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  1. #1

    Default NS90A runs on two cyls

    Hi all,

    I took a chance on an old NS90A and I'm kinda stumped.

    I have it on a test stand, and it runs on the top and bottom cylinders at all speeds. It will sort of idle and every 5 seconds a big pop comes out the exhaust, like the fuel charge for the second cyl is getting pushed into the exhaust/leg and igniting.

    Compression 125/115/125 cold, can't really get it warm.
    Carbs are clean (spotless)
    Grounds are OK
    Tried it with rev limiter disabled /enabled (black wire at CDI connected/disconnected) with no change.

    I figured that I had an ignition issue, but I put an adjustable gap spark tester on that cylinder and am getting a good looking spark that spans a 7/16 gap while cranking. It does not look intermittent. I swapped coils between cylinders with no change. Spark on other cylinders looks the same as #2.

    After verifying that I had spark, I figured maybe I had a reed issue (not pulling fuel charge in), so I checked those and they are perfect. Checked the bearing drain check valves and they are fine. No obvious signs of a vacuum leak anywhere.

    After all that, I started thinking that maybe I had a water leak into the combustion chamber. The #2 plug didn't look wet but I was running out of possible explanations, so I pulled the head, and the exhaust cover, but didn't see anything unusual. The cylinder does not have any signs of water or steam.

    So, I'm not sure anymore. I ordered a DVA adapter and will run through the ignition test sequence in a few days. Perhaps I am getting spark while cranking but not while running, but that seems unlikely. The only other thing that comes to mind is a major vacuum leak at the block seam, or that the timing on #2 ignition is off (CDI?).

    Did I miss anything?

    Thanks...

  2. #2

    Default Re: NS90A runs on two cyls

    Not to be insulting but I recently had a spark issue and it turned out to be the spark plugs.

  3. #3

    Default Re: NS90A runs on two cyls

    Ha, no offense taken - Since this was new to me, I changed the plugs first thing and actually tried a different one on #2 just in case the new one was dead. No dice.

    I had a look at it again today in the garage out of the sunshine and I can see an excellent spark from all three coils, exactly the same, I had it jumping 3/4 inch gap without issue. Having pulled the head and the exhaust cover, I can't see this being a water leak as there are no traces of water or steam washing, and no milky oil etc. I feel like a water leak to completely kill one cylinder at idle would leave traces.

    I am wondering if it might be timing related - Maybe a bad pickup that is fooling the CDI into firing early or late on #2. At idle it does "kick" quite a bit, almost seems like it might be advanced timing on that cyl.

    One other issue - As you guys know the pulser coils are hidden under the flywheel, and I can see some metal filings on the end of the coil. It might be from the starter gear, maybe just from being stored in a shop where someone was doing some grinding etc. I'm thinking that the iron powder might affect pulser coil function.

    I'm going to wait for my DVA adapter to come in and then see what kind of pulse I get out of the pulser coils. If it's nice and even, I might throw a timing light on it and see what kind of output I get.

    Am I correct in seeing that you have to buy the whole timer base complete? I can't replace one coil?

    Thanks,

  4. #4

    Default Re: NS90A runs on two cyls

    So, the DVA adapter came in and everything tests fine. I get 4.5 to 5V out of the pulser coils, 150 V out of the exciter, and 145 to 150V to each coil, and a bright blue spark while cranking for the test. So, I decided to tear the engine down to see if I might find a vacuum leak or some other issue, and I found a few things wrong:

    - Nearly all of the vacuum/oiler line check valves were burned up inside, I'd tested them with a syringe, I guess my test wasn't as good as I thought because they are definitely not working.

    - I found the lower crank seal plate just kind of "there", the two bolts that are supposed to secure it to the block were long gone. Not sure if it was leaking, but it was not tight up against the block.

    - I think someone has been into the engine before. In addition to the missing bolts, the crank halves were sealed with a pink sealer that did not look factory, and there are tool marks where the cases meet indicating that someone separated the halves before. Does Tohatsu use a pinkish translucent case sealer?

    That's it for now. Kind of relieved that it's not a timer base or CDI, that stuff isn't cheap!

  5. #5
    Join Date
    Mar 2010
    Location
    Hamburg, NY, USA
    Posts
    2,917

    Default Re: NS90A runs on two cyls

    Who knows what RTV was used by the previous owner. Better go through everything at this point.
    Paul
    Certified Tohatsu TLDI Technician
    Hamburg, NY USA

  6. #6

    Default Re: NS90A runs on two cyls

    Yeah glad I tore it down.

    I'm measuring cylinders - Pistons have some scuffing and I should probably deal with since I'm in there. Am I correct that there is no cylinder wear allowance in the manual? 3.386 that's it, or did I miss it somewhere? I have some wear, about 2 thou taper, nothing earth shattering.

    Thanks...

  7. #7
    Join Date
    Mar 2010
    Location
    Hamburg, NY, USA
    Posts
    2,917

    Default Re: NS90A runs on two cyls

    If you have it apart anyway, good time to true up the cylinders. You can also get 0.5mm oversize pistons and rings if really bad. Sounds like you just need new rings and a good honing.
    Paul
    Certified Tohatsu TLDI Technician
    Hamburg, NY USA

  8. #8

    Default Re: NS90A runs on two cyls

    Thanks Paul. I looked at it again and I can see some pronounced wear between the transfer ports in one spot. It's not an area that can be measured, but I can see a dip in that area. The pistons are really too rough to reuse so I am looking at replacing, I think I'll go oversized.

    Looking at OEM pistons, as you said, 3B7000042 is 0.5mm oversized. Unfortunately I don't see a ring set for the oversized pistons, it looks like you need to buy the rings individually (1st - 3B7000140 and 2nd/3rd - 3B7000150). I had priced these parts for my NS70A2 and, through my Canadian dealer I am looking at $227 CAD for the piston, $35 CAD for the first ring, and $21 CAD for each second and third ring. That's a lot of money to put into an old motor. Looks like US pricing is about half of Canadian (using Boats.net to estimate but not taking into account the exchange rate).

  9. #9
    Join Date
    Mar 2010
    Location
    Hamburg, NY, USA
    Posts
    2,917

    Default Re: NS90A runs on two cyls

    Any US dealer can sell to you. Even me.
    Paul
    Certified Tohatsu TLDI Technician
    Hamburg, NY USA

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