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What going on here....(Carb Syncing)

Discussion in 'XJ Technical Chat' started by S10gto, May 4, 2020.

  1. S10gto

    S10gto Member

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    Hello everyone , I'm finishing up my 750 and having a strange experience. I have the block off tool and have used it to sync my xj900. Works great! So today I warm up the 750 after a complete carb rebuild and rejet. Has trouble starting but starts and runs good after warm. I calibrated my sync tool the shut it down and installed my blocking tool. When I restart the bike the top end ticks like crazy. Sounds like the timing chain is falling off but the noise is at all 4 cylinders. With a couple thousand rpm the noise is gone. Only at idle. Scratching my head I went back to the last thing I did. I uninstalled the blocking tool. On restart the bike sounds great so I just synced the carb without the tool.

    Anyone have a idea what's going on here? I swear its detonation but cant see how blocking the yics would matter.

    Thanks
     
  2. SQLGuy

    SQLGuy Well-Known Member

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    (Way) too lean at idle?

    With YICS blocked, each cylinder only pulls through it's only carb on the intake stroke. Normally, each cylinders pulls through its carb, plus a bit through the other three via the YICS ports. I would guess that blocking YICS makes the mixture leaner, and, if it were already too lean, that might be enough to push it into knock territory.

    Anything else special on the bike that might add to this? Non-standard TCI, for instance?
     
  3. S10gto

    S10gto Member

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    All stock. PO drilled a bunch of holes in lid. Running paper filter. Dynojet jet kit and I went up one size on pilot. Mix screws 3 turns out.
     
  4. S10gto

    S10gto Member

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    Was thinking it has to be lean. The choke didnt seem to be enough. This also supports super lean. I could have propaned the manifold but I rather have the peace of mind so i just pulled the carbs back out and removed the manifolds. Gaskets are all intact but super thin and crispy. I will make some up and go from there.
     
  5. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    I don’t use my YICS tool. No need to. It still in the sealed plastic bag that it came in 15 yrs ago. I look at it every now and then just to remind myself where it is and what it looks like.
     
  6. S10gto

    S10gto Member

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    I agree. It does make a difference but it is very little. I synced my xj900 without then installed the tool. If was slightly off.
     
  7. Rasput

    Rasput Member

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    I'm not intending to argue with you, I just want to make sure I'm understanding the YICS system correctly. Since the ports are after the carbs, wouldn't blocking them off decrease the air and cause the mixture to be richer?
     
  8. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    Basically, the port is just a port that allows for a little extra balancing and swirl after the stick is removed. But unless you are intending track times and need that extra 1/1000th of a second advantage to win, you’ll never notice a difference
     
  9. Polock

    Polock Well-Known Member

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    when you put the"tool" in there's a possibility you made a vacuum leak.
    when the tool is out, anything that goes through the yics port is a A/F mixture, , it's past the carbs.
    for the tool to leak from one cylinder to another and make it so lean as to detonate, how was that lean cylinder running in the first place.
    i rode my 750 a few miles without the vacuum caps on at all, didn't run too well but it ran without detonation
    if my idle gets down to about 800 the cam chain rattles like crazy
    if it is detonation you should feel it through the whole bike, like someone hitting the frame with a big hammer
     
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  10. S10gto

    S10gto Member

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    There is no chance of my tool leaking. It slides in completely and you screw the factory end plug back in. Has orings at correct locations. Haven't ridden this bike yet but without tool engine starts instantly when warm and idle nice. No rev hang. Was getting and intermittent miss on cylinder 3 or 4 but that resolved itself after carb adjustments.

    Thanks for all the insight. Getting gasket paper today. Gaskets looked wet all across so I'm suspicious of them.
     
  11. S10gto

    S10gto Member

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    Gasket and tool
     

    Attached Files:

  12. S10gto

    S10gto Member

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    Ok so today I replaced the intake gaskets. No change. Bike syncs and runs perfect without the tool. With the tool instant rattle from top end. I can enrich the mixture screws until it has a low rpm bog and I can lean the screws to a point that it wont take throttle. In between the settings it runs awesome.

    Really wish I knew what's going on here. What am I missing? My 900 is just about identical and I have no issues with or without the tool.
     
  13. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    What happens when you set everything where it runs really good, then go for a ride and do a plug chop. What do they look like?
     
  14. S10gto

    S10gto Member

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    When for a ride and she runs good but I feel it is lean. Feels low on bottom end power. Didnt do a chop but looked at the plugs and also looks lean. White with a hot ground. Pistons light tan. My 900 plugs in comparison are white with some tan and half a hot ground with pistons lightly dusted black.

    Engine seems to run hot too. Just took the 900 for a rip for comparison. No temp gun but if feels the same temp as the 750 that's been sitting for that last 20 mins as I rode the 900. 900 does have a oil cooler so this probably means nothing.
     
  15. S10gto

    S10gto Member

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    Was thinking my aftermarket pilot 42 jets might be the issue. Pulled carbs out again and swapped to the factory 40's. No change. Ordered a color tune. I'm done guessing.

    As another thought can the ignition timing be altered at the trigger in these bikes? PO had bike from new and was serviced at dealer but I am just out of ideas.
     
  16. joejr2

    joejr2 Active Member Premium Member

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    Sometimes one of more of the pilot fuel circuits could be restricted causing a lean condition
    you really need to blow them out good
     
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  17. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    That's not a YICS synching tool, it's a YICS eliminator tool that is designed to be left permanently in the engine, perhaps if you are leaving it in then it's rattling around in there? I would assume that it expands a bit with enough heat to actually fully seal those o-rings against the inside of the chamber (and heaven help you if those o-rings overheat and melt inside that chamber..........). The real YICS chamber blanking tool uses "expanding" rubber seals (not o-rings) that are squished by the cam-action of the handle after installation, and thus expand and seal off the chamber during the synch process.
     
  18. S10gto

    S10gto Member

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    Tool fits tight. Yes I know it's a elimination tool. I use a motion pro for sync. It's not rattling. Definitely top end. Fits just the same in my 900 and the only thing that bike does with it installed is a lower idle. Tool works and my motion pro proves it.
     
  19. S10gto

    S10gto Member

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    I hate when nothing makes sense and I can't put my finger on it. Carbs are completely rebuilt including oem shaft seals. Dyno jet 124 main and needle. Sprayed cleaner through pilot circuit and they flow. At 25years as an autotech this stuff gets personal real quick. I will figure out whats happening here.
     
  20. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    Have you checked your exhaust collars to make sure they are tight, and the exhaust gaskets too? Leaks there can sound like this..... been there often enough
     

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