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2008 Yamaha 70 hp 2 stroke problem

GifisherGifisher Posts: 70 Deckhand
Engine was running fine all day in Biscayne Bay, and suddenly lost power on the way home. Would only go 1800 rpm. Any more throttle than that and it would labor. Luckily I was close enough to the ramp, and made it back ok (after a while). All plugs had spark, and compression was good (125, 125, and 120) when tested later. Started up fine, and idled fine in the channel. Could rev out of gear up to 5000 rpm. Changed the lift pump as it was only ~$40 (and 11 years old), and changed the plugs. Tried it again tonight and same problem still exists... 

Two days before all this happened, it ran great in Flamingo (burned 12 gallons). Engine has 600 hours on it. Oil reservoir sensor was changed in May, and it was also serviced at that time. Overheat alarm never went off either. Tach shows no abnormal warning or flash.

Gonna have a professional take a look at it, but was wondering if anyone had any other thoughts. I guess the carburetors need to be looked at next, but I'm no mechanic. Odd to me that it would run fine all day, and then all the sudden lose power if it's a fuel problem. I guess bad gas (I only use ethanol free btw) is a possibility, but I would think it would sputter, knock, kick etc which it doesn't. 

Any constructive suggestions would be much appreciated.


  • MelbourneMarkMelbourneMark Posts: 3,557 Captain
    Almost sounds like you spun the hub on the prop?     That would explain the low speed, but not necessarily the low rpms while underway?

    My motor has the high-temp low rpm function, but I am not sure if I could rev it up in neutral.  So, I doubt it is that. 
  • ReelthiefReelthief Fort Lauderdale Posts: 127 Deckhand
    Sounds like it's going into safe mode. Had an older (90`s) Yamaha70 chafe a wire that was routed around the cylinder head. Drove me crazy because we were on vacation at the time. The wire was not loomed and was just laying in the housing. It was putting the motor into safe mode. In gear I only got about 1500-1800 rpm out of it. Would rev fine out of gear and sounded no alarms. Ive also heard of corrosion at connections causing the same issue. Wish I could remember the color code of the wire involved but it was a long time ago. Give your motor harness a good once over and check all of the wire connectors. Good luck and please post your solution. 
  • GifisherGifisher Posts: 70 Deckhand
    edited August 2019 #4

    Thank you both for those responses. I considered a spun hub (which happened 2 years ago after hitting a pile of sargassum with a 4x4 piece of lumber on the way in one night), but then I figure it would rev ok in gear-which it doesn't. It only revs ok out of gear, and not under any load. Good thought though...

    I also thought it was running in safe mode, and looking at the motor harness closely is a great idea that I haven't done yet. I did look at some of the sensor wiring (oil, overheat), and didn't find any worn insulation or severed wires. Was gonna take the boat to my mechanic last night, but Dorian has postponed that idea. I will post the resolution of the problem soon, hopefully by the end of next week.

    Thanks again!

  • GifisherGifisher Posts: 70 Deckhand
    Mystery solved! After my mechanic tested the output and resistance of all the coils (ignition, pulser, and others), the pulser coil had high resistance (319 ohms instead of 198) and reduced output (3.7 instead of ~5). Yamaha allows for 3-6% variance I believe, and those values were therefore not acceptable.

    Because it's a two stroke, the replacement part came from California, was installed after pulling the fly wheel, and the engine runs great now. Apparently the pulser coil (with the white plug-in wire harness) can affect acceleration (which I had lost). It is also a trigger that tells the CDI box when to fire. It's uncommon for it to fail, but it does happen... 

    He replaced all three spark plug boots as well; apparently there is a built in resistor that can go bad on the later 2 stroke models. 

    Took a while, but it's great to be back on the water. Thanks for your interest!
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