1987 JS300 wonít start (full rebuild)
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  1. #1
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    1987 JS300 wonít start (full rebuild)

    Well, I bought this thing for nothing about 7 years ago, and couldnít get it to start. I disassembled everything, and then life happened and itís all been sitting in separate baggies in a box for 7 years....

    I finally got around to putting everything back together from the crank case to the exhaust. Now I still canít get the ******* to start.

    Its getting fuel, compression and spark (possibly weak) and I can only get it to backfire every now and then.

    Me I tested my spark plug cap to ground and got 6.33 ohms which is in spec. The only issue I can find is when I test the black and red wire from the stator (excitor coil) which should be just above 300 and Iím reading 0.22. I was hoping the red wire was shoring somewhere but I canít find anything.

    I REALLY donít want to buy a new stator as they are around $100 and I have spent way more than this thing is worth to get it going. Anyone have ideas or suggestions?

  2. #2
    I dream skis
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    Re: 1987 JS300 wonít start (full rebuild)

    You could try running extra grounds...try your best battery fully charged...post a video of it trying...

  3. #3
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    Re: 1987 JS300 wonít start (full rebuild)

    Check spark under compression, and make sure you didn't shear the flywheel woodruff key.

    ....but not having spec. resistance on the coil is not a good thing.
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  4. #4
    Frequent Poster Pb300's Avatar
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    Re: 1987 JS300 wonít start (full rebuild)

    I've been through all that and my issue ended up being that the cranking/starter motor was too weak to generate a fast enough rotation to evoke a strong enough spark...

    It would turn over okay without a load but under compression it was struggling, I did not realize because I did not have a reference for it at the time. Post a video of it cranking with the spark plug tight. Maybe another with the spark plug out of the hole and grounded.

    Exciter coil (red to black wires on stator plate) should measure 168-252 ohms.

    Take apart the cranking motor and clean all of the graphite dust out of the wingdings and such. I am not sure if carb cleaner is recommended but I did end up using it after a good spray down with the air compressor to get what was left behind. Test the armature to make sure there isn't a short. Should crank better after a cleaning.

    Edit: I have testing specs. and information on tear down of cranking motor from manual if needed. Let me know. Might have a video of a better cranking 300 engine too.
    Last edited by Pb300; 06-08-2018 at 12:01 PM.

  5. #5
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    Re: 1987 JS300 wonít start (full rebuild)

    Wow guys, thanks for the fast replies.

    I did run an extra ground from the ebox to the engine. I am using a car battery with jumper cables that is fully charged (car battery is 12v, as long as the car isn't running the electrical system should be fine)

    How do I check the spark under compression? I just pulled the flywheel last night to inspect the stator and the woodruff key is fine.

    I was wondering that about a slow crank with the starter as well. It does turn over but like you said i have no reference. I am still thinking that my exciter coil is bad, but I want to try everything else that is free before I replace it. I will clean out the starter motor tonight and get a video up if it still doesn't work. I will also do some more Ohms testing and get some pictures.

    Thanks again!

  6. #6
    Frequent Poster Pb300's Avatar
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    Re: 1987 JS300 wonít start (full rebuild)

    Definitely ohm rest the exciter coil. Just pull the plug that’s in the ebox and test the red and black wires.

    To check spark under compression, you need an inline spark tester available at auto parts store. 10-15 bucks. Spark plug stays in engine. Let’s you know if it’s sparking or not.

    My spark was weak out of engine. Sometimes intermittent. But my all my coils were fine. Sometimes it can be your spark plug cap. Cut it back to expose new wire, only a1/4” of becessary. Reinstall boot.

  7. #7
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    Re: 1987 JS300 wonít start (full rebuild)

    Just remove the plug wire from the plug, insert another plug into the boot, and ground that plug to the already installed plug. Crank the motor. That checks spark under compression without a dumba$$ tool.
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  8. #8
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    Re: 1987 JS300 wonít start (full rebuild)

    My spark plug wire is new. And I tested the spark with compression and it’s there. I just pulled the starter and cleaned everything inside it. Here is a video of it turning over: http://www.youtube.com/watch?v=PF2UsH4UpM0

    I also just tried to prime with premix by poring it in the cylinder park plug hole and still nothing. I’m gonna buy a compression tester, I don’t trust the **** one that I rented from autozone.

  9. #9
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    Re: 1987 JS300 wonít start (full rebuild)

    Well I guess itís the compression... Highest reading I could get is 90 psi. Spec is 150 psi. I donít get why itís low after a full rebuild: bore, piston and rings. Maybe a bad head or base gasket? Iím about ready to give up.....
    Last edited by DMan II-40; 06-15-2018 at 01:51 AM.

  10. #10
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    Re: 1987 JS300 wonít start (full rebuild)

    Did you check the bore size, and order the correct pistons?
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