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'83 XJ750 MK suddenly running rich

Discussion in 'XJ Technical Chat' started by byob_fil, May 29, 2020.

  1. byob_fil

    byob_fil New Member

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    Hi folks,

    I am currently wokring on my 750 Midnight Maxim and I have been doing all sorts of jobs on it to get it into shape.

    Recently, I have taken apart the whole exhaust, took out the fuel tank to respray it and also removed and cleaned/lubed all the linkages (throttle, choke and clutch). The tank was drained in the process and filled with fresh fuel after assembly. I have also been messing with various electrical connections although mainly lights, it's not impossible that I have messed something there.

    I did all this in parallel so the bike was not running for some time and when I finished doing all of the above and went to start it, I noticed something's wrong. The bike ran very smoothly and well before all that so I must have messed something up.

    First of all, it starts cold with no choke but requires some throttle to do so. Runs somewhat okay when cold but when it warms up, it starts to misfire, pop in the exhaust and dies down unless given plenty of throttle (air). At this point applying any choke stalls it.
    All sparks are black with dry soot even though they were okay before.
    It also tends to hang revs and drop them very slowly.

    My conclusion is that it is running very rich, so far I have done the following:

    * made sure that choke valves are closed completely when choke should be closed.
    * made sure throttle cable is well adjusted and has just a tiny bit of slack
    * tried running it dry by disconnecting fuel to empty the carbs
    * made sure there are no exhaust leaks or blockages
    * changed spark plugs to new ones
    * checked if the ignition coil has ground etc.
    * checked and cleaned the air filter, also tried running it with no filter, no difference
    * messed arund with the idle screw but did not do any good
    * checked vacuum hose that feeds the fuel tap for leaks
    * checked if the fuel tap is operating correctly (on PRI and ON)
    * checked if all four cylinders are firing


    My whole intake + carb setup is completely stock.

    I have not gone to check the most obvious culprit - the carbs, as I am somewhat reluctant to dissassemble the thing given how complex and time consuming it is. Before I do so, I wanted to ask you guys if perhaps I am missing something here.

    I am slowly leaning towards the idea that some gunk from the tank got into the carbs since it was moved and shaken around quite a bit during paint stripping, sanding and painting but I really hope it's not the case.

    I want to also try and check (although not sure how) the ignition pack (next to the voltage regulator).
    I've heard that freezing it in a plastic bag might help? Sounds crazy though...

    Please advise!

    /Phil
     
  2. Franz

    Franz Well-Known Member

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    You say it is running rich. Do you have a Haynes manual for the bike? What about your mixture screws? You didn't mention them. Check them first.
     
  3. Mezzmo

    Mezzmo Active Member

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    Hi Phil, I think the problem is most likely to be the carbs. Have you checked the carby slides go up and down freely? Doesn't take long for issues to arise if a bike is left idle for a while, particularly if most of the fuel has been drained.
    Good news is you can check my YouTube channel on how to do this... I also have another video on cleaning carbs too. It's not as hard as you think....


    Hope this helps,
    Cheers,
    Simon
     
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  4. byob_fil

    byob_fil New Member

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    @Franz I do, what do you mean check them? I have not touched them at all and the bike was running great before. The Haynes manual explicitly states not to fiddle with those screws.

    @Mezzmo Thanks for the video, it looks super useful once I inevitably get to working on the carbs. I will still give myself a day to figure out something else but then I will give up and try the carbs.
     
  5. Franz

    Franz Well-Known Member

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    @Franz I do, what do you mean check them? I have not touched them at all and the bike was running great before. The Haynes manual explicitly states not to fiddle with those screws.

    That is what I meant check the manual to see what it states.
     
  6. byob_fil

    byob_fil New Member

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    I yielded and dissassembled the carbs, took out every little bit apart from the mixture screws. I will clean it all thoroughly and see if it helps.

    There was some residual gunk in the bowls and some in the passageways but nothing too crazy.

    Also, on of the floaters seems to be sitting much higher than the other 3, I will bend it to spec during assembly...
     
  7. byob_fil

    byob_fil New Member

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    So I put the carbs back together, set the floats dry to spec (measuring at the highest point of the float with the carbs upside down ofc).
    I synchronized the butterfly valves and the carbs seem to be level.

    The bike starts but it now shows different symptoms, as if it now has too little fuel. It requires full choke to start but then when it's running it will spike up the revs up to almost 5k if I don't adjust them using the choke.
    When I close the choke it stalls. It also doesn't sound right whatever that means...

    Idle throttle screw is set to open the butterfly valves just a tiny bit, but I did not get the engine warm enough yet to set it properly.
    I did not touch the mixture screws at all during this whole process.

    I checked the fuel level on the first carb with a hose and it is definetly far less than the spec mentioned 3mm below the bowl rim. Less in that there's less fuel.

    Perhaps the floats must be set differently than the spec says (17.5mm)? They were quite far off to begin with.
     
  8. XJ550H

    XJ550H Well-Known Member Premium Member

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    dry setting floats is a starting point not that accurate. new float needles are longer than oem.
    you have to do a wet set.

    Setting the fuel levels

    the mixture screws have a little oring that may need to be changed. the reason manual states do not touch mixture screws is it was an epa rule set at factory to meet emisions test.
    pull them inspect oring washer and spring. reinstall to 2-1/2 to 3 turns from soft bottom.

    color tune or plug chops are next to set proper mixture.
    bench sync carbs then do a running sync.

    when using choke to start bike as it warms up rpms will climb it is normal. you should not have to use throttle at all to start bike.
     
