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Hesitation and Stumbling at Idle - With Video

Discussion in 'XJ Technical Chat' started by Shannonside, Mar 4, 2012.

  1. Shannonside

    Shannonside Member

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    I hope it's ok for me to repost these movies as a new thread with a more accurate title.

    Beneath the videos are the details which I have already given in another thread, of what has been done to the bike to try and eliminate this problem.

    Appreciate any thoughts...

    In the first one, there is a stumble/hesitation to be heard at 15secs, 26secs, and 28secs; and you should hear a slapping noise at 32secs, 43secs, and 58secs.

    If anything, it sounds more pronounced in real life.

    [​IMG]


    In the second one, there is a clunking at 10secs; at 30secs a clunking and the revs drop; and the same at 35-38secs.

    [​IMG]

    This only happens on start-up. Once she's been runnning for a short while, she idles like a champ.


    To recap on what has been done so far -

    Valves

    Inlet Valve range is given as 0.11 to 0.15 in Haynes.

    I have readings of 0.15, 0.16, 0.15, 0.15


    Exhaust Valve range is given as 0.16 to 0.20.

    I have readings of 0.22, 0.21, 0.21, and 0.19.

    Carbs

    cleaned, including soaking all jets in carb cleaner for a week; clunk-tested; float heights set using clear tubing; balanced using Morgan Carbtune and ''Oily Rag'' YICS.


    Using a Colortune, I can see bunsen blue in all pots except when stumbling/hesitating - the spark goes bright orange. All four cylinders the same.

    Have used tissue paper in front of exhaust downpipes to find leak - nothing.
     
  2. Shannonside

    Shannonside Member

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    Are the links to these movies working for everyone ?
     
  3. skoster

    skoster Member

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    Working for me, I just have no idea how to help you. Sorry.
     
  4. mook1al

    mook1al Member

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    Sounds to me like the coils are just barely putting out enough. Pull the plugs after this brief idleing and stumbling and post pics.

    When you wet set the floats, did you set them to the low side?
     
  5. BluesBass

    BluesBass Member

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    Definitely check your coils. If OEM, they could EASILY be way out of spec and produce a weak spark.
     
  6. RickCoMatic

    RickCoMatic Well-Known Member

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    There's a couple of things I think you should do.

    One is:
    Look at the Plug Wires with the Engine Running, ... in complete darkness.
    See if the Plug Wires are OK, ... or if you see "Discharges" from the Wire to the Frame, Head or between Wires.

    Next:
    RE-Colortune!
    Add some additional RICHNESS.

    Turn the Pilot Mixture Screw OUT until you get the Pretty Bunsen-Burner Blue.
    When you "Find" the introduction of BLUE, ... Keep TWEAKING Out until the BLUE begins to fade-out --> ON THE RICH SIDE.

    "Blue" ... you need for Idle.
    The Engine actually requires additional richness to NOT hesitate as the Engine transitions from Idle to Off-Idle.

    The additional Richness should resolve the "Poppities"
    Room-room-room-pop-room-room-room-poppity-pop, ... should disappear.

    One Turn is like a Minute on a Stop Watch.
    One Full Sweep of the Second-hand = 60 Seconds.
    The Tweaking you need to do amounts to about: 1.5 -to 3 Seconds.
    About the width of a Nickel.
     
  7. Shannonside

    Shannonside Member

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    Thanks for replies.

    I forgot to say that I had already wet the coils and leads with sprayed water and run the bike in total darkness - no sign of any arcing or flashover.

    I wonder does the performance of the coils change much with temperature ? I ask because the bike idles very well after it's been running a while.



    Colortune is a whole other drama. I screwed up the Colortune before, and have somehow chewed up the mixture screw on 1. It's jammed pretty solid in there, with little chips of brass and alloy :-(

    I guess the carbs are going to have to some off again, or there will be no tweaking done.

