I'm not sure exactly what was done. It had about 700 hours on it then. The pump seems to be pumping good pressure, not signs of overheating, but I'll check the temp with my hand next time out. The other motor always starts just fine, no problem, but this one acts up. I'll ask about what was done last time, and check on the water temp. I haven't been using that engine for the past couple of weeks, but someone else has. I did look underneath the cowling and couldn't see where any fuel lines were out of place, etc, compared to the other engine that might allow for hot fuel, etc. I'll find out more about when the water pump was replaced too. Thanks
     
  8. blackwaterkatz

    blackwaterkatz Active Member

    Messages:
    3,659
    State:
    Andrews, SC
    Ok, the water temps feels cool to my hand after running at speed, then stopping. I did notice something from the exhaust port that looked a little like steam (or maybe white smoke) but it went away within a few seconds of coming to a stop. Could it be that a little water entered the exhaust opening just below the water pump telltale momentarily when I came off plane, causing steam?
    The water pump was changed lasted year. The motor doesn't seem to be running hot or anything; I looked good under to cowling but couldn't see anything obvious. I'm not sure it's a carb problem, cause it starts, idles, and runs great; just doesn't want to restart when it's been run for awhile. If it sits up for a short time (20-30 min?) it will restart ok. ?????
     
  9. blackwaterkatz

    blackwaterkatz Active Member

    Messages:
    3,659
    State:
    Andrews, SC
    After several trips to at least 3 other repair shops, I ended up taking the boat to the dealer where I bought my personal boat. They took the extra steps to troubleshoot by running the boat on their test lake to simulate the conditions. Finally diagnosed as corrosion (?) buildup (from salt water, most likely) where the water tube enters the transition plate beneath the engine, not allowing enough water flow at WOT.
    Also found a situation of too much fuel pressure at idle and corrected that. The engine has 1081 hours on it as of now and runs great.