    Last edited: Jun 10, 2020
  9. Minimutly

    Minimutly Well-Known Member

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    Xj55h beat me to it - dry set is only a starting point, you have to carefully measure and record the fuel level and then take them off and re-adjust the floats - and then recheck....
    most do this on a bench rather than refitting every time. Fuel level has probably the most effect (other than the mixture screw) on idle fuel mixture.
     
  10. Mezzmo

    Mezzmo Active Member

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    This might help too,
     
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  11. byob_fil

    byob_fil New Member

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    Thanks for the tips.

    I set the wet fuel levels to spec and checked it back on the bike, they are all good.

    The bike is back to running a bit rich but I can tune that however now the 4th cylinder is not firing.

    It has spark, the bowl is full of fuel and yet it doesn’t fire. The plug comes out completely dry and clean as if there is no fuel at all coming to this cylinder...

    could it be that the some passages got clogged in this particular carb? I did clean the carbs properly after all.

    I suspected the spark but then it would be wet and the inside of the cylinder is black and dry and the spark plug is clean.

    I have set all mixture screws to 2,5 turns.

    it seems that every time it’s a different issue
     
  12. XJ550H

    XJ550H Well-Known Member Premium Member

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    try opening up that mix screw about a dimes with to start with.

    could be the spark plug could be out of spec needs to be ohmrd out. possible trim spark plug wire a little incase wire is corroded.

    checked valve shim clearance?
    did you bench sync carbs?
    Have you running synced the carbs yet?
     
  13. byob_fil

    byob_fil New Member

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    Spark plugs are the NGK BPR7ES which I believe are the right kind for this particular setup and it says on the ngk pipes that those are 5k ohm. Anyhow that wasn’t an issue beforehand.

    I cleaned and gapped all the spark plugs. If it was an issue with the spark, wouldn’t it get wet from the mixture not firing? It’s completely dry and clean, in the same state it was after I cleaned it. I also tried switching the plugs but to no avail.

    if by bench synching you mean synchronising the butterfly valves and making sure the carbs are level to each other then yes.

    I did not do any running synch since I first wanted my 4th cylinder back alive.

    all the spark plug wires and coils were changed to new ones a few years back by my father they look pretty much new to me.

    just so that I get it clear, if I turn the mixture screw clockwise as if tightening am I making the mixture richer or leaner?
     
  14. XJ550H

    XJ550H Well-Known Member Premium Member

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    ccwise richens it up clock wise closes it and leans it
     
  15. byob_fil

    byob_fil New Member

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    Okay, as I thought.

    BTW can the running sync be done without a proper tool? All I see everywhere on the internet is that they use those fancy gauges that show you the vacuum levels...
     
  16. XJ550H

    XJ550H Well-Known Member Premium Member

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  17. byob_fil

    byob_fil New Member

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    Looks simple enough, I might try this.

    I am still struggling with getting the 4th cylinder to run though.
    I have switched to a brand new spark plug, messed with mixture screw back and forth, it just doesn't fire.

    I changed the ground wires going from the lamp harness and going to the battery terminal becaused they seemed crusty.
    I switched the coil leads between the cylinders and consistently got 4th cylinder not running.

    I am entirely positive it is not a spark related issue.

    I was able to get the bike to idle more or less (very rough) and I still am getting a too lean mixture (pops from the exhaust, dry, white'ish plugs) so this needs further fixing but this 4th cylinder is what bugs me the most.

    I even took the rubber going from the airbox to the carb on that 4th one and sprayed some starter fluid but into the carb, still nothing happened.

    I warmed up the engine so that the heat transferred to that 4th cylinder to possibly help evaporate whatever is in there but it changed nothing.

    Any other suggestions as to what it might be?
     
  18. byob_fil

    byob_fil New Member

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    Of course, it was my fault after all because I switched the pickup jets during reassembly of the 4th carb and the main one was 40 and the secondary one was 120...

    I corrected that and all four cylinders now work.

    So tuning is ahead of me, I’m going to diy a synch tool and get myself a colortune plug.

    should I synch the carbs first or setup the mixture?
     
  19. byob_fil

    byob_fil New Member

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    Hey guys,

    I’ve progressed some more on the topic.
    I have bought a colortune plug and some vacuum gauges.

    I synced the carbs as best as I could but there is one issue, fourth cylinder vacuum levels are much lower than the other 3 meaning I cannot sync it to the rest and it rises much less when I apply throttle than the other 3. Should I look for a vacuum leak between the cylinder and the point at which I plug in the gauge
    or should I go straight to valve adjustments?

    As for the mixture tuning, I find it surprising that an optimal (blue) flame is present at a very wide spectrum of the mixture screw rotation, as in there is very little variance when I turn the screw. I only get yellow/red when I really open the screw by a lot and I almost never get white flame suggesting a lean mixture... Is that how it’s supposed to be? I read that it is very sensitive.

    I am not very happy with how the engine runs even after all those adjustments but I will hold with further questions until I resolve the 4th cylinder vacuum issue.
     
  20. k-moe

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

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    If the valves aren't in spec then you can't colortune or get a proper running synch.
    Valve shim clearances are the most ignored bit of maintenance on motorcycles.
     

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