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super high idle

Discussion in 'XJ Technical Chat' started by Nacho_Supreme, Apr 8, 2014.

  1. Nacho_Supreme

    Nacho_Supreme New Member

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    I recently bought an '81 xj650 and it was not running well. It was dying every time I stopped, after the first carb rebuild while waiting for parts. I replaced the throttle shaft seals, floats, float needles, float chamber gaskets, fuel line o rings and cleaned the crap out of the carbs. I also checked all of the valve clearances, which were all within spec but most at the Low end. Anyway it is running very well, maybe too well. When I start it up now it will idle well as long as the choke is on partway. Once the choke is turned off though the rpms increase to about 7000 rpm. It will only slow down if I put on the choke again. I have tried changing the idle screw, all the way out with no change. I originally set the pilot mixture screws(after rebuild) at 2.5 turns out. I moved them all into 1.5 turns in and then out to 6turns out with no change. All the manifolds seem seated well and the boots are seated in the airbox all the way. Choke seems in the right place too. Throttle cable normal as well. Any thoughts?
     
  2. PilotSmack

    PilotSmack Active Member

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    Just dial back the idle throttle setting. It's a long screw behind the 2nd and 3rd carbs, close to the top of the engine case. Do it while the engine is warm, and wear gloves.
     
  3. adrian1

    adrian1 Active Member

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    Welcome! You need to sync the carbs. Oh and pls list your bike in your sig area
     
  4. BruceB

    BruceB Active Member

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    A couple of reasons for high idle after rebuilding carbs...

    1) Air leaking into carbs. Possible from butterfly shaft seals if you didn't use amble amounts of silicone grease. Possibly leaking intake boots. Vacuum nipple plugs/caps are bad. Test with propane around intakes and carbs to see if air is leaking in.

    2) Butteryfly valve was not seated correctly on rebuild.

    3) Bench sync was not done properly. Re do bench sync with closer tolerances.

    4) floats are not within spec. Check for gas in your oil (sniff test)

    5) verify diaphragms with clunk test

    6) check linkage is connected correctly and is not binding.

    probably more, but those are the biggies...

    hope this helps
     
  5. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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    Running vacuum sync is the next step if you accurately set the float levels.

    Unless you used American feeler gauges when you checked the valves; if they were at the "tight end" using American numbers they're out of spec.

    If you fudged on the valve clearances you'll have a harder time getting a good vac sync.
     
  6. KrS14

    KrS14 Active Member

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    If you run 7k rpm, make sure you don't have the choke and throttle cables backwards.

    You need open throttle plates to get 7k.
     
  7. junkmn

    junkmn Member

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    Floats in upside down can will result in high idle. Been there done it
     
  8. Nacho_Supreme

    Nacho_Supreme New Member

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    Thanks for all of the advice!

    Floats are installed correctly to the best of my knowledge(as shown in the 'Setting your Fuel levels' PDF on here). Also fuel levels were set like shown as well using clear tubing, tangs bent and adjusted as needed to be at the required fuel levels.

    Choke and Throttle cables are not backwards, choke chokes and throttle throttles. None of the linkages that I can tell are binding or anything.

    I checked the diaphragms with the clunk test while rebuilding, and checked them against the light for any pinholes or tears. All good there.

    No gas in the oil (sniff test), and oil level is still at the same spot on the sight glass as when I changed it the other day. Before when I was getting gas in the oil(old float needles...) the level increased dramatically and was easy to smell.

    I checked for air leaks around the intake manifolds and all around the airbox boots by using a propane torch and moving it all around both sides of each rubber piece. No change in idle speed during this. I also checked it with carb cleaner aerosol spray, no change in idle speed there either. Also around the vacuum nipple plug caps with no change so it looks like those are good as well.

    I replaced the butterfly/throttle shaft seals during cleaning(they were pretty much disintegrating and maybe half there when I opened it up) and used plenty of grease when putting them back together. As well I tried to seat the butterfly valves securely upon rebuild, and checked for gaps with a light behind them. None were 100% blocking the light, but there was barely any gaps and seemed like there wasn't much I could do to make it any better. I tried rotating them some, made sure they were in the correct way(not upside down/backwards...) and got the best possible fit I could.

    At this point I need to vacuum sync the carbs. I tried using a friends vacuum gauge today but it did not have any bleeder valves to calm the needle down and didn't move from 0, and I don't know what the hell his digital vacuum gauge was telling me. I think tomorrow I will make a gauge like the ones shown on here using ATF fluid (like the one greggvickrey posted) and see if I can't get it fixed that way.
     
