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Help Compression - FZR250 3LN1

Discussion in 'Yamaha 250cc In-Line 4's' started by ROVER LARRAGUIBEL, Mar 23, 2021.

  1. ROVER LARRAGUIBEL

    ROVER LARRAGUIBEL Active Member

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    Hi friends.
    I need help please...

    1HX Engine (3LN1)
    Assemble the engine, original rings, original gasket, valve seat and valve regulation are changed.
    I did the compression measurement:
    Cylinder 1: 90PSI
    Cylinder 2: 80PSI
    Cylinder 3: 80PSI
    Cylinder 4: 80PSI

    Is compression ok?
    Will the engine start at that PSI or does it require even more compression?

    What else can I do to increase compression?

    Thanks!
     
  2. ROVER LARRAGUIBEL

    ROVER LARRAGUIBEL Active Member

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    B8FB44ED-952C-4BCE-B999-AA94052B7B3A.jpeg 80464E3C-95F8-40D5-A2A6-957D6BDDB710.jpeg 566A8EB6-9242-4EDA-B029-308DE97DC7A7.jpeg 60862239-65A3-4520-B7E1-5DAA3356A3B8.jpeg D979C248-9C89-4421-811D-65F433360315.jpeg 3381796C-9C24-4975-A0A7-54AC185E6C08.jpeg F8B9F357-407A-4D6A-934C-C17CD921B946.jpeg
     
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  3. Murdo

    Murdo The Good Doctor Staff Member Contributing Member Ride and Events Crew

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    How were you testing to get those figures?
     
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  4. ROVER LARRAGUIBEL

    ROVER LARRAGUIBEL Active Member

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    were tested with a pressure gauge ... with starter motor only
     
  5. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    This comes up a lot. The volume of the cyl head is about 5cc. Your compression gauge probably has more than that in the hose so the numbers mean next to nothing. The consistency is good though.
     
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  6. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    They are consistent as both @Murdo and @maelstrom have noted, the difference between them may be down to valve clearances
     
  7. ROVER LARRAGUIBEL

    ROVER LARRAGUIBEL Active Member

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    So with the compression 90 - 80 - 80 -80 PSI, the engine will start?
     
  8. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    How do you know the compression is 90 psi? What is the volume of the compression gauge? How accurate is the compression gauge? The chances of 90psi being an accurate figure are somewhere between very slim and zero.
     
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  9. ROVER LARRAGUIBEL

    ROVER LARRAGUIBEL Active Member

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    this is the pressure gauge ... i bought it on aliexpress and had to put a motorcycle adapter ... I don't have more information ...



    52E50701-9906-4B51-8D37-ABD35AB1ACE7.jpeg
    60253C45-8839-4883-97B5-5E7B88361BFF.jpeg
    16FE3A82-6605-4DBE-956F-CE51ABE8E050.jpeg
     
  10. DannoXYZ

    DannoXYZ Well-Known Member

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    Very simple solution:

    1. remove engine
    2. rebuild engine with new rings and valves
    3. re-install engine


    Have you re-tried compression-test with squirt of oil?
     
  11. ajb0235

    ajb0235 Active Member

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    I had the same problem with my 3LN1. 33000 Km. it would not start.
    Compression check using Gunson comp. tester and adapter 10 mm

    Dry bores. 8 bar all cylinders. ( 116 psi)

    Oil in bores. 10-11 bar all cylinders. (145-159 psi)

    Conclusion: increased pressure with oil in bores indicates a piston ring sealing problem

    I checked all the valve clearances. Two of the inlets were down to 0.004”. I was able to lap the shims on 240 wet or dry paper to get the correct clearances. Exhausts all OK. Didn’t have to buy any shims.
    upload_2021-3-24_14-13-57.jpeg

    upload_2021-3-24_14-15-0.jpeg

    lapped in all the valves with fine paste and checked seats with engineers blue. Very little lapping required on all valves.

    Removed cylinder block from crankcase. There are no lips at the top of the bores.There is some slight surface deterioration in the barrels, possibly rust at some time. Also some slight marking from rings stuck in position over a long time?

    Measured a couple of ring gaps, cylinders 1 and 4. Top ring gap is 0.018" at the bottom of the bore. recommended limit is 0.011" Rings are worn. This is the reason for 8 bar compression, rising to 10 bar with oil in the bores..

    Decided to get a new set of rings (Aliexpress £20 posted!). Rings were branded AHL, which is apparently a good manufacturer.
    I deglazed the bores using 400 wet or dry paper, then cleaned the bores very carefully with detergent, and then repeated the cleaning exercise.
    Fitted the new rings and checked the gaps, all OK 0.08”to 0.010” of the box.
    Reassembled the engine and it started
    Taking care not to squirt too much fuel down the intake when starting initially. upload_2021-3-24_14-13-57.jpeg upload_2021-3-24_14-15-0.jpeg upload_2021-3-24_14-13-57.jpeg upload_2021-3-24_14-15-0.jpeg With such a small combustion chamber it’s easy to get to a hydraulic lock condition when cranking.

    I would recommend new rings and a valve clearance check on any bike with this mileage, particularly if the bike has been stood for a long time.

    Starts first time now.The engine is really crisp, and stops dead when stop switch is used.
    I’ll do another compression check when the run in period is completed.
     
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  12. ROVER LARRAGUIBEL

    ROVER LARRAGUIBEL Active Member

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    Hi.

    I bought new piston rings and gasket at megazip.com

    be1459f1-91ca-4b1c-b8bf-0f4168ab629d.jpg

    install new rings as indicated in manual

    be3877e9-56e3-4152-b320-c013f1fc4fa8.jpg

    Install gasket:

    b6840e2e-b85a-4ecb-8913-492a819bee5a.jpg

    I did the valve lapping and tested it with Prussian blue paste and also performed a leak test, introducing gasoline through intake and exhaust, there were no gasoline leaks through valves.

    18be65f3-36f1-4e60-8120-95fb5dcf324d.jpg

    after all this, perform the valve clearance measurement and install new shim

    Captura de Pantalla 2021-03-24 a la(s) 23.12.35.png

    done all this, I measured compression and the result is the one mentioned above 90-80-80-80 PSI
     
  13. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    and you honed/deglazed the cylinder bores?
     
  14. ROVER LARRAGUIBEL

    ROVER LARRAGUIBEL Active Member

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    I did a cylinder burnishing
     
  15. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    Maybe something lost in translation, burnish means to polish.
     
  16. ROVER LARRAGUIBEL

    ROVER LARRAGUIBEL Active Member

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    I think maybe it's just that the pressure gauge is broken ...
     
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  17. DannoXYZ

    DannoXYZ Well-Known Member

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    no, with new rings, you'll need to run engine ~500km to break it in. Rings and cylinders need to wear into each other for good seal.
     
  18. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    What Danno said, except I think it will take about 12 minutes to 'run in' your rings.
     
  19. DannoXYZ

    DannoXYZ Well-Known Member

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  20. ROVER LARRAGUIBEL

    ROVER LARRAGUIBEL Active Member

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    Engine ignition ok

     

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