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  1. #1

    95 slt jet boat conversion cutting out at low speed

    If someone could please give me some advice I would really appreciate it. I have installed the hull and engine of a 95 polaris 750 in a 13ft skiff. The engine compartment is homemade but all of the components are laid out almost exactly how they were in the ski. It has a center console which is made to be straddled and extends to the engine compartment. The only ventilation I haver installed so far is three vents (the ones that look like shower drain covers) in the front of the console. The last time I had it out to test it ran fine at high speed but would die out with my finger off the throttle after a little while and not start back up. The first time this happened I found that the choke cable had come loose, after putting it back on and tightening it, I was able to start it again and run at idle for awhile then it cut out again. It would not start back up until I opened the engine compartment. After having this happen a few times I idled with the compartment open and no more problem. The confusing part is however that when it dies, it is like someone hit the kill switch instead of sounding like it is starving for air. There is no sputtering, the engine is not hot, I did have a problem with a leak in the fuel selector valve but Im almost positive I fixed it, but I cannot imagine that this engine, at idle would create a vacuum in a compartment that does not even have seals around it yet. Can someone please help?


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    Welcome to the Hulk

    Show us some photos of the engine area, and the whole boat

    Electrical - you may have something loose or a connection/wire that isn't quite right, and taking the engine compartment cover off may have just been a coincidence.

    Carefully check ALL the engine and electrical grounds.

    Remove the positive battery connection, then use an ohm-meter to confirm zero ohms from the battery negative to;
    Engine block
    Black wire terminals inside the electrical box
    Grounding tabs for CDI, LR module and ignition coils

    Make sure the lanyard kill switch is working properly. If the switch or wiring is shorting, that will kill the engine and not allow re-start.

    When the engine dies and will not start, does it still crank at normal cranking speed?
    Does it have spark?

    Spark plugs - correct NGK spark plugs installed?

  3. #3
    I also thought that it may be an electrical issue. The engine cranks at normal speed when I restart it, the spark plugs are correct and I am pretty sure they are all firing. When I took it all apart I had to cut the kill switch wires, I rewired myself and I cannot see any shorts or issues that could cause this. I have not yet checked all the grounds however, the fact that this problem occurred many times when the engine compartment was closed, and never when it was open, I want to say that it has something to do with it.

    If the lid was too close to the spark plug wires would that have some effect? (the hood is made of wood, laminated in fiberglass)

    Directly above the flywheel, in the front of the engine, there is a rubber plug that looks like it once had wires running from it but they are gone. Would this be a problem?

    I am using a full size marine battery, and due to clearance and wire length issues the control box is in a separate battery box than the battery and they are sitting side by side (they are not stacked on top of one another. Would this cause grounding issues?

    The gauge is either shot of the fuse is blown, haven't been able to get to that yet. Could that be a cause?

    Is there a chance that a fuel issue would cause the engine to die like this? Like I said it sounds like it would if you hit the kill switch, or disconnected the battery.


    Ill have some pictures tomorrow to elaborate

  4. #4
    Moderator beerdart's Avatar
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    Welcome to the Hulk.

    It could be a fuel vent issue what fuel tank are you using? Oh and before we can help you any further we need pics??

  5. #5
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by jleworth View Post
    I also thought that it may be an electrical issue. The engine cranks at normal speed when I restart it, the spark plugs are correct and I am pretty sure they are all firing.

    When I took it all apart I had to cut the kill switch wires, I rewired myself and I cannot see any shorts or issues that could cause this.

    I have not yet checked all the grounds however, the fact that this problem occurred many times when the engine compartment was closed, and never when it was open, I want to say that it has something to do with it.

    If the lid was too close to the spark plug wires would that have some effect? (the hood is made of wood, laminated in fiberglass)

    Directly above the flywheel, in the front of the engine, there is a rubber plug that looks like it once had wires running from it but they are gone. Would this be a problem?

    I am using a full size marine battery, and due to clearance and wire length issues the control box is in a separate battery box than the battery and they are sitting side by side (they are not stacked on top of one another. Would this cause grounding issues?

    The gauge is either shot of the fuse is blown, haven't been able to get to that yet. Could that be a cause?

    Is there a chance that a fuel issue would cause the engine to die like this?

    Like I said it sounds like it would if you hit the kill switch, or disconnected the battery.

    I'll have some pictures tomorrow to elaborate
    There are no wires that go through the access plug on top of the flywheel housing. That hole normally has a plug seal to keep water out of the engine.

    Have you checked inside the electrical box? There should be zero moisture and no corrosion in there. The MFD fuse is a 1/4 Amp fuse. Replace with another 1/4 Amp fuse, not a higher rated fuse.

    If you cannot get the MFD to work then unplug it from the connector on the back of the MFD. Sometimes a dead MFD can cause strange problems.

    Large Marine battery? What voltage do you measure when the engine is running? Should be right around 14.5 volts. If it is below 13.5 volts then the battery charging system on the engine may not be working properly.

    How close are the spark plug caps to the bottom of the engine cover?

    Before the donor PWC was cut apart, was the engine running as it should?

    Has the engine been tuned up?
    Carburetors rebuilt?
    Fuel lines replaced (including the little ones between the carbs, fuel pump pulse hose, etc)?

    If the fuel lines have been replaced, was the fuel restrictor discarded from the return fuel line?

    Are you certain it was a 1995 model PWC? Is there a model code sticker on the engine?

    How is the fuel tank vented? These PWC fuel systems have a very specific venting system that retains mild positive air pressure.

    Click my signature link to see the common items that need attention with the Fuji engine.