    The carbs were put back on at 2.5 turns out, with bunsen blue on all 4 cylinders now. Except when she stumbles.
     
  8. Shannonside

    Shannonside Member

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    Will do the pics thing, hopefully today if I can.

    I set the floats to about half a mil either side of the datum line. If any are on the low side, it won't be by more than that.

    Thanks.
     
  9. RickCoMatic

    RickCoMatic Well-Known Member

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    2.5 Turns Out is inadequate.

    That's a Lean Idle with trouble getting out-of-the-hole.

    You need to be at 3.0 or mighty close to 3.0 (Preset).
     
  10. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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    Why are you settling for running with your valves out of spec?

    Did you replace the throttle shaft seals?
     
  11. Shannonside

    Shannonside Member

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    Haynes specifies 2.5 turns for my bike, the 900F, which uses the Mikuni BS36;

    and actually specifies 2 turns out for the 31A, which uses the BS35.

    I may well be a little lean, all the same. The Motad 4into1 is probably not the bolt-on they claim it to be (I asked them personally by email) and I have a bit of discolouration on the exhaust.
     
  12. Shannonside

    Shannonside Member

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    I didn't replace throttle shaft seals since I don't have a vacuum leak with a high and hanging idle. I've sprayed all around the carbs and boots with cleaner with no increase in revs. And I didn't want to break the carbs off the rack unnecessarily.

    As for the valves - I have been waiting on the Motion-Pro tool and was really only covering all other bases while waiting.

    I don't have any problem with buying the shims, but being so expensive, if they turn out not to be the cure, I'm going to be really cheesed off :lol:

    So they are the last option, and I will pay out for them happy that I have tried everything else first.

    Will be testing the coils tomorrow and have ordered a new air filter. While I'm waiting for that, the carbs will be coming off again.

    Do you know if the coils are temperature sensitive, performance wise ?

    Thanks as ever to you all for your help.
     
  13. Bobbybonez

    Bobbybonez Member

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    The alternative would be a lot more expensive.
     
  14. streetbrawler750

    streetbrawler750 Member

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    Try shimming the needles in the carbs. Cured my hesitation and doggy low end. On a stock 750 with 4-1 and stock airbox.
     
  15. RickCoMatic

    RickCoMatic Well-Known Member

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    Again, ...

    2.5 Turns (Factory Preset) is inadequate.
    The Idle Mixture is too Lean.
    Bikes entering the Country were set to 2.5 Turns to meet California Emissions requirements.

    2.5 Turns out is fraught with running problems.
    Insufficient Mixture to sustain combustion at the critical point of transitioning from Idle to Off-Idle.

    The Engine "Stumbles" ... as Yours IS doing on the Video ... because there is a momentary starvation causing a misfire. (The Poppity's).

    You do not have to add too much more Richness to correct the problem.
    Nudging the Mixture Screw OUT, ... --> Almost Imperceptibly <-- resolves the too Lean Air~Fuel Ratio.

    In reality, ... about THIS far --> || <-- (Maybe less).

    Let the Spark Plug colorization be your guide.
     
  16. Shannonside

    Shannonside Member

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    Are you saying that blowing up my engine is just as likely with the valve clearances a tad on the loose side, as it would be with them being too tight ?

    I didn't know that.
     
  17. Shannonside

    Shannonside Member

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    Hi, thanks for that. What did you use to do that with ? Do small adjustments make noticeable changes ?
     
  18. Shannonside

    Shannonside Member

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    Hi Rick,

    Thanks for the reply, appreciated.

    I will adjust the mixture screw outwards as you suggest.

    Can you tell me though, just out of technical curiosity, why the bike will after a few miles of riding show absolutely no sign of any hesitation or stumbling if I pull over and let it idle ? I mean, it is rocksteady.

    And just to clarify - I'm in Europe and I don't believe the bike I'm riding was ever Stateside. I don't think European bikes would have been set up for California Emissions law. I will research that a bit more though.
     
