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82 XJ750 no power off-idle under load, then fine

Discussion in 'XJ Technical Chat' started by jpacman, Jun 21, 2015.

  1. jpacman

    jpacman Member

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    In reading some other posts, this is definitely a fuel delivery problem. The bike (82 XJ750) starts immediately and idles smoothly (11-1200 rpm) once warm. It will rev in neutral with no load just fine. But once in first gear, it bogs down when I let out the clutch. Only when I rev to 3500rpm and let out the clutch slowly will it get under way. Once I'm moving, it's much better. Still a slight hesitation between 10% and 30% throttle input. It also seems to get better the more I ride and the warmer the engine gets.

    I had been using premium fuel earlier in the week (problem persisted) and switched to 87 octane mid-week. The hesitation problem still occurs but the bike runs much better on 87. Go figure. I also add some StarTron fuel treatment every third tank or so. Good stuff. I like it better than Seafoam.

    K-moe posted a reply to a carb problem and mentioned sticky slides. Although I cleaned everything with Carbout, I did not polish the slides or bodies.

    And before you ask:
    Church of Clean with full rebuild kits (all passages, no holes in diaphragms, enrichment circuits in the bowls, etc.), bench synched, wet set floats (pvc stand), valves in spec, Colortuned twice, synched with 2-bottle method twice.

    Also, how do I add what bikes I have to the bottom of my posts?
     
  2. k-moe

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

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    The bike sems to run better on 87 because your bike doesn't have a high enough compression ratio to gain an advantage from the higher octane. Bigger numbers ≠ better.

    Sounds to me like you're lean. check your plug color to confirm the color tune, and then start looking for vacuum leaks. If you don't find any, and the mixture is good then you should pull the airbox boots and see how the slides react since you didn't check that when the carbs were apart.
     
  3. jpacman

    jpacman Member

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    Icy blue in the Colortune, right? Not orange-ish?

    I used an unlit propane torch to check for vacuum leaks in the carb boots and didn't experience any rpm surges. While doing the rebuild, I made new gaskets between the carb boots and the head as I didn't want to buy new boots. There were several cracks in two of the boots that I cleaned and then filled with black RTV then wrapped with black silicone tape. I also replaced the three vacuum caps just to be sure they didn't leak either.

    I did read that very small changes in the adjustment screws can make a big difference in the burn color so maybe I didn't let the carb settle out before changing the screw again. It was a warm day and even though I had a big fan blowing on the engine, I was in a hurry. I'll slow down and do it again.
     
  4. k-moe

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

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    Take them to white (lean) then bump them back out until you see a nice bunsen-burner blue. Go slow and let the mixture settle for a second before making another change.
     
  5. Stumplifter

    Stumplifter Well-Known Member

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    I thought the Gunson directions said to go to yellow (orange) and then back in to blue . . . .
    Do the XJ's prefer white to blue?
     
  6. k-moe

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

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    Either way works. What you're doing is finding the sweet spot between rich and lean; same as you would tuning by the idle drop method. Gunson's instructions make sure that they are not blamed for holed pistons since it errs on the side of being rich.
     
  7. Luis

    Luis Member

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    At the top of this page, the header, where you see your name, select your name, the next dialogue box, select signature, then you will be able to add the info.... I'm just curious because I don't know any better, but, does it act the same way if your petcock is on prime? Also just curious where your mixture screws are sitting right now after the colortune, how many turns out are they, one last thing, at idle running fine, if you blip the throttle, how does it react..I'm just curious
     
  8. jpacman

    jpacman Member

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    I tried it on prime to rule out slow flow from the rebuilt petcock but it's the same lag under load. Not sure how far out the mixture screws are now but when I pulled the old ones out, they were out 2.5 to 3 turns each. So when I rebuilt the carbs, I put the new ones in about that far to start then did the Colortune from there. At idle with no load, the rpms ramp up just fine.

    Tomorrow I'll check the Colortune again and report back on the results.
     
  9. TECHLINETOM

    TECHLINETOM Member

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    Please check the valve lash then sync the carbs, Re-colortune and then see. A bench bleed is good but out of sync can really foul up the off idle response.
     
  10. jpacman

    jpacman Member

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    The valves are good and in spec. I adjusted them over the winter (800 miles ago) but I double checked them last night since I had the tank off.

    Plug #1 was a nice beige color while 2, 3, and 4 had slight white coloration (lean as K-moe suggested).

    I Colortuned again and I guess I was just going too fast the last time as I was worried about overheating the Colortune plug and the engine. I took more time per cylinder and opened 2-4 quite a bit until I achieved a "bunsen burner blue". I could already hear the engine running better. Then I did another 2-bottle synch and it got even better.

    It wasn't too late in the day to take a spin so I geared up and took it out. What a difference!!! No hesitation at all from idle under load. Even stopped on a steep hill to fully test it out and it pulled away strongly with no lag. Sweet!!!
     
    k-moe likes this.

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