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To Rebuild or Not to Rebuild

Discussion in 'XJ DIY How-To Videos' started by Joshua Olkowski, Aug 2, 2018.

  1. Joshua Olkowski

    Joshua Olkowski Member

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    Has anybody out there ever rebuilt an 81 xj550 Seca Engine. I have one that's running...okay, but I also have access to one in a junk yard for 350 bucks. Was thinking of doing a complete Seca teardown and restoration taking the best of both bikes and going to town. However, to really make it worth it I feel I would also have to rebuild the engine otherwise you're just slapping old parts together. I am a novice mechanic and this is my first venture into bikes. The 81 Seca is one of the sexiest bikes I've ever seen and I really want to go for it. Any vids, tutorials, articles would be very helpful. Thanks. Josh
     
  2. XJ550H

    XJ550H Well-Known Member Premium Member

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    first consideration is the amount of money you are going to spend. Complete restoration is a costly project.
    what milage do you have on motor? test the compression. all you should have to do is rings and valve job.
    consider that the valve job is usally just a refurbish of the head

    ground up rebuild of motor would have you spending money you do not need to.
     
  3. Joshua Olkowski

    Joshua Olkowski Member

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    Thank you for responding. Money is definitely an issue since I want to do it myself. However, I like your idea of just doing a rings and valve job. Are there any tutorials you could direct me to? Thanks.
     
  4. k-moe

    k-moe Pie, Bacon, Burbon. Moderator Premium Member

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    How many miles are on the engines?
    A runner typically won't need to be rebuilt, and there are many things that need to be checked in order to determine if the rebuild is necessary.
    There is also the issue of parts avaliabllity. So few of them have needed rebuilding that Yamaha stopped providing pistons, oversize piston rings, and plain bearings a long time ago. Anything you find now will be from someone's stash of NOS parts, or from an engine that's being parted out.
     
  5. Joshua Olkowski

    Joshua Olkowski Member

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    Got it. Unfortunately, I don't know the mileage because the studs on the metal ring that make the speedo wheel turn were bent. Wish I knew that before I bought the bike. Nevertheless, this bike has had a full life, I'm sure. I only got 96 miles on a full tank. Most of that was city driving but I'm guessing I won't get much more then that on the highways. If I do a top end rebuild or any rebuild for that matter will I get better gas mileage or am I just stuck with a beautiful gas guzzling machine from the 80's.
     
  6. Joshua Olkowski

    Joshua Olkowski Member

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  7. XJ550H

    XJ550H Well-Known Member Premium Member

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    you should get 45 to 50 mpg out of the city
     
  8. Joshua Olkowski

    Joshua Olkowski Member

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    I'm not sure if it will. As it stands I'm only getting 25.
     
  9. XJ550H

    XJ550H Well-Known Member Premium Member

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    yes but how much 6th gear time do you get in the city
    and once you get shims in spec and carbs cleaned and tuned your milage should improve

    do the compression test see what you get ( should do this after shims)

    90% of my riding is 55 mph and above
     
  10. Joshua Olkowski

    Joshua Olkowski Member

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    Pardon my ignorance but are there any tutorials you could direct me to on a compression test.
     
  11. Rooster53

    Rooster53 Well-Known Member Premium Member

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    From the Information Overload Hour:

    https://www.xjbikes.com/forums/threads/the-information-overload-hour.27544/

    - COMPRESSION TESTING:

    Yamaha recommends that you perform a compression test every 5,000 miles or so, and that you should record the readings, per cylinder, for future comparison and evaluation. The acceptable readings (specified at sea level) are as follows:

    To do a compression test properly, you should first make sure all of your engine valves are properly adjusted to their recommended clearances, as valves that are too "tight" (not enough clearance) will allow the intake or exhaust valve to be open more than is necessary, or at the wrong time within the compression stroke cycle, thus bleeding off compression that would otherwise be developed.

    http://www.xjbikes.com/forums/threads/compression-test-xj650.113151

    Do not use thread adapters or the like on your pressure gauge, as the added volume of air space within the adapter will reduce the indicated pressure readings.

    a) make sure the engine is warm (at operating temperature).

    b) remove all spark plugs, and then stick the plugs back into their caps and make sure the plugs are grounded to the cylinder head (or even better, disconnect your TCI unit).

    c) remove the airbox filter lid and the air filter.

    d) make sure the battery is FULLY charged, and remains so throughout the course of these tests! It is actually recommended that for purposes of compression testing that the TCI be un-plugged and jumper cables to a large capacity battery (i.e. car battery) be used to make sure that the cranking speed remains pretty constant between each reading. Slow or sluggish cranking speeds will reduce the indicated compression pressure. Yamaha specifies their compression pressures at 300 rpms (which is why the battery needs to be in good shape).

    e) open the throttle FULLY and keep it open during testing.

    f) crank the engine over until the needle stops advancing.

    g) Let the starter cool down for a minute or so, then do the next cylinder, etc.).

    h) If the readings are below spec, then shoot about a teaspoon amount of motor oil into each cylinder, crank the engine over a few revolutions with the starter (to spread the oil around), and then re-test each cylinder using the above procedure.

    i) compare the two results and analyze.

    j) keep all of your figures, and note the date and mileage from your odometer, so you can compare the next time you take readings (every 5,000 miles or so).
     

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