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XJ550 hard to restart when hot

Discussion in 'XJ Technical Chat' started by stfano, Aug 10, 2021.

  1. stfano

    stfano New Member

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    Hello guys it's the first time I write in the forum so I think a little intro is a must here.
    First of all I'd would like to thank you all for this awesome community 'cause I read and learned so much from all of your experiences.

    I recently bought my first bike, 81 Europe XJ550 (4v8) the one with double front disc brakes. It has 40k km and it's in great shape for a 40years old bike.

    I've been working on the bike for a couple of months and until now these are the jobs done:
    - master cylinder cleaned and rebuilt
    - front calipers cleaned, rebuilt and repainted
    - new SS brake hoses
    - tank derusted internally and coated with proper epoxy resin, also rebuilt the gas cap (cleaned and new gasket)
    - rebuilt petcock
    - had the carbies cleaned and rebuilt professionaly (completely disassembled, acid bath for the bodies and ultrasonic cleaning for all the other parts, new orings, gaskets float needles and seats)
    - new plugs and oil
    - new intake boots
    - new clutch cable and adjusted for zero slack

    I ran her for about 70km in a couple of days to burn the deposits away (she's been sitting for at least 2 years in the garage of the previous owner).
    So far runs pretty decently as far as my lack of experience on bikes could tell. The problem is that she's hard to restart when really hot. Most of the times she wouldn't pick up as I crank her and when she does I have to keep playing with the throttle to keep her alive. After some tries she kind of unlock and the throttle start to respond normally again and she idles fine. After that she runs normally. In the posted video I couln't reproduce the problem as she was super hot but started right away (of course...). The outside temperature these days is between 30° C and 40° C, actually pretty hot and humid.

    Carburetors aren't synched and I didn't check the valves, yet.

    I have the luck to own a thermal camera so I took some pictures of the bike running, I noticed that exaust pipe n3 is a little colder then the others. Could be lack of synchronization or valves off? Both?
    Could the difficulty to restart be caused by them?

    I leave you with some media...
    IMG_20210810_095518.jpg

    IMG_20210810_095544.jpg

    IMG_20210810_095610.jpg



    img_thermal_1628582521140.jpg

    img_thermal_1628582654233.jpg

    img_thermal_1628582656956.jpg

    img_thermal_1628582660120.jpg
     
  2. StarGeneral

    StarGeneral Active Member

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    Beautiful bike! Check the valves first, worry about everything else after. Make sure you get a new cover gasket and valve cover bolt seals too so it doesn't leak everywhere when you finish up.

    My bike ran "fine", but four of my valves were dangerously out of spec, and it was very hard to restart when hot without using the enricher or bumping up the idle. Once you're sure your valves are in spec if it's still having that issue we can move on to other suggestions.

    I've been told that hard to restart when hot is a common symptom of out of spec valves.
     
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  3. Polock

    Polock Well-Known Member

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    i'll take a guess that your sync is off, #3 pipe is colder and #3 is the base for your sync
     
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  4. stfano

    stfano New Member

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    I measured my valves clearances and they're way off
    1 2 3 4
    Intake (mm) 0.06 0.13 0.10 0.05
    Exhaust (mm) 0 0.08/0.10 0.08 0.05

    Hopefully that could be the cause of my problem :)

    One question: I took all my readings with the lobe pointing 180° away from the shim. For curiosity I measured some clearances with the cam lobe in other positions but still not pressing against the shim and the readings differs. Is that normal? Shouldn't be the cam perfectly round besides the lobe?
     
  5. stfano

    stfano New Member

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    I think I did a stupid mistake and now paranoia got me. To check valves clearances I use the ziptie method, and I always turned the engine forward (counterclockwise). Turning the engine always forward means that both vavles, intake and exhaust, were open simultaneously at some point. Wouldn't be that a catastrofic event?
     
  6. jayrodoh

    jayrodoh YimYam Premium Member

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    Yup that is normal, the cams are not perfectly round which is why you check it with the lobe 180 degrees out.

    You concerned that the valves would hit each other? A zip tie or wire is not going to hold the valve open far enough to allow them to interfere.
     
  7. stfano

    stfano New Member

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    Yeah pretty much that. When I did the job I had a lot of clearance to get the shims out. Anyway, tomorrow I'll get new shims to swap and I'll know if I f***ed up compression or what :|
     
  8. k-moe

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

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    A cam has what's referred to as a ramp on both sides of the lobe. This allows the valve to be opened and closed gradually. A cam lobe is more like the shape of an egg than it is a circle with a bump on it. Measuring valve clearance is always done with the high-point of the lobe 180º from the shim.

    Bigfitz's AIRHEAD VALVE ADJUSTMENT with Pics - parts I & II

    From what you describe about how you did the work to check the shim sizes, you have nothing to worry about.
     
  9. stfano

    stfano New Member

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    Thanks for the clarification k-moe, I followed what the yamaha service manual for my bike said (lobe at 180° from the shim). I asked because I saw an other manual where valve clearances are checked with the piston in TDC, which means lobes are pointing away from each other, sloppy mistake probably.

    Yesterday I measured clearances again and the results are the same of the first measurement. If I had accidentally bent a valve from interfering with each other I would have measured something different, probably looser.
    After that I got them all in spec, still waiting for a new head gasket to arrive tough. Also compression feels much better now.

    I'll report the results when I'll get everything back togheter. Thanks!
     
  10. Toyobaru866

    Toyobaru866 Active Member

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    Is this different for different models? For my X I always measure with the piston at TDC. I hope I didn't do it the wrong way... Screenshot_20210825-222639.jpg
     
  11. stfano

    stfano New Member

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    I've no idea really, is this a yamaha manual or a 3rd party? I'm following yamaha's service manual for my bike.

    Btw, I've got valves in spec and synched carbs. That made a whole world of difference, I can hear the valves playing that beautiful ticking music now, and the bike runs much better overall. I set it to idle nice and smooth as well.
    Also the problem it had about starting when hot went away.
    I think something isn't quite right with the mixture still... I checked the plugs after riding and some of them where white indicating a lean mixture. I'm new to carb tuning so I can't really appreciate the variations of the engine when regulating the pilot screw, and I can't seem to find a colortune to buy (out of stock everywhere).
    I'd like to get it done correctly because I don't want to cause any damage to the engine.
    Right now I have the screws to 2 - 1/2 turns out, It feels and sound ok to me, but I'm a newbie and I might be wrong :)
    Any advices?
     
  12. Toyobaru866

    Toyobaru866 Active Member

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    It's the original factory service manual.

    As for your plugs, a lean mixture causes the engine to run hot. I would turn the screws 1/4 turn out on the plugs that looks white. Do some riding and check them again. Adjust and check again if necessary untill they are all the right color. Al least, that's the way I do it because I found the colortune very unpractical for the X.
     

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