25/35 impeller housing know-how

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  • johnyrude200


    Replies: 782
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    #2581

    Last winter, I was given some words of the wise about the various pressure relief/water passages of the older 18-20-25 impeller housings, which, if clogged, would basically stop water flow through the motor (and there is 1 in particular the size of a nearly a pin hole which if clogged is like flipping a light switch on water flow).

    Are there any of these present in the newer 20-25-28-30-35hp motor housings that I should know about?

    I only ask because I have an ’82 35hp short shaft that I’m getting ready to tear apart tomorrow because there is basically NO water circulation from the gearcase, and on it’s initial run, had solid water flow, then became intermittent, then has stopped completely. I installed a new impeller, and the old one looked totally fine. I closely inspected the impeller housing and didn’t see any clogged holes, and I am guessing it is a collapsed water tube guide and/or upper exhaust/lower power head water tube grommet that is no good.

    Appreciate the help!


    frankr

    US Member - 1 Year (includes $3 online payment fee)
    Replies: 6715
    Topics: 51
    #24189

    I think you are referring to the angular hole between the pump housing’s water tube grommet area and the cup surrounding the drive shaft. The purpose of that hole is to keep the cup filled with water. And the purpose of keeping the cup filled with water is to provide a water seal around the hole that the drive shaft passes through. If it were not for that water seal, exhaust gasses under slight pressure in the area could blow through the drive shaft hole, into the pump housing killing the pump prime.

    With all that in mind, I just happen to have a 35hp plastic pump housing laying here. It does not have such a hole to provide a water seal. Instead, it has a "real" seal–a conventional lip type seal pressed into the pump housing. Also, I believe the drive shaft runs in a tube isolated from the exhaust, and vented / drained to the outside world.

    So, I suppose your pump housing seal could be shot, allowing air from the outside world to enter the pump. Or maybe the pump housing is not sealed to the drive shaft isolation tube. BTW, that shaft seal is not perfect. When everything is correct, water still flows out the drain holes in the bottom of the isolation tube (located near the lower rubber motor mount). In fact, it is a surprising amount of water. It is a wonder to me how that much water gets past that seal. I must not be seeing the whole story.

    Hope this helps. At least lots of food for thought.

    EDIT: Now that I sat here and typed all those words, I just checked and the pump housing I’m holding in my hand is not for a 35hp. I don’t know what it fits. But anyhoo, you got a dose of education and something to consider as you look at your motor.


    johnyrude200


    Replies: 782
    Topics: 186
    #24196

    What’s great about this particular housing is it is from one of those years where OMC decided to have the shift shaft go through the housing. This one does have a rubber lip that goes around the driveshaft, but I didn’t see any holes leading into the impeller area. I’ll update once I get the issue resolved.


    fleetwin

    US Member - 2 Years
    Replies: 4737
    Topics: 46
    #24198

    Again, I just don’t want you to overlook that darn overboard cooling telltale. Does this engine show signs of overheating? Or, is it possible that there is just some debris getting caught intermittently in the telltale elbow/nipple. Again, don’t try to unthread that plastic elbow from the exhaust manifold, it will probably snap off and create a mess for you.
    So, this engine has the thru hub gearcase with the shift rod that goes through the impeller housing, correct? That shift rod must be properly sealed through the impeller housing or there will be an air leak into the pump, especially at high speeds. Post some pictures if you have questions, this procedure is kind of hard to explain on paper.


    johnyrude200


    Replies: 782
    Topics: 186
    #24202

    This is a different 35hp motor from the one I worked on last week (that one seems OK to go now, thanks again).

    This is an ’82 35hp SS/Tiller model with ES.

    It is getting no water into the water jacket – no water passing through the telltale. I pulled the tell tale right off the plastic ‘L’ and all it is spitting is air, or nothing at all. The cooling system was acting erratically even on the initial test run.

    With this motor, some knucklehead overtightened the aft electric start screw (the one that goes on the front of the crankcase), stripped the orifice, retapped it poorly with a larger screw, and restripped it. It is so mucked up that I doubt JB welding a stud in there would hold the starter in the right position (and I’ve tried a couple of things to shim that starter, but it is still working it’s way out of alignment). So I am probably going to have to just switch out powerheads regardless before I can fill this order for the customer because he needs a short shaft motor with ES/remote for his whaler.

    So this powerhead is getting switched out no matter what, and will be used as a manual start motor next year regardless.

