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  1. #1

    Red face Yamaha 30 hp not running at full power

    Hey Guys,
    Looking for some advice and what to look for.
    96 Yamaha 2 stroke 30 hp with 3 cylinders is running well with new spark plugs. After 2-3 weeks or approx 10 hours of run time, cylinder 1 and 3 plugs are getting wet fouled (black deposits) and engine is losing power. I am using spark plugs specified in the manual. Set up a tiny tach and cylinder 2 is showing WOT 4200 RPM, but 1 and 3 are only reaching 2000 RPM at WOT. Compression is 120 on all 3 cylinders.

    Any help would be much appreciated.

  2. #2

    Default Re: Yamaha 30 hp not running at full power

    Whenever you've got 1 or 2 fouled plugs and the other(s) are good, it's a good idea to time and sync carbs from scratch. Or at the very least, reset the pilot screw for each carb. The picture below comes out of your engines service manual and it details the number of turns for each pilot screw. Important to note, each carb has a different number of pilot screw turns to spec. These are spec of course and shouldn't imply where each screw should ultimately be. Think of it as a starting point for each pilot screw. As the engine ages, it may like the mix more lean or more rich.

    First start up engine and get it warmed up before making adjustments. Remove plugs and clean. Reinstall plugs. Turn in each pilot screw until it seats (gently), noting the number of turns (if you find that carbs 1 & 3 pilot screws are turned too much IN, this is probably the reason of fouled plugs). Follow the specs for each pilot screw turn out. Start engine and listen to how it runs. Don't be concerned with the idle speed, as this adjustment is done last. Start the pilot screws adjustment by turning them in or out 1/4 turn increments. Let engine catch up with your adjustment. If engine is bogging down to stall, turn each screw 1/2 turn the opposite direction from your initial turn. Make small adjustments. Once she's purring nicely, if the idle speed (rpms) need adjusting then turn in (increase rpm) or turn out (decrease rpm) the idle speed screw. Make small adjustments.

    If it's been a few years or more since you last pulled the carbs and jets for cleaning, this will make a big difference in performance. Especially since you're losing power. The orifice of each jet is tiny and doesn't take much debris for it to become clogged. I wouldn't put too much faith in the tiny tach. They are helpful at times, but unreliable. Other than a true tachometer, nothing is more helpful than a good set of ears to listen to what the engine is telling ya. Good luck.



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    Last edited by ClassicAQ; 09-07-2017 at 11:27 PM. Reason: xxxxxxx

  3. #3
    Join Date
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    Default Re: Yamaha 30 hp not running at full power

    I agree, do a proper carb reset. While you have the carbs off, inspect your reed valves and make sure they still look decent. The tiny tach can give you a false reading if it is too close to the other coil leads.

  4. #4
    Join Date
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    Default Re: Yamaha 30 hp not running at full power

    Trouble shooting 101----Do a compression test and check for STRONG spark on the leads.----Strong spark is able to jump a gap of 3/8" or more.

  5. #5

    Default Re: Yamaha 30 hp not running at full power

    Quote Originally Posted by racerone View Post
    Trouble shooting 101----Do a compression test and check for STRONG spark on the leads.----Strong spark is able to jump a gap of 3/8" or more.
    Not disagreeing with your suggestion, compression and spark are 2 of the 3 systems necessary for top performance. But the 3rd system, fuel, is responsible for 90% of all engine issues. Even more so with 2 stroke carb'd outboards. Fuel system should always be where troubleshooting begins.

  6. #6
    Join Date
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    Default Re: Yamaha 30 hp not running at full power

    Sorry, most carburetor " problems " turn out to be something else !

  7. #7

    Default Re: Yamaha 30 hp not running at full power

    Thanks everyone,
    Just figured there is no warm or hot water coming out of the pee hole even after the motor runs for a while. It is just cold water all the time. Could a thermostat be the problem ?

  8. #8

    Default Re: Yamaha 30 hp not running at full power

    Quote Originally Posted by racerone View Post
    Sorry, most carburetor " problems " turn out to be something else !
    Hi all,
    Changed the thermostat but the issue is the same. The spark plugs are getting fouled after a month (10 hours or so) and engine is losing power. Checked spark and compression, they are fine. Engine is running fine when i put in new spark plugs. Opened the air intake cover and noticed the butterfly valves on the three carbs are always open, at neutral, idle and at full throttle. They are only closed if the choke lever is pulled. Is this normal ?

  9. #9
    Join Date
    Sep 2010
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    ontario
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    Default Re: Yamaha 30 hp not running at full power

    Are you sure you are looking at the throttle butterflies ?----Or the other set which is the choke flappers ?---And does spark jump a gap of 3/8" or better on all 3 leads ?

  10. #10

    Default Re: Yamaha 30 hp not running at full power

    Quote Originally Posted by racerone View Post
    Are you sure you are looking at the throttle butterflies ?----Or the other set which is the choke flappers ?---And does spark jump a gap of 3/8" or better on all 3 leads ?
    Using Spark plugs that are factory recommended. BH7S 10. I am looking at the ones that are clearly visible as soon as the air cover is removed. May be they are choke flappers. The butterfly valves behind them are opening 3/4 th at full throttle amd fully closing at neutral. Should they be fully open 90 degrees at full throttle ?
    Last edited by Repsolboy1; 10-01-2017 at 08:18 PM.

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