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Hot start issues on XJ650 RJ

Discussion in 'XJ Technical Chat' started by DimaV, Jul 17, 2021.

  1. DimaV

    DimaV Member

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    Hi guys,
    Bike: Yamaha XJ650 RJ 1982

    Having trobles past week or so with hot engine start. To make it work, I have to fully turn on throttle, hold the start button for 2-3 seconds and only after that engine shows some life signs. Meanwhile, no problems with cold start whatsoever.
    The thing is the more it stays hot the harder it starts. For ex. if you leave it for 5 min it will start much quiker then when it's been left for 30-60-90 min and so on. Unleess the engine will be cool enough so you can put your hand on it. Then you can start it with no issues.
    Shortlist of what's been done with it lately:
    • Valves adjustment 10000 km ago
    • New carb manfolds + clearned carbs + sync (this season)
    • Fresh oil/filter, air filter, fuel filter with hose (this season)

    And yes, the compression is not so decent (around 7-8 points and varies from cylinder to cylinder) but it stays the same for a bunch of years now and Ive faced this hot start issue only a week ago, so I highly doubt these are connected.

    Finally, the question: What can cause such problem?

    PS: Could not find the relevant topic, so asking your advise in a new post, thanks in advance:)
     
  2. Jetfixer

    Jetfixer Well-Known Member

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    One thing to check , when bike is hot pop open the fuel cap and see if it makes any difference. The fuel vent could restricted and this can cause fuel starvation to carby .
     
  3. Mechanic1978

    Mechanic1978 Active Member

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    -high temp vapor lock (fuel seeping into cylinder and turning to vapor)
    -ignition coils breaking dow under heat
    -too small spark plug gap/cracked plug(as it heats up, gap expands open allowing spark to arc to engine)
    -weak/worn starter brushed/armature housing swolen from too much cranking
    -low carb bowl fuel level(repeated cranking uses vacume pump to resupply it)
    -battery cells low level/bad
    -whats the actual engine temp(is it running hotter than normal) plastics and metal expan and contract with heat and cold, you obviously have a "Heat" issue... take your pick, and let us know any results

    if its "not" getting fuel during this heat cycle, use some carb cleaner or either starting fluid to spray it while starting, see if it helps....
     
  4. XJ550H

    XJ550H Well-Known Member Premium Member

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    at 10k km it is time to check shims again. were any of your shims on the tight side last time you checked? you know Just in spec?
    tight shims will give a hot start problem
     
  5. DimaV

    DimaV Member

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    Yes, some of shims were tight, so had to change some of those according to specs. Well, 10k km is not so big mileage but anyway, maybe I will check sometime soon.
     
  6. DimaV

    DimaV Member

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    This is a new one for me, thanks, will check
     
  7. DimaV

    DimaV Member

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    I’ve increased a spark plug gap a bit so far, maybe this hotfix will do the trick. Thanks
     

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