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Stalling because of low to no idle

Discussion in 'XJ Technical Chat' started by Wkidtoy, Jun 22, 2021.

  1. Wkidtoy

    Wkidtoy New Member

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    This may get "wordy" as I try to explain,

    85' 750 Maxim X still restoring it, recently my bike has started stalling if I let off the throttle after about 20 min. Temp gauge has been in different locations it seems. It starts most times without choke unless its cold out very easily and purrs away at about 1100ish, responds to the throttle without hesitation. Drive around or let idle and the RPM without throttle will drop till it shuts off. Let it sit for some time(I've never timed it)it will behave perfectly again. If I keep using it and not shut it off it will eventually die and wont start for a period of time. on one occasion while trying to make it home while at a light I was full throttle and bike barely idling. Never duplicated this, stopped pushing my luck.
    New T-stat, coils, wires, plugs, cleaned tank, new lines and petcock, checked the gas cap for vapor, cooling fan maybe a little weak and haven't pulled the temp sensor or pressure flushed rad. Running out of ideas with my limited knowledge, the things I've replaced so far were just part of the restoration and all OEM. Any suggestions would be appreciated, currently thinking heat soak maybe temp sensor is reading wrong.
     
  2. Jetfixer

    Jetfixer Well-Known Member

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    You may have a vacuum leak , could be the butterfly seals, you could if you get bike to idle spray some starting fluid at the butterfly stub if you get an increase in idle this could be problem. The other thing is to inspect the intake boots for cracks and the clamps for tightness. One other simple thing make sure the vacuum caps are in good shape and if you have the vacuum petcock make sure the hose is in good shape a vacuum leak can cause idle problems as well as hard starting . Cheers.
     
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  3. Minimutly

    Minimutly Well-Known Member

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    Full throttle just to keep it running? That's no vacuum leak. Sounds like fuel starvation - a faulty petcock or vacuum signal failing to keep it open, tank not venting, can't think of anything else right now. It's defo not throttle shaft seals - unless of course you have enough of a leak to steal the vac for the petcock? Nah, sorry, I can't see that - put the petcock on PRI next time it happens?
     
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  4. Beck

    Beck Member

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    Your bike issues mirror mine. I’ve been playing with carb adjustments lately and am nearly at a loss. So many say to start with mix screws to be set at 2.5. Mine will run when cold but won’t keep running once warmed. Acts like the choke is still on. So I backed out them out 1/2 turn. Didn’t help. Put them back and went in 1/2 turn from original and can keep it running when warm but not well enough to not have to play with the throttle to keep barely running.
    85 xj700x Valves done, clean gas/plugs/carbs.
     
  5. Minimutly

    Minimutly Well-Known Member

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    Someone will be allong in a minute to tell you to start a thread for your bike/issues.
    But in the meantime, in is weaker, out is richer. You have to vacuum synch all of your carbs, and at the same time tweek your mixtures (so your bike needs to be hot not for synching, more for the mixtures).
    In my usual way of diagnosing faults, rather than taking part in the self flagelation often seen on here, I personally would be trying to rule out the worst case scenarios. So, if someone tells me I have a vacuum leak I would be using some gas/fluid/spray to carefully test this. Then, if no change, get an oil can full of engine oil and squirt it at each throttle shaft spindle - this should temporarily seal any minute leakage if the seals are shot. If no change, wipe away the mess, breath a sigh of relief and move on to the next possibility.
    Forums are a great learning source, and someone will allways know more than you, but you still need to use your own brain - sometimes.
     
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  6. Beck

    Beck Member

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    Thank you for this. I basically made a comment in order to keep following the original poster. So you’re saying since choke is rich and out is richer, I’m on the right track by turning them in for less fuel more air. Which has been a sticking point for me, because every reading is worded differently on every post and dyslexia doesn’t help. I did find a slight fuel leak at the petcock and subsequently discovered a vacuum leak. I did a rebuild kit with new diaphragm and am feeling much closer but still not there yet.
     
  7. Minimutly

    Minimutly Well-Known Member

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    Be careful of generic rebuild kits - many are just poor copies. Get your stuff from @chacal .
    Yes, in is less fuel, if you're careful and methodical you can screw in and out individually for best idle - you need practice and a good ear.
    Edit to add, this is pointless if your fuel levels haven't been checked and are in spec.
     
