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  1. #1

    Default 1992 2st 4hp throttle issues

    Hi..I just bought a used 4hp 2 stroke 1992 yamaha. I'm having some issues, I think, with the throttle. It seems to me the tiller throttle isn't affecting the operation of the motor, but the choke does? I'm totally new to this engine, and can't seem to find an owner's manual on it without paying $30 dollars I don't have..so I'm kinda stuck trying to figure out how to even use this thing, so I could just be doing it completely wrong.

    I was under the impression that I close the choke to start it, then open it after it starts..but it won't start closed, only open. Here's a video to show me starting it / how it's working.

    This video shows the cover off, and appears that the throttle cable is hooked up..just don't know why it's operating the way it is. Is this normal? Am I forgetting to use one of the other 10 levers on this thing that I can't actually identify? haha..plz help.


    it's worth noting that this is the first start of it, and I haven't used it on a boat yet..I had planned to try it tomorrow on a lake anyway, just to see how it goes. Could the low water in the bucket affect the throttle ability? I really can't find any info on operation of this motor for some reason..but I know I really want to take my 12 ft aluminum boat fishing tomorrow


  2. #2

    Default Re: 1992 2st 4hp throttle issues

    Furthermore, can any one tell me..
    Should #1 freely move, or does it have to be running to move it? Assuming this is forward / reverse? Cuz mine is seemingly stuck.

    What is #2, #3 for?

  3. #3

    Default Re: 1992 2st 4hp throttle issues

    Quote Originally Posted by sogboat View Post

    Furthermore, can any one tell me..
    Should #1 freely move, or does it have to be running to move it? Assuming this is forward / reverse? Cuz mine is seemingly stuck.

    What is #2, #3 for?
    You're in luck. A free digital download of your engines service manual is here:

    #1 is the gear shifter. It should move into forward and reverse freely, though shifting into either gear should only be done when crankshaft is spinning (engine running). In reference to the 2nd video...that is the starter neutral in gear protection. Its purpose is to prevent the engine from starting if the gear shifter is not in neutral.

    Not sure what #2 is. #3 looks to be the manual trim/tilt lever.

    In reference to your 1st video. I find it best to use the word "engaged" when referring to the choke, as to not get confused with the carb butterfly as open or closed. So if the choke is engaged, the butterfly is closed, thereby decreasing the amount of air into the intake, thus causing a rich mix for cold starting. The choke should be engaged for first start of the day. After the engine is warm you should no longer need the choke engaged. From the video, it looks as though the tiller throttle is not functioning. When the engine is running in neutral as it is in the video, when you turn the throttle grip the rpms should increase or decrease.

    With the engine off, neutral gear, and cowling off, twist the tiller grip to max throttle then back to no throttle. Watch the throttle cable where it attaches to the throttle lever near or on the carb. As you twist the throttle grip, the cable should be also pulling the throttle lever open and releasing to close. If not, then you've got an issue with the throttle cable.

    RE: Smoking
    Looks to be an excessive amount of smoke. However, it looks as though the prop boss was not completely submerged. When submerged, exhaust exiting prop boss would not be seen since it's underwater. Another note...it's an old 2 stroke. Excessive smoking is not uncommon. Just be sure you've got the proper fuelil ratio mixed per yamaha's suggested ratio.

    Now that you have a service manual, follow the instructions in the section "periodic adjustment and maintenance" for applying grease. You'll need a tube of marine grease, a grease gun, and a zirk fitting for the gun. Grease all points. Who knows how long its been since the engine was properly greased. It could be the reason the shifter is not moving.
    Last edited by ClassicAQ; 10-01-2017 at 08:02 PM.

  4. #4

    Default Re: 1992 2st 4hp throttle issues

    I've come to realize..it starts in neutral, then I can engage gear and it goes..but throttle issue still exists. The cable looks good..but I feel like maybe internally it's somehow stuck on full throttle..and opening the choke gives it the air to go wide open. Check out this video..any idea?

  5. #5

    Default Re: 1992 2st 4hp throttle issues

    And thanks for the manual..mines a 4mshq is that the same as a 4n/5n? I couldn't find anything anywhere..so even if not, I'm sure they're similar

  6. #6

    Default Re: 1992 2st 4hp throttle issues

    Re: Service Manual
    It was the only 1992 4hp manual available, so I'm assuming its content applies to your model.

    Take a look at #9 in this parts blow up, control engine. I have the number circled in red. This knob on the tiller is a friction control knob. Is it possible the knob is turned too far in the direction of less friction? Try turning the knob clockwise, if this is the direction to increase tiller handle friction. Turn counterclockwise if not.

    I'd remove the carb intake and take a look at what's happening with the carb throttle plates and butterfly when you're engaging/disengaging choke, as well as when your turning tiller throttle. When choke is engaged, the carb butterfly should completely close, or if you engage only halfway then the butterfly should only close halfway, etc. As you turn tiller handle to increase/decrease throttle, you should see the throttle plates inside the carb move back and forth.

    I really can't explain why rpms are increasing as you engage more choke, somehow acting as the throttle body. Who knows what has been tinkered with in the past with this engine. It's quite possible you'll need to disassemble the chokes articulating parts and tillers throttle cable, then reassemble following the service manual.

    Click image for larger version. 

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  7. #7

    Default Re: 1992 2st 4hp throttle issues

    There should also be a lever located on the carb, where the throttle cable attaches. You can manually push this lever down, either with engine on or off. Flip the tiller over, exposing the underside with the cables. Push this carb throttle lever down and watch what happens with the cable. Also, push this carb lever down when engine is running in neutral. What happens...increase in rpm, nothing?

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