Mariner/Mercury 40hp 4cyl 2-Stroke Idle woes

Discussion in 'Outboard Maintenance' started by Bryan_G, Sep 22, 2014.

  1. Bryan_G

    Bryan_G I Love!

    This is my first foray into the micro skiff world. It's a great site here, and I'll post more up about the boat later.

    I wanted to start with the engine though. I recently picked up a used IPB 16 with a 1989 Mercury 40hp.  When I got it, the prop was spun, and there wasn't water coming out of the telltale on the muffs.  I replaced the water pump gaskets and impeller, and put on a new prop first thing.

    After that it was hard to start and there still wasn't any water. I changed the plugs and dumped most of the gas. Still sucks to start, but I'm figuring that out. It requires a lot of throttle and choke to start.  I also put a vacuum on the telltale and it started pumping water.  So I thought maybe it was just the muffs.

    All that, and I still wasn't really happy with it, but I put it in the water yesterday evening.

    Problems I found on the water:
    Forward movement in neutral
    Won't idle ~500 rpm's according to tach.
    Steam/smoke out of telltale until in gear and hitting about ~1200 rpm
    No big surprise - overheats while idling/moving slow.

    It planes around 3000rpm and seems happy to run there.  The water is pretty hot coming out of the telltale, but it has a good flow.

    I'm new to outboard repair, and don't have a lot of experience with them.  Are idling issues like this common for this year and type of motor? Do these impellers normally not move water at idle and lower rpms?

    Where do you guys think I should start next? I have the service manual and it's serial C159200 - C221499
  2. Brett

    Brett > PRO STAFF <

    Check the water tube grommet to block seal.
    Did you replace the entire water pump housing?
    Set idle rpms with engine in forward gear, while tied to dock.

  3. timemachine

    timemachine I Love!

    As for the idle, I would go through each of the carbs and clean out the insides before doing anything else.

    After 25 years, and ethanol, they may be pretty well gooed up.
  4. HaMm3r

    HaMm3r Well-Known Member

    500 seems like a pretty low idle RPM. I'd adjust that up to 700ish. I'd also replace the thermostat, it may be heavily corroded and sticking, which might explain your water pressure issues. As a quick test, you can pull the thermostat out and run it to see if it pumps better, just don't leave it that way.

    As for whether or not it should be pumping water at low RPM, well it varies. I've had motors that didn't pump much at idle and others that were like a fire hose. It all depends on the design and when the thermostat gets hot enough to open up.
  5. Bryan_G

    Bryan_G I Love!

    Thanks for the ideas. After some more research, I think I'm looking at 3 problems.

    I think the shifter is in wrong so it's not going into neutral. I'll pull the lower unit again and fix that.

    I think i have a leak in the exhaust manifold gasket or the top of the water tube where it goes into the power head. I've pulled the exhaust plate off and will replace those gaskets. Hopefully that's it.

    I'm assuming the carbs need a good cleaning as well.

    I'll post back when i get it back together and can test again
  6. Bryan_G

    Bryan_G I Love!

    I replaced the gaskets in the exhaust manifold and it still doesn't shoot water at idle. Once it's rev'd up there is enough pressure and it starts coming out.  I think there is still a leak at the top of the water tube.

    I fixed the shifter and it was definitely stuck in gear.  It will run in neutral now, but at low RPM it still tries to stall out.

    So I still need to clean the carbs, and I need to figure out where the water is losing pressure.

    For such a simple water system, it is quite finicky.
  7. DuckNut

    DuckNut Brandon, FL

    Telltale - I think you have a blade or two of the impeller going the wrong way.

    I had this happen to me and I never figured out how it flipped after being great for a couple months - but it did.
  8. Dillusion

    Dillusion devilray snob

    Have you cleaned out the thermostat and housing? You could have seagrass and shells stuck in there.

    Also, your thermostat doesn't always allow water to flow through immediately when the motor is started up. Sometimes it takes a minute or two for the block to heat up and the thermostat to open allowing water through.
  9. Capt Dan Medina

    Capt Dan Medina Well-Known Member

    Mercury motors are notorious for running warm. The thermostat regulates the water coming out of the pee tube. While it is possible there is another issue, I would start with pulling the thermostat, and let the motor run on. The muffs. You should see water come out of the thermostat. Turn off motor... Replace thermostat cover without thermostat installed. Start motor and it should pee a little better. If it does, you've identified your problem. Also, check the tell tale water line. Physically blow back thru the tube. If yours has a plastic grommet, you can stick a piece of weedeater string in the small hole.

    As for your idle, that motor should be adjusted in the water and under load to get the correct back pressure. Typically, 700 rpm is a good starting point. As one of the other posters said, secure it safely to the dock and put into gear.

    Now, before I'd focus on dealing with the idle issues, I'd focus on the carbs. Ensuring you have a properly working carb(s) will ensure the idle is set correctly. You can do the carbs yourself, but if too much, take to a good shop who will do a chemical bath to clean it up. Some good carb/brake cleaner can break up a lot of the junk sitting in the jets, and evinrude makes a thing called engine tuner which is also great for doing carbs and other chores. It comes in both a can and by the gallon depending how your going to use it.

    Where are you located?
  10. Bryan_G

    Bryan_G I Love!

    Thanks for the suggestions.

    It doesn't look like the '89 has a thermostat. But the hose is clear, and it works once it gets going.

    I have just been running it as is for now.  Maybe once it gets a little cooler, I'll take the carbs apart and make sure they are clean. After that I'll probably try to set the idle better. It's still dying if I don't give it enough gas.

    Oh, I'm in Austin, Texas.
  11. Bryan_G

    Bryan_G I Love!

    I wanted to give an update since a lot of these repair threads never have any follow up.

    I had the carbs cleaned and now it idles pretty well. It still takes a bit of effort to start, but it's electric start so I don't mind. I've been running it for a while now and once it's running it hasn't died.

    I also had the mechanic pull the powerhead and replace the exhaust gasket and plate. It still doesn't shoot water very well at idle. I don't really trust that he did a good job though. He didn't give me a parts list or tell me exactly what he replaced, plus he kept talking about stuff that just wasn't true for this motor.

    Anyone know a good Mercury mechanic in Central Texas?