1. Hello Guest. You have limited privileges and you can't "SEARCH" the forums. Please "Log In" or "Sign Up" for additional functionality. Click HERE to proceed.

750 seca carb mystery

Discussion in 'XJ Technical Chat' started by joejr2, May 23, 2018.

  1. joejr2

    joejr2 Active Member Premium Member

    Messages:
    713
    Likes Received:
    109
    Trophy Points:
    43
    Location:
    Santa Fe,NM
    what's a plug chop? I'm the self taught lone wrencher.
     
  2. joejr2

    joejr2 Active Member Premium Member

    Messages:
    713
    Likes Received:
    109
    Trophy Points:
    43
    Location:
    Santa Fe,NM
    So the consensus is start at 2.5 turn out. Turn the mix screw in until the colortune turns white, then turn out until it turns blue,
    then synch the carbs take the bike for a ride then colortune and synch again ?
     
  3. joejr2

    joejr2 Active Member Premium Member

    Messages:
    713
    Likes Received:
    109
    Trophy Points:
    43
    Location:
    Santa Fe,NM
     
  4. tabaka45

    tabaka45 Well-Known Member

    Messages:
    2,348
    Likes Received:
    494
    Trophy Points:
    83
    Location:
    Brunswick, Georgia
    The colortune is only used to get the initial setting for the pilot screws. From there you open the pilot screws —no more than the width of a dime— and ride, read and adjust as needed—never more than the width of a dime. I think I synced each time.
     
  5. k-moe

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

    Messages:
    19,613
    Likes Received:
    6,703
    Trophy Points:
    113
    Location:
    The City of Seven Hills
    Running the machine in the rev range you want for the fuel delivery you want to check, then cutting the engine off (ideally) and pulling the plugs to read. How you read them depends on what tools you have, and what sort of engine you have. For 2-strokes the typical mehtod is to cut the threads off so you can see the smoke ring deep in the insulator. For 4 strokes you can just read the ground strap and the base ring of the threads (insulator color can be read too, but only on engines that have run for a good while). There was, at one time, a magnyfing flashlight that let you see down into the deep recesses of the insulator without cutting the plug. Dad had one, but it went missing many years ago :(
     
  6. k-moe

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

    Messages:
    19,613
    Likes Received:
    6,703
    Trophy Points:
    113
    Location:
    The City of Seven Hills
    I use it to set the mixture. No more turning needed.
    Bunsen blue my friend. Bunsen Blue.
    A tad more rich won't hurt though.
     
  7. joejr2

    joejr2 Active Member Premium Member

    Messages:
    713
    Likes Received:
    109
    Trophy Points:
    43
    Location:
    Santa Fe,NM
    Yesterday, I pulled the carbs and replaced the aftermarket jets ( aftermarket 122+43 for stock 120+40) with the stock jets for xj750.
    With the carbs back in, it was harder to start and the engine quickly surged to 5k rpms. I tried to control the surge by backing of the enrichment
    but the engine quit. Today I'll pull the carbs again and replace the aftermarket jets (122+43) . The mix screws were 2.5 turns out during the test.
    What would the surge be from, too much air or too much fuel ? I've gotten to the point where I can pull the carbs, exchange parts and
    get them back in less than 2 hours I'm retired anyhow, so I can take the time to experiment.
     
  8. k-moe

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

    Messages:
    19,613
    Likes Received:
    6,703
    Trophy Points:
    113
    Location:
    The City of Seven Hills
    Too much air. Lean makes her race, rich makes her bog.
     
  9. Rooster53

    Rooster53 Well-Known Member Premium Member

    Messages:
    1,852
    Likes Received:
    723
    Trophy Points:
    113
    Location:
    North, FL
    Are you running pods or modified exhaust? Sante Fe is pretty high up there (7200 ft), and normally would require a reduction in jet size.

    Decrease main fuel jet size by 2 sizes per every 2000' above sea level.

    Decrease pilot fuel jet size by 1 for every 6000' above sea level

    Probably not enough to affect the colortune issue with the pilot, but might be overly rich once you are up and running if you are running stock.
     
    k-moe likes this.
  10. joejr2

    joejr2 Active Member Premium Member

    Messages:
    713
    Likes Received:
    109
    Trophy Points:
    43
    Location:
    Santa Fe,NM
    It has an air box and standard pipes. By your reconning the pilot jets should be 39 and the mains 113 since I'm at 7200 '. I just tried the 120+40 stock standard jets
    and the engine surged big time. Too lean ? Where can I get 113+39 jets
     
  11. Rooster53

    Rooster53 Well-Known Member Premium Member

    Messages:
    1,852
    Likes Received:
    723
    Trophy Points:
    113
    Location:
    North, FL
    Len has a good supply of jets, and then there is also Jets R Us. I don't think I would worry too much about the pilots as a change of 1 wouldn't seem necessary when you have the adjustment with the pilot circuit screw. Maybe a change in the mains if all of your riding is done at the higher altitude and the plugs indicate overly rich.

    I was just really curious as why you had gone up in size from stock given the altitude of Santa Fe.
     
  12. Polock

    Polock Well-Known Member

    Messages:
    9,751
    Likes Received:
    2,093
    Trophy Points:
    113
    Location:
    Beaver Falls, PA
    did you try low speed screws with that setup? back off the idle knob? i've tried all kinds of jets and never found that racing problem.
    maybe look for air leaks or throttle linkage jams
     
  13. TheCrazyGnat

    TheCrazyGnat Well-Known Member

    Messages:
    684
    Likes Received:
    323
    Trophy Points:
    63
    Location:
    Central PA
    I know I'm late to the party here, but don't use the original Simple Green on aluminum. It can oxidize the carb bodies. There is a purple formulation (pro HD) that is safe for aluminum. Just my experience.
     
