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No to low power when cold, mega high revving when warm/hot

Discussion in 'XJ Technical Chat' started by Derek Hildonen, May 18, 2025.

  1. Derek Hildonen

    Derek Hildonen New Member

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    Location:
    Ipswich, MA
    1982 XJ650J
    I attempted a tune up on my bike for high revving and very slow return to idle.

    Valve clearances for Intakes are at 0.13 and Exhausts are at 0.18
    super deep cleaned the carbs
    polished the pistons (not the gold bodies) so the diaphragms pass the clunk test
    float heights are spot on at 3mm
    boots are in good condition
    colortune is a nice solid blue
    carb sync constantly checked during the colortune
    because I replaced the mufflers, I re-jetted from 110 to 112, the mufflers have no baffles, straight shot through and very loud.

    When I took it on a test ride, I couldn't stand the noise, but the power was very boggy from the low to mid range
    it wouldn't pickup until 5000rpm
    once it got to a decent speed, the exhaust noise was nasty loud, but the big problem is when the throttle was closed, the revving would take more than 10 seconds to return to idle, basically very sluggish throttle response.
    when I got home after 20 minutes, the engine sounded like it wanted to grenade unless it was under load

    things I have checked:
    I used an unlit propane torch all around the boots, butterfly o-rings, and the entire carb area, no changes to the rpms
    checked to make sure the throttle cable is free and not binding
    the throttle returns to closed very snappy

    questions or things I thought of:
    Muffler, because it is un-baffled and loud enough to cause local problems, I am going to install a baffle/DB killer. They do have a small glass packed area, that I re-wrapped with better stuff.
    -Could the lack of back-pressure cause some of these behaviors?

    -What could I be missing here?

    I have scoured this site since I joined a few years ago and it has been a tremendous resource when I resto-modded my bike.
     
  2. Minimutly

    Minimutly Well-Known Member

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    Well you can discount your exhaust mods - no engine will run without air and fuel going in...
    So I would be looking at the carbs again.
    Are you sure all the diaphragms are seated correctly? Drop test with finger over the AR inlet port will check this.
    Recheck your carb synchs - out of synch carbs cause all kinds of issues. I repeat - engines don't race without air and fuel..
     
    Timbox likes this.
  3. Derek Hildonen

    Derek Hildonen New Member

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    Whelp, I removed the carbs, popped the top covers for the diaphragms, #1 was wet with gas between the diaphragm and carb, the venturi side/bottom. When it's mentioned AR port, is it the rounded slotted port on the top of the intake side. When I move the piston up and cover that hole, the piston returns slower to the resting position, but never is stopped from moving.
     
  4. Derek Hildonen

    Derek Hildonen New Member

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    additional DFQ, the diaphragms are really saggy/floppy, probably original, would this also contribute to my issues? They sit in the groove just fine
     

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