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  8. Toyobaru866

    Toyobaru866 Active Member

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    On the X's I've had and the one I have I've always found the steady idle is achieved by no vacuum leaks and a good running sync. Of course the fuel level and pilot screw settings are important for the total performance of the bike but (unless they are way off) this doesn't affect idle too much. Like for now I know my fuel levels are too low and the pilot screw settings are set to a base of 2,75 turns, so tuning needs to be done. The performance can be better but the idle is steady at 1050 rpm, warm or hot.
     
  9. Minimutly

    Minimutly Well-Known Member

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    The fuel level setting is important everywhere, but the "pilot" screw settings are for idle and just off idle....
    Once the twist grip is opened, the pilot is neither here nor there...
     
  10. tabaka45

    tabaka45 Well-Known Member

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    I agree that it is most likely a fuel starvation issue. Have the carbs been cleaned? Do you have a fuel filter that might be almost clogged?
     
  11. Polock

    Polock Well-Known Member

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    ever look at the valves?
     
  12. Wkidtoy

    Wkidtoy New Member

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    Not sold on the fuel thing or vacuum to be honest unless these could happen suddenly and consistently over a set amount of time. I say for the first 15 min my bike runs just a smooth as a new one with awesome throttle response. 15-20 min later, Highway speed or city speed things start to go to Sh. That’s why I’m confused, it runs so good for a while I’m still going to keep checking the simplest to hardest of your guys suggestions. If I figure it out I’ll be sure to post.
     
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  13. Wkidtoy

    Wkidtoy New Member

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    I haven’t not sure about the previous owner, I don’t know much about that as I haven’t researched it. Its been running so good with a smooth sounding engine I was have trouble wrapping my head around why I should touch it.
     
  14. hogfiddles

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

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    Because over time, the valves recede into the head.
    - the valves get closer to the cam
    - the valves open sooner and sooner
    -the valves close later and later
    - the valves stay open longer and longer
    - the compression drops farther and farther

    your performance degrades slowly and you don’t notice it until you get things back in spec.

    you DONT want a very quiet engine. You want the “singer sewing machine” sound.

    why should you touch it ? So you DONT have to TOUCH IT MORE.
    dfox
     
  15. Toyobaru866

    Toyobaru866 Active Member

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    For sure I'm not an expert so I believe what you're saying. Forgive me my ignorance but why do people check their sparkplugs color after a ride and adjust the pilot screw settings accordingly if they only affect idle? Just trying to understand.
     
  16. Huntchuks

    Huntchuks Well-Known Member

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    Fuel starvation can happen gradually as you ride. If there is some restriction, fuel will not be able to keep up with demand and eventually there is not enough to run the engine smoothly.
     
  17. Polock

    Polock Well-Known Member

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    it doesn't only affect low speed but it affects low more than mid or high speed. if your general riding around shows rich or lean, you can change the screw setting to compensate a little and still have a good idle. if your idle screw can't get you to happiness, then your into changing jets.
    did you try opening the gas cap while it's acting up? might be a clogged vent
    maybe replace the petcock vacuum line with a real vacuum line
    start reading about setting the valves
     
  18. Toyobaru866

    Toyobaru866 Active Member

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    Thnx Polock, my idle is really good and the bike runs good :) It's just that I said before that changing the idle mixture a little doesn't affect my steady idle. I was responding to Minimutlys response to my earlier post.
     
  19. Minimutly

    Minimutly Well-Known Member

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    The ports from the idle mixture screws are just that, the empty the fuel and some air mix ino the ports downstream of the throttles. If turning them in or out by half a turn doesn't change anything then its likely your carbs need a look at, or you need to listen more intently.
    On the subject of valve clearances, welcome to the other (after the carbs) downside of xj ownership, except you have it double since you have an X. But, on the upside, I believe they don't suffer with valve recession as much as the 8valvers.
    Clearances are easy to measure, and as suggested you should at least do this.
    Your symptoms tell me you have fuel starvation though, and thats what I'd be chasing to at least eliminate it. Get your carb drain screws working, measure the volumes, you've said the issue is consistent, so you can prepare to do the checks - a float bowl volume check when the issue happens would be an easy one. If your carbs are empty, open the filler cap or put the petcock on pri. If fact does this happen on pri?
    Stop listening to idiots like me and start investigating is the best advice I can give you, learn about your carbs and valve clearances is the next two bits.
     
  20. Minimutly

    Minimutly Well-Known Member

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    This exactly is what usually happens, when idling you use very little fuel, andnthere is loads of vacuum, so it takes ages, if ever, to occur
     

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