  14. joejr2

    joejr2 Active Member Premium Member

    Messages:
    713
    Likes Received:
    109
    Trophy Points:
    43
    Location:
    Santa Fe,NM
    I ordered a complete rebuild kit off ebay it had #122 main jets and #43 pilots. the stock jets were #120 main and #40 pilot.
    On a past try I put in the stock jets and I couldn't get the bike to idle off choke and I had to keep feathering the choke to keep it from surging.
    I swapped jets with the #122 main and #43 pilot, and blocked off any screw holes etc. into the airbox also I sealed the boot/airbox joints with
    strips of duct tape, no obvious leaks. Checked compression, #1-130, #s 2-4 are 125. this time I could get it to idle off choke at 1200 rpms by playing with
    the idle speed knob. But when I flicked the throttle it surged up to 5000 rpms . the colortune showed more blue and the engine sounded the best at
    2+ 3/4 turns out. By your reckoning Rooster I should have #113 main jets ( #120 stock minus 7 sizes for altitude ) I can get # 114 off Jets R us. Do you
    think a # 40 pilot and # 114 main jet would help ? I can order the # 114 mains and swap back to # 40 pilots. Also I can caulk the boot/airbox joint
    with a non hardening gasket cement, since the tape made a difference. ( gasket cement recommendations ? ) "81" xj750 seca.
     
  15. joejr2

    joejr2 Active Member Premium Member

    Messages:
    713
    Likes Received:
    109
    Trophy Points:
    43
    Location:
    Santa Fe,NM
    Actually, Jets R us offers a choice of either #112 or #116 mains Should I go with the #116 ?
     
  16. Rooster53

    Rooster53 Well-Known Member Premium Member

    Messages:
    1,852
    Likes Received:
    723
    Trophy Points:
    113
    Location:
    North, FL
    Well I lived in Alamogordo for 5 years (4300 ft) and drove up to Cloudcroft (8600 ft) a couple of times and never had any drivability issues that I remember. That was with the stock jets 120 main and 40 pilot. Carbs had never been touched at that point and were still at their factory settings. I have to admit I never even looked at the plugs back then since I did not have any running issues.

    So, I think I would try going back to stock where it should run OK, and then perhaps evaluate the plugs once you have it sorted.

    Maybe others can weigh in here on needed altitude adjustments, and also on the carb to air box boots. My guess is a slight air leak at the carb to airbox would not cause the poor idle conditions you described.
     
    dirtmax and k-moe like this.
  17. k-moe

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

    Messages:
    19,613
    Likes Received:
    6,703
    Trophy Points:
    113
    Location:
    The City of Seven Hills
    There really should be no need for a rejet on a stock bike (even with an aftermarket exhaust) to account for altitude. There will be a small loss of power at high altitudes (above 6,500 feet if memory serves) due to increased richness, but no other drivability issues.
    I didn't have an XJ at the time (the bike did have CV carbs), but when I lived in Oregon I'd regularly ride from sea level up to Timberline Lodge (6,000 ft asl) without any troubles.
     
  18. joejr2

    joejr2 Active Member Premium Member

    Messages:
    713
    Likes Received:
    109
    Trophy Points:
    43
    Location:
    Santa Fe,NM
    Yes, the bike was set up with pods and the stock jets by the PO.The main jet #120 and the pilot jet #40. The frame was bent and the forks were mangled but it started
    up with choke. I didn't start it up again until I rebuilt the carbs, mounted the engine in a good frame and added rebuilt forks. At that point The jets were still #120 and #40
    I replaced the dried up shaft seals with the fattest "o" rigs I could squeeze in without binding the shafts. It started up with max choke. I had to feather the choke to keep it from
    stalling or surging but I could never close the choke off. Later as an experiment I switched the jets with the ones in the rebuild kit. (#122+#43 ) This time I could get it to idle off choke
    at about 1200 rpm . All the pipes got hot but it surged to 5000 rpm when I flicked the throttle. I was thinking of ordering new stock jets. Even though the old jets had been cleaned
    well and the holes were open and clear, something may have been less than perfect. Since I do most of my riding over 5000' maybe I should go with #118+ #40.
     
  19. k-moe

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

    Messages:
    19,613
    Likes Received:
    6,703
    Trophy Points:
    113
    Location:
    The City of Seven Hills
    What size o-rings did you use?
    There really needs to be a double seal for the throttle shafts to not leak, so the usual alternative to the correct throttle shaft seals is to fit two #11 o-rings in each position.
    If the o-rings you installed were distorted too much, that would cause a vacuum leak that might only show up with the throttle opened.
     
  20. Minimutly

    Minimutly Well-Known Member

    Messages:
    1,746
    Likes Received:
    748
    Trophy Points:
    113
    Location:
    West Wales, uk
    All this stuff is the very reason peple on here advise not to fit pods.
    My advice is to find a std airbox, find some std jets and then if it still doesn't behave get some chacal shaft seals fitted.
    Then if it doesn't behave get the carbs serviced by someone who knows.
    Fiddling about with jets is for the experienced, and as kmoe has saidmore than once, you should even then only start from a well running bike. Otherwise good luck, I'll be suprised if you get it anywhere near running right, and this is from someone who has no issues with pods per-se, but appreciates how difficult it is to get them working well.
     

Share This Page