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Yamaha 9.9hp 4-stroke erratic spark at high rpm

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  • Yamaha 9.9hp 4-stroke erratic spark at high rpm

    I’ve read a lot of great things on these forums after finding this site recently, but still can’t seem to nail down my problem. Motor starts easy on first pull, but runs rough with misfires at higher rpm, and timing light shows no spark with the miss-fires. I’ve checked resistance and peak voltages (with DVA adapter) of every electrical component (there aren’t that many on these simple little motors) and its all in spec per service manual. Compression is about 98PSI on each cylinder. I’ve run with kill switch and oil pressure sensor disconnected to rule out any wiring issue there. Timing does advance properly to 30 BTDC, so CDI box is doing its job. I resorted to just buying a new ignition coil to try and fix the issue, but no luck with that. Even temporarily borrowed a known-good CDI. I assumed it had to be electrical since the timing light shows the symptom, but can a fuel issue indirectly cause an electrical misfire just by throwing timing off? This short YouTube video shows the problem with more details in the description of testing I’ve already done.
    https://m.youtube.com/watch?v=IXGMdeszy40
    If anyone can help me from going insane with this motor I’d greatly appreciate it!

  • #2
    Compression is LOW.----Test with another gauge and throttle wide open.

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    • #3
      charge coil under flywhell or pick up sensor. there is a problem

      68T-85520-00-00 68T-85580-01-00

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      • #4
        Originally posted by racerone View Post
        Compression is LOW.----Test with another gauge and throttle wide open.
        Thanks for the reply.... My compression test was done with the throttle wide open, but I've got about the cheapest gauge a person can get. I've got the carb off the intake now, so I'll try to borrow a higher quality gauge and re-run the test. This motor is a pull-start, and I tried to pull the rope as fast as I could 5 or 6 times when checking compression, but maybe readings are typically lower when there's no electric start on the motor?? One other question: if compression was the problem, would it cause my timing light frequency to be erratic like I'm seeing? Or would the spark be normal? (I don't have enough experience with these small outboards to understand how compression could cause bad symptoms on the electrical/ignition system like I'm seeing with the erratic flash frequency).

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        • #5
          Originally posted by raczekp1 View Post
          charge coil under flywhell or pick up sensor. there is a problem

          68T-85520-00-00 68T-85580-01-00
          Both the charge coil and pulsar/trigger passed tests for resistance and peak voltage, but now that I think about it, the DVA adapter wouldn't be able to pick up a high frequency miss as long as the proper voltage popped right back into range. I suppose I'd need a scope to watch that (which I don't have). I was hoping I wouldn't have to pull the fly wheel off, but might just have to do that to at least visually inspect the charge and pulsar coils. Is it common for those coils under the flywheel to have intermittent faults that a DVA peak voltage reader would miss? And thanks again for the advice!

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          • #6
            On a boat does it make the same ? Look like the rev limiter is on because it running in the neutral ….

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            • #7
              Sign up today
              Originally posted by Sylva-Ranger View Post
              On a boat does it make the same ? Look like the rev limiter is on because it running in the neutral ….
              Thought I’d login and give an update to the original post in case it helps anyone avoid a similar headache, but I see that Sylva-Ranger beat me to it....
              The problem turned out to be no problem at all. It was the built in rev-limiter. I initially discovered the “problem” while running the motor on water muffs after cleaning the carb and replacing the impeller on the lower unit. Put it back in the water under load, and no more misfire. I didn’t know a small motor like that would have over-speed protection, but it does. On a side note, I also learned you should never assume a plastic carb float is good if it floats and isn’t cracked. Mine was mis-shaped just the slightest (maybe from freeze/thaw cycles). It looked fine but was about 1mm out of spec which was enough to cause the engine to flood at certain rpm ranges.

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