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Issues With Acceleration and Idle

Discussion in 'XJ Technical Chat' started by BKthickburger, Jul 29, 2022.

  1. BKthickburger

    BKthickburger Member

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    Hi Everyone,

    My XJ has been running great this last season, but after the last ride I went on, it started showing performance issues. When I started it up again I noticed that the throttle was pretty sluggish with acceleration and deceleration, I'm also getting some pops from the exhaust at idle.

    What I've done so far:

    Checked spark plugs - They all look great, pretty much the perfect air/fuel ratio. Cylinder 1 might be a tad rich, but nothing concerning. I replaced the ignition coils a couple of years ago and it looks like they're still producing spark on each cylinder.

    Removed and cleaned the carbs - Again, these looked great and super clean, still having issues after reinstalling them. Manifolds all look good too.

    Replaced gas in the tank - was wondering if it was a fuel issue. I replaced all of the gas, but no luck.

    Checked air filter - filter is really clean and not causing any issues.

    What else am I missing that would be good to try next? Last season I checked valve clearances and they will all good. I was wondering if it could it be that the carbs are out of sync?
     
  2. Franz

    Franz Well-Known Member

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    Fuel tank cap vent needed cleaned causing fuel starvation?
     
  3. Franz

    Franz Well-Known Member

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    Open the fuel cap slightly to see if idling improves. Does it idle properly if you turn the fuel tap to prime assuming you have a vaccum tap? Might be a faulty tap. Do you have an in line fuel filter? Might need replaced.
     
    Last edited: Jul 29, 2022
  4. k-moe

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

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    Which XJ?
    There are many important differences from model to model.

    Is there an inline fuel filter fitted?

    There are many things that can cause your symptoms, from a vacuum leak to fuel delivery, and even an exhaust leak.
    Start with what has been suggested so far though.
     
  5. BKthickburger

    BKthickburger Member

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    I went on a long ride a couple of weeks ago, and everything was normal. I tried starting it again a couple of days later and while it ran, it was pretty rough, and had some pops at idle and higher rpms. Here's what I've done so far:

    Checked the air filter - it's super clean with no damage
    Cleaned the carbs - Again, super clean. Everything looked perfect when I took them off
    Synced the carbs - Didn't seem to change much for performance, still ran rough
    Replaced the gas - Wasn't sure if it was being picky about the gas type. I drained the tank, refilled it with premium, but no difference.
    Checked engine compression - all good and the same reading across all four cylinders.

    Right now I'm experiencing two problems.

    1) After I synced the carbs the bike won't start at all. It turns over, but is not firing. I'm not getting spark from the ignition coils. Is this just weird timing, or is there something that could have happened during the syncing process that would have caused this? It seems odd that I would lose power to both sets of coils at the same time. I also just replaced them a couple of years ago.

    2) I think I found the source of the performance issues and rough sounding idle/acceleration. When it turns over it sounds like there's air escaping from around the spark plug on the first cylinder, even though the plug is tight and torqued correctly. I know the threads on that cylinder are already a little messed up so I was wondering if it's time to retap the hole. If so what would be the appropriate heli-coil kit to use on this bike?

    Any help or advice would be greatly appreciated!
     
  6. BKthickburger

    BKthickburger Member

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    Sorry I should have specified. It's an '81 XJ650 Midnight Maxim. It doesn't have an inline fuel fitter, just a screen for the petcock.
     
  7. BKthickburger

    BKthickburger Member

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    I have tried setting it to prime and it doesn't make any difference, still runs the same whether it's using the vacuum or not. I haven't tried slightly opening the fuel cap, but definitely can.
     
  8. BKthickburger

    BKthickburger Member

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    I noticed recently that there's a compression leak coming from around the spark plug on cylinder one. I'm wondering if that's the reason it's still able to run, but is so choppy. Should I try cleaning up those threads/retapping it?
     
  9. k-moe

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

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    Start by pulling the plug and checking that the crush washer is not missing, then check the top of the plug hole seat for debris.
     

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