  9. lacucaracha

    lacucaracha Member

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    Seems like an out of sync issue if all the rest is in order.

    I bet one of your butterflies are getting snagged somewhere. Either they got mixed up and the wear patterns don't jive, or they're not seated correctly. If you can force the throttle shaft closed and it slows down you have the culprit.
     
  10. BruceB

    BruceB Active Member

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    If your bench sync was done correctly, your bike would not idle @ 7000 rpms. If you have backed off the idle adjustment screw and it has no effect, and all the checks have been done that you stated in your prior posts, then it is your butterflies that are not seated or installed in the wrong orientation. I don't think a vacuum sync, though needed for final turning, will affect an "idle" of 7000 rpms.

    I had the same problem after a carb tear down to do fuel rail and throttle shaft seals. It ended up the #4 carb butterfly was not seating all the way. I unhooked the spring, loosened the butterfly screws and reoriented in the carb. Tightened everything back up, installed back on bike and she fired up and idled down where she was suppose to. Finicky little devices needing exactness and perfection to run correctly.

    hope this helps
     
  11. Nacho_Supreme

    Nacho_Supreme New Member

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    Took it apart and checked the butterfly valve seating, made some adjustments and bench synced again. Checked against a bright light to make sure they were fully seated and no light was getting through. Also took the float bowls off and double checked that everything in there was good and put together correctly. Now the idle is only around 4-5k.
     
  12. rocs82650

    rocs82650 Well-Known Member

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    Nacho,

    Pop the tops on the carbs and make sure the diaphrams and springs are in good shape. I had a bent spring once. Hope this helps.

    Gary
     
  13. Nacho_Supreme

    Nacho_Supreme New Member

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    Ya, did that last night also, springs were good, diaphragms were good, seated well in the groove, and checked them for tears. Clunked during the clunk test.
     
  14. rocs82650

    rocs82650 Well-Known Member

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    Here's a dumb question...is the petcock turned to prime?

    Gary
     
  15. Nacho_Supreme

    Nacho_Supreme New Member

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    The PO must have installed a new petcock at some point because it only has On, Off, and Res. There is no prime and no vacuum line.
     
  16. Nacho_Supreme

    Nacho_Supreme New Member

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    And there are no dumb questions :) I appreciate all the help I can get!
     
  17. rocs82650

    rocs82650 Well-Known Member

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    Cool. That's what we're all here for right. Sounds like what others have advised...your sync is off. Odd that opening the ec will idle it down unless it's getting too much fuel flooding the plugs which would cause it not to fire at all.

    Gary
     
  18. BruceB

    BruceB Active Member

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    I don't want to sound like a broken record, but since your rpms came down 3000 rpms after readjusting the butterflies, you are on the problem. It really doesn't take much for them to be off. If you didn't put them back in the same carbs as they came out of you may have to play musical butterflies until you get them in correctly. This one problem drove me nuts. I think it was the ninth or tenth time I had the carbs off to finally get the butterflies shut and synced and the bike to idle. it was ugly and wasn't no fun...I know..
     
  19. Nacho_Supreme

    Nacho_Supreme New Member

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    I took them apart, cleaned, and put them back together one at a time so that I had no way of interchanging parts in between the carbs. At least the butterflies are in the same carbs as they came out of.

    I have noticed that when it is running out of gas the engine will race and not even the choke will calm it down. Very similar in rpms to the "normal" racing. By that train of thought, maybe it is not getting enough gas when trying to idle and it is causing it to run extremely lean. Applying the "choke" bypasses this and adds more fuel richening the mixture and causing it to idle normally. Pilot screws have had no effect when adjusted so the blockage(if there is one) would be upstream of these somewhere in the pilot circuit.

    Does that sound like it makes sense or am I way off base?
     
  20. BruceB

    BruceB Active Member

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    As it is running out of gas there is more air...more air..higher rpms...opening the enrichment circuit adds more gas, less air, idle comes down. You are chasing an air leak from the needle valve forward one or all carbs. Shaft seals, butterflies, vacuum nipples, intake boots. They are the problem areas creating extra air, causing the vacuum to open the diaphragms, therefore increasing fuel and rpms.

    I ended up seating all butterflies without the springs set, then bench sync to .15 mm, then backing off the idle adjustment screw until the butterflies were shut.

    I did not reconnect the fuel supply but used starting fluid around each carb boot while trying to start the engine. #4 tried to fire off...that led me to concentrate on # 4 only. I redid everything including the shaft seals and reinstalled. Fired her up and she idled down.....

    Your getting close...don't give up now :)
     

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