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1982 XJ750 Seca - won't start- defies laws of universe

Discussion in 'XJ Technical Chat' started by aaronoz80, Jun 25, 2022.

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  1. aaronoz80

    aaronoz80 New Member

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    Brief history
    I bought the bike a few months back, it's only done 15k Kms.. apart from some dodgy paint, the bike is in immaculate condition (inside and out). Took it for a run and it was okay, but once it warmed up the throttle would run away. After much trial and error and issues with the carbs, and a lot of reading here, I had them professionally rebuilt by an expert here in Australia.

    At some point it wouldn't start any more.

    Symptoms

    I press the starter button and everything sounds great, but it just won't fire. If I put 'Start ya bastard' (love this stuff) into the airbox, it'll turn over until the spray dissipates.

    Theory
    All a bike needs is fuel, spark and compression, right?

    Science
    1. Carbs are immaculate - as in brand new - not just taken to church, but standing at the altar in virgin white lol - they are shiny inside and out - I've also verified everything is in spec. Float levels are text book, all jets are clean, they are for all intents and purposes - brand new.
    2. Brand new OEM manifold boots (not ebay specials)
    3. Battery is brand new and charged daily to 100% using a high-end charger. Tested with 3 different batteries from my other bikes - all have between 180-320 CCA.
    4. Not related, but added a brand new Rick's Lithium rectifier so I could use a lithium battery. Won't start with that or the old one (which is also newish).
    5. Fuel - plenty of fuel is coming down the line and the carb bowls are filling as expected.
    6. Spark - brand new spark plugs all round and have confirmed a strong spark on all by grounding them to the engine. Just for giggles, I've ordered a complete Dyna Coil setup which I wanted to do anyway.
    7. Valve clearances - I do need to validate the shim sizes, but the measurements on all valves are 0.10 - 0.15 (with the exception of one which is 0.15 to 0.20)
    8. Compression - here's where things get weird, I've got 74, 75, 75, 76 - I added a teaspoon of oil to each cylinder and the result only changed by 1 or so on a couple of them. Note, I followed Chacals guide to the letter to test this - it could be a faulty gauge.. but.. ?
    Thoughts?
    Either my gauge is wrong or there's an issue in the engine causing compression on all cylinders? Apart from that, I'm lost.
     
    Last edited: Jun 25, 2022
  2. k-moe

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

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    Borrow another gauge, or have yours checked for calibration.

    Is fuel coming out of the carb bowl drains when you open them to check for fuel flow?

    Did the machine start after the rebuilt carbs were reinstalled?
    If so, about how manu times did it restart and run properly before the problem occured?
     
  3. aaronoz80

    aaronoz80 New Member

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    Yes, fuel drains out of the bowls when I open them.

    I want to say about a dozen times - but it always ran away - it was during this process that we discovered how cracked the manifold boots were - so there was always going to be a huge vacuum leak. Basically, started (over-revved), stopped. Adjusted. Repeat. Swore at it. Went for lunch, came back and it no longer started.

    Hasn't started since without the 'Start ya Bastard'
     
  4. McTavish

    McTavish Active Member

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    what condition is your airbox boots ? this happened to me.

    resize_SANY2220.JPG

    if you adjusted the settings after the high rpms- it might not start.
     
  5. aaronoz80

    aaronoz80 New Member

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    Nah, the airbox boots look almost new. The settings are a possibility. I'll go check the bench synch again, what are people using as a guide for that? I've read everything from a business card to a paper clip - I've tried bench synching at 0.05 (paper thickness) to around 1mm.
     
  6. Huntchuks

    Huntchuks Well-Known Member

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    Just use something small that will work the same in each carb. Bench syncing is only for initial starting until a vacuum sync can be done.
     
  7. k-moe

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

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    Do this instead.
    [​IMG]
    [​IMG]
    [​IMG]
     
  8. Jetfixer

    Jetfixer Well-Known Member

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    That is low compression if it has sat for awhile it might be stuck rings . If you have access to an aircompressor and the equipment to do a leak down test you can then see if it is a ring leaking . The other thing could be burnt valves of course you would possibly only see that on one cylinder . I agree with K-Moe try another compression gauge .
     
  9. Carl LaFong

    Carl LaFong Member

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    Try pulling plugs then put a shot of motor oil in each cylinder then try to start. Low compression is a known hard to start condition. Good luck.
     
  10. Timbox

    Timbox Well-Known Member

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    How is the inside of the gas tank? How did you fix the air leaks in the boots?

    If you have a bad fuel supply and it got into the carbs, that will mess them up again. Go back to basics, spark, compression and fuel. If you can't get it to start on spray, either you lost spark or compression on all four for some reason. I can't see you losing compression on cylinders without a catastrophic failure.

    I would guess the throttle flies are stopping the spray from getting to the spark.

    Keep trouble shooting and see what you find.
     
  11. aaronoz80

    aaronoz80 New Member

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    Hey @Timbox ! I've done all the basics alas, I've had it running on an aux tank while working on it, so the original tank hasn't been connected. As I posted above, it starts fine on spray, but dies as soon as it dissipates. Strong spark on all - tried with 3 batteries..

    The carbs are spotless (aka professionally cleaned / rebuilt / validated).

    Spark (Check - validated by grounding plugs to case)
    Fuel (Check - all carb bowls are full and refilling when bled)
    Compression (Need to validate)

    I have no ideas lol apart from rebuilding the whole engine to find what the hell is wrong lol.
     
  12. Franz

    Franz Well-Known Member

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    Are the air jets in the right place and throttle shaft seals done during carburettor rebuild?
     
  13. aaronoz80

    aaronoz80 New Member

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    Will verify the air jets, but yeah, throttle seals replaced
     
  14. Jetfixer

    Jetfixer Well-Known Member

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    When you did valve clearance check did you use a metric feeler gauge set ? If you use SAE it could be off . To me it still could be stuck rings low compression will cause hard starting.
     
  15. Franz

    Franz Well-Known Member

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    Agree with Jetfixer. My bike was the same until I did the valve clearances. That or gummed up rings or both.
     
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  16. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    1. There’s at least one potential contributing factor....INTAKE valve in-spec clearance range is: .11-.15mm. EXHAUST valve in-spec clearance range is: .16-.20mm
     
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  17. aaronoz80

    aaronoz80 New Member

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    Interesting! I bought a new compression tester so will check that and the clearances.
     
  18. a100man

    a100man Well-Known Member

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    You could check you're compression tester against another 'known good' motor, however my bet will be the choke circuit, 'professionally rebuilt by expert' means nothing.
     
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  19. Timbox

    Timbox Well-Known Member

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    I too am betting it is the carbs. When I first started working on these bikes, I pulled a set of carbs six times as I didn't have them cleaned. As the shop that you took them to might not have them cleaned as well as they say. If they are not used to working on XJ carbs well, they can miss some stuff.
     
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