  19. MercuryMan

    MercuryMan Active Member

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    Wow that sounds like me in the morning, always starting out kinda slow but once I get warmed up! I'm going!!

    All joking aside both your potential problem areas (fire & fuel) are known to be affected by temperature. Coils are most efficient from about 5 to 50 degrees Celcius, each degree of C raised is worth about .5% loss in efficiency. So it's not your coils, but your battery is another story-it doesn't like being cold at all. Vacuum will increase as the motor warms up and especially so if you have some dry, cracked, or tired rubber that is sealing better as it swells up. Your fuel will flow better warm also but is less efficient in producing power.

    If it is running fine once you get going, then you're not far from having it right. I would double check all your rubber (including the vacuum nipples), check your ignition system primary and secondary resistances, your cold battery voltage, and follow Rick's advice on your mixture settings. You will get it.

    I don't believe you will blow your motor with a loose valve, but you can't get it synched or running right that way. Too loose is still bad since they don't open all the way. So Yea get those baby's in spec.
     
  20. streetbrawler750

    streetbrawler750 Member

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    Ya, the needles make a difference. But I guess it shouldn't if it only does it cold.

    To do it, take the carb hat off, pull the slide and needle. Remove the plastic nut. Pull needle out (remembering how it came out) put a tiny washer, just big enough to go around the needle but small enough to fit back down in the hole. The higher the needle is raised the richer. Then reassemble.

    But, that probley won't help if this problem is only when its cold. Maybe you just arenn't waiting long enough for it to warm up? I don't know if valves could cause this problem, but they aren't that hard to do and well worth the money.
     
  21. Bobbybonez

    Bobbybonez Member

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    I dont know about blowing it up, but I know its called "spec" for a reason. This is where your clearances are supposed to be and Yamaha has reasons behind it. I know if they are too tight then yes "blowing it up" is a great possibility. Also having it too loose, you will be letting in and out more mixture and exhaust then your supposed to be. If all the gas doesn't combust in the chamber, because too much is being let in and out then that could also lead to the back firing. If your letting too much gas into the chamber your going to be using more gas then you should be and with the price of gas these days there is an expense I would rather not have.

    I dont know how correct I am on this as I am just learning too. Some of the other guys here could probably tell us if im right or wrong. But like I said, I do know they want you to be in spec for a reason.
     
  22. parts

    parts Member

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    +1 for the batt.

    +1 for the pilots @ 2.5 out-just a start point.

    ++1 on valve clearance, if not correct then the carbs cannot be
    sync'ed. period.

    My bike is just a tad out of tol on one exhaust shim and on the line
    on one intake. I knew this when my idle started a slight stumble. Putting
    her on the carb stick (a quick way to check these things) and she was
    off.

    A quick check of the valves told me why. I'm just waiting for the $$
    to adj but I'm stuck with a poor idle until fully warm just as you are.
     
  23. Shannonside

    Shannonside Member

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    Hey guys, many thanks to you all for the excellent info and advice.

    What I found last night while checking the HT leads is that the cap just effortlessly came away from the lead on no.1. The spike is absolutely rotten with corrosion ! I had to cut back over half an inch of the lead to get to copper.

    Will post pics of them tomorrow.

    Anyway, I'm off work for a couple of days now, and the Motion Pro tool arrived last week so am just going to cut to the chase with the shims.
     
  24. RickCoMatic

    RickCoMatic Well-Known Member

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    Be sure you have enough slack before trimming the HT Leads back.

    Another way to get the Spark Plug Wire connection restored is to take a 6-Inch Length of the solid copper Ground Wire from Household Wire.

    Sand a point on the Copper Wire
    Smear some liquid detergent on it.
    Stuff it as far up into the middle of the Spark Plug Wire as you can get it using Needle Nosed Pliers.
    Clip-of the excess.

    The Plug Cap will take a nice bite into the Copper.
     

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