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NSF 9.9b compression

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  • NSF 9.9b compression

    I have a new-to-me NSF9.9B, serial 10430, shortshaft estart tiller. I have had several 4/5/6 4-stroke Nissans, but this is my first twin cylinder. I don't yet have a service manual, but will get the factory one soon.

    Checking everything over before I go and try to start. It only cranked over 1/2 turn with the pull start when I was buying it, but I found out someone had laid it down the wrong way and the bottom cylinder was full of clean oil. I pulled the plugs and pull-cranked it to clear out the oil, then put the plugs in and it appeared to have compression in both cylinders (just from the pull starter effort).

    Getting it home, I checked compression in both cylinders. Bottom one was 180+ but the top one was <50. I was using the estart to crank. Used a leakdown tester and the top cylinder had 8% leak and it was leaking out the intake. Bottom cylinder was ~2-4% and no obvious leak any direction.

    I'm not 100% sure I was at TDC though.... if the dots (one per cylinder) on the cam gear are pointed towards the crankshaft, is that the right spot for TDC for each cylinder?

    I am going to pull the carb to clean it, not knowing how dirty or clean it is but especially because of the oil issue.

    Can you pull the cover to check out valve clearance without removing the powerhead? Any other model-specific suggestions or known issues that would cause a cyl #1 (is that the top cylinder?) issue?

    Also... when laying these engines down, is it the same as the 4/5/6 series... only lay down on the handle side?

  • #2
    Sounds like the decompression may not be your friend here. It's possible that the decompression for the lower cylinder may not be activating. Your owner's manual shows how to lay the motor on its side. Yes, tiller handle down.
    Buffalo NY USA


    • #3
      Sign up today
      Thanks Paul.

      I did get both the ops an service manuals last night. I took the valve cover off and you're right - the decompression mechanism for the lower cylinder is not as loose as the upper cylinder, so it doesn't toggle back and forth easily. Not sure exactly why or what I can do about it... the mechanism is just 'stiff'. It certainly was not active when I did the compression test. Valve clearances appeared to be within cold specs.

      After I had put it back together and also took the fuel pump apart and cleaned it (looked pretty good anyways), I redid the compression test and leakdown. Using the estart motor, I was getting 175+ on cyl 1 and 185+ on cyl 2. Leakdown was 6-7% on cyl 1 and leaking to the intake, and still 2-4% on cyl 2 with no apparent direction. Now I feel better and wonder if the intake valve on #1 is just a bit dirty or slow from sitting for so long.

      Next task is taking the carb apart and cleaning whatever is there, and then starting it up. Looks promising!

      Geez...these motors are HEAVY, but appear well built. I may do the 15HP upgrade to the carb at some point to make all that extra weight easier to justify!!