  6. #6
    On a previous run, before this problem started to occur, the o ring in the hose connector, to flush the engine came out without me knowing. When I ran the boat the water was leaking out of there into the hull. I did not realize this at first but is caused the engine to kind of bog out. You had to pump the throttle to get it going, like if the engine was cold. It sounded like it was not getting fuel. I checked it and found the control box was just about submerged in the water in the hull. I know this box is sealed, cause I have had it apart before but I have not yet checked it since it has been submerged but I have relocated it to a marine battery box. On the next test, I got the boat really moving, up to about 35-40, which is half throttle for this thing, and as i was going around a turn it cut out on me, at high speed which sent me cruising, with no steering, into a patch of cattails lol. When I finally came stopped, the engine started right back up and I was amazed that I was able to cruise right outta them as if this thing were a fan boat. I found that the wires to the battery were loose so i tightened them. When this occurred a second time and sent me barreling into some dense lilly pads, clogging my intake, I headed in because I was unable to free the weeds and the sun was setting. So, on this test it was cutting out at high speed, on the next test (the last one) it was cutting out at low speed.

    Concerning the gauge, I believe after reading what you guys have said, that this problem started occurring as soon as i plugged the gauge in. I didnt think, because the gauge would not turn on at all that it was a possible cause of the problem. I will try changing the fuse, if that does not work I will unplug it.

    According to the previous owner the ski ran fine before I bought it. However, take that for what its worth, the guy was trying to get rid of the thing and he only wanted $250 for it.

    The breather lines, which if im not mistaken there are two of them, were disconnected from the rubber ends right before this problem started. I accidentally drilled a hole that was too big for it so I decided to exchange it for a larger, bulkhead style breather. Anxious to get out for another test, I did not hook the line to this breather because the existing hose is too small. I figured though that since the lines were still attached to the tank they would still function. Can I use a larger breather line? Does it matter where they are placed on the boat? How much positive pressure does it need? Is it more than 1 atmosphere (14.7psi)?

    To my knowledge the carbs have not been rebuilt and I believe the restrictor is still in place, unless I could have lost it when I took the lines off the engine. When I took the engine out of the donor ski the only fuel lines I disconnected were the ones that ran from the gas tank. I did not mess with any that I didnt need to to separate the gas tank from the engine.

    Concerning the fuel selector, mine is broken, the knob came off and the screw that holds it in will not come out. I noticed that this was leaking a while back because one of the clamps was broken. I replaced the clamp and no more visible fuel leak, dont know about an air leak necessarily tough. Do I need this thing? Can I just connect the three lines together?

    Im 100% sure hull of the ski was a 95 polaris slt 750. As for the engine, it is the blue one, three carbs, would be surprised if it wasnt stock. I am using the stock fuel tank too, I have made almost no changes to the engine or any of the critical components, just rebuilt the compartment it sits in.

    Second thought about the spark plug wires, without looking at it right now I would say that the plug to the back #1 I believe ( the one with the short wire) is closest to the hood, but i would say there is at least 2 inches clearance between it and fiberglass covered plywood.

    I really appreciate the quick responses pics will be up later on today.

  7. #7
    Click avatar for tech links/info, donation request K447's Avatar
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    Hmmm - cutting out unexpectedly at high and low speeds - sounds electrical to me.

    Engine starts right back afterwards - sounds like an intermittent electrical connection (loose/dirty/corroded) or an intermittent short (engine kill)

    The diameter of the fuel tank vent hoses is not critical, but the one-way check vales are important. There are two check valves, one lets air into the tank, the other releases excess pressure from the tank (the vent hose that leads outside the hull). The valve releases at between 1 and 2 PSI. Not much pressure, just enough to help the fuel get to the fuel pump and carbs.

    The check valves have small arrows on them and sometimes get stuck when they get old. If they are original, you should consider replacing them anyways as they are not expensive.

    Make sure those vent hose ends are well away from water and kept above the level of the fuel tank top.

    A missing fuel return line restrictor will not cause sudden engine shutdown. You should still check if you need one (not all Fuji models used it) and whether it is still in there (if you should have it).

    Fuel selector valve provides a safety function. If the fuel tank vent check valve sticks closed the fuel tank can build up excess air pressure (when warmed by the sun) and force liquid fuel through the carbs and flood the non-running engine. That is a fire/explosion risk, not to mention possible engine damage.

    An air leak at the fuel selector (or anywhere else in the fuel system) would not cause the sudden shut down problem you have described. Still worth your while to replace the selector valve.

    If you want to change to an ON-OFF valve and skip the Reserve function, you can cap off the MAIN fuel nipple at the tank and only use the Reserve nipple. They are marked in small print around the tank fitting.

    Spark plug wires - 2 inches of clearance is plenty. Among the common problem fixes is to unscrew the plug caps, trim off about /4 inch, then screw the wires back into the plug caps. This ensures the threaded tip makes good contact with the wire core.

    Carburetor rebuild should be on your to-do list
    May or may not help your current problem, but not rebuilding the carburetors has burned down many of these engines.

    Left field question: Are you sure there is zero water in the fuel tank?

  8. #8
    I like Boobs. wetwolf's Avatar
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    How about an exhaust leak, or a burned exh hose that might be bubbling on the inside?

  9. #9
    Polarisitis loonatik's Avatar
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    Quote Originally Posted by K447 View Post
    Hmmm - cutting out unexpectedly at high and low speeds - sounds electrical to me.

    Engine starts right back afterwards - sounds like an intermittent electrical connection (loose/dirty/corroded) or an intermittent short (engine kill)
    jleworth,
    I had similar problem on a Yami, it turned out the kill switch's rubber seal was bad, water got in rusted it and caused it to intermittently kill the engine, then either started right up or just cranked till the battery is drained. I would definitely take the kill switch apart and check it.
    Just my 2c

  10. #10
    SPEED KILLS, BUT YOU GET THERE QUICKER Keddano's Avatar
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    WE WANT PICTURES

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