    Just figured I’d ask about if there were any water cooling system issues other than the collapsed water tube guide I should know about before getting more invasive.

    Thank goodness for parts motors!


    fleetwin

    US Member - 2 Years
    Replies: 4737
    Topics: 46
    #24204

    Well, the exhaust manifold is the first stop on the cooling system powerhead circuit, so problems "down the line" in the powerhead wouldn’t really affect water flow into the exhaust cover/cooling elbow. So, if there is no flow to the exhaust cover/cooling telltale elbow, then the problem is before this area. So, that would leave the exhaust tube/grommet/water tube guide/water tube/water pump as suspect.
    I suppose it is possible that a blown head gasket/exhaust cover gasket could create enough back pressure to stop water flow but not likely. And, leaky gaskets would probably create an internal water leak/poor running as well.
    Post some pictures of the problem threads on the block, perhaps someone will have a creative repair that will save this powerhead.
    If the powerhead is salvageable, then I would recommend pulling it off and going through the entire cooling system to make a reliable runner out of this engine, unless you fear corroded fasteners might create a nightmare, or the owner is unwilling to make the investment in the engine.


    johnyrude200


    Replies: 782
    Topics: 186
    #24205

    This motor is a unit I am selling, so I am not afraid of tearing it apart to make it right for the customer. My main concern is getting the cooling system working of course.

    With the stripped threads, I can just repurpose the powerhead (which is running tops) onto another pan/leg and make it strictly a manual start motor. It has a provision on the carb for manual choke so no big deal. It just won’t ever be able to take an electric start without swapping out the front crankcase half.


    fleetwin

    US Member - 2 Years
    Replies: 4737
    Topics: 46
    #24207

    OK, guess I’m confused.
    In any event, I would try to find the cause of your problem first. So, I would begin by pulling that gearcase back off and pulling the water tube out to check for bends/kinks/breaks. Then, try to have a look up inside the exhaust tube to see if the grommet/guide are messed up. If that stuff appears to be OK, I would get back to the water pump.
    Does the engine seem to cool OK at low speeds/in neutral/at the dock? I guess what I am getting at is trying to pinpoint a possible air leak problem. This water pump is a tricky item, especially due to the added burden of having to seal that shift rod through the housing. Is the SS impeller plate mating surface on the gearcase in good shape, perhaps it is pulling exhaust through there.
    Again, let’s pinpoint the actual problem first, I surely don’t want to lead you down a bad path.
    Is this a fresh or salt water engine? Does it look like others have had the head/exhaust cover off before?


    dan-in-tn

    US Member - 1 Year (includes $3 online payment fee)
    Replies: 961
    Topics: 78
    #24208

    First of all I agree with most of what has been said here, but the exhaust cover consist of a stainless baffle plate that warps badly on the first overheat. When this happens it opens the cooling system for exhaust pulses to blow the water out of the water pump. At idle you may notice the pump trying to get water to powerhead, but only if the plastic tube around the driveshaft above the pump is in good shape (o-rings) and all. That is something that very seldom occurs with an overheat of this motor. The pump housing itself is prone to warp or the cup turn in the housing. When these motors overheat, lots of things have to be inspected before you get back to square one and have a good cool running motor.

    Dan in TN


    johnyrude200


    Replies: 782
    Topics: 186
    #24209

    This is a situation where the very first time I ran it, plenty of water was pumping, then it became sporadic, then drips, then steam, then nothing. I will say that the water tube is barely staying up in the upper exhaust housing. It doens’t feel as if it’s grabbing that rubber gromet at the top.

    I just pulled apart a frozen 35hp parts motor to get better acquainted with these motors and see that pastic gromet. I can see how if the motor overheats, then the inner exhaust housing heats up, that plastic melts and/or the grommet deforms/stops water flow.

    Either way I have to pull the powerhead and switch it with another because the front half crankcase orifice for the starter is no good, and I have to use this short shaft exhaust housing as per the customers request. So I’ll be examining that water tube and grommet. Hopefully the lower exhaust tube screws aren’t stuck. Will report back tomorrow. Beach time for now!

    Dan, where is the stainless exhaust baffle you’re talking about (parts diagram #)?

    This motor is pumping basically nothing at all up from the gearcase, so its something before the overboard water indicator that is the culprit.

    Fleetwin, this does not appear to be a salt motor, and it looks to be with an intact original exhaust/cylinder head.

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