Idling motor, blip throttle, then suddenly motor revs to 4k and stays?

Discussion in 'Carb/Tuning' started by c0zgrove, Apr 8, 2019.

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  1. Apr 8, 2019 #1

    c0zgrove

    c0zgrove

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    I've been having this phantom problem where I blip the throttle while the bike is idling and sometimes it will rev up to 3-4k rpm and just stay there. Is this a vboost problem maybe?

    Background: Shotgun carb clean has been done, carbs are synced, intake boots replaced, valves adjusted, throttle cable replaced, I've sprayed carb cleaner around to try and find a vacuum leak with no luck. The bike idles fine otherwise but when this happens I basically have to aggressively adjust the idle screw or turn the bike off and turn it back on.
     
  2. Apr 8, 2019 #2

    MaxMidnight

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    I don't see this as a V Boost issues as it would be the throttle butterflies (as opposed the V Boost butterflies) that would affect engine revs.

    When did this problem start and had you been doing anything to the bike prior to the issue?

    I would check some of the basics like throttle cable lubrication and routing, throttle linkages and air slides for free movement.
     
  3. Apr 8, 2019 #3

    c0zgrove

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    I've been slowly bringing this bike back from the dead so everything I listed above has been done recently. I feel confident the throttle cable is ok. I replaced upper pull/push cables yesterday and was careful to make sure they moved freely no matter what position the bars were in.

    I'll have to double check the slides. I cleaned everything up and lubed them when I did the shotgun procedure. They seemed to move well afterwards but I haven't since taken off the airbox and double checked they were still moving ok.
     
  4. Apr 8, 2019 #4

    sdt354

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    If it were me, I'd disconnect the cables from the junction box then retest. Just to eliminate the possibility. Also inspecting how the cables are routed to the carbs.
     
  5. Apr 8, 2019 #5

    one2dmax

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    If you removed the intakes (for instance to replace the vboost to intake boots) did you make note which 2 bolts where the shorter ones? If not you won't get proper clamp up on the intake and can cause a vacuum leak.
     
  6. Apr 8, 2019 #6

    c0zgrove

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    I double checked that the slides were moving OK then went for a test ride. I noticed that the idle of the bike would drop maybe 200rpm or so if it was idling while leaned over on the kick stand vs upright, which I thought was strange. While I was out riding the bike seemed OK at first then it started to sputter at idle like it wanted to die and again after few blips it was pegged at around 4000rpm. I stopped for gas and when I went to start the bike one of the cylinders backfired and it sounded like a gun shot. Maybe there's a spark issue going on as well? Anyways, I did manage to make it home by just letting the clutch out and using the front brake to manage the engine speed.

    Sean, I didn't see your post before I went out but that's something I'll have to add to the checklist.
     
  7. Apr 8, 2019 #7

    c0zgrove

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    Forgot to add, I did double check that the vboost servo was moving properly when I turned the key on but during my test ride the bike seemed really down on power, especially up top.
     
  8. Apr 9, 2019 #8

    Edward

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    You said in a previous post you lubricated the slides. Gasoline lubricates the slides and if it hasn't washed off the lubricant you applied, that may be causing them to stick. Your overrevving symptoms are indicative of sticking slides.

    Re the idle change when leaning the bike: Probably due to incorrect adjustment of the floats. Also a probable cause of the backfire.
     
  9. Apr 9, 2019 #9

    c0zgrove

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    I had seen some debate on whether you should lube a slide or not online yesterday so I figured I would clean them completely and try running them dry. They seemed to be moving fine beforehand but I'm trying anything at this point. I think I can rule that out for now, same behavior in both cases.

    I'll have to go back and check my floats.
     
  10. Apr 9, 2019 #10

    cwkerr007

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    If you want to check the slides, after you install them, take the upper air box off. WHITHOUT the bike running use an air blower if you don't have a compressor just use the can of duster for electronics. Now hold it away from the rear of the carbs ,the upper slot on the rear of the carbs blow into it GENTLY and watch the slides move now make sure they raise at the same rate as each other once that is done and none of them are sticking then check the needle is secure in the slide by holding it up with your finger and take a clothespin and see if the needle pushes up in the slide. This will eliminate your slides as the culprate Hope that helps ya out
     
  11. Apr 10, 2019 #11

    c0zgrove

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    Quick update, I tore into everything last night and found that on one side of the motor there was a small tear in one of the rubber vboost boots. I also noticed on the other side of the motor a hairline crack in one of the intake manifolds. I also went through and confirmed/adjusted the floats while I had the carbs off. Anyways, parts should be here Saturday and we'll see if I've finally got this bike sorted. Thanks for all the suggestions everyone!
     
  12. Apr 11, 2019 #12

    BrianB.

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    I have been dealing with a very similar issue and your thread has given me a few things to look into here before riding season. Thanks c0zgrove.
     
  13. Apr 11, 2019 #13

    c0zgrove

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    That's what makes this forum such a great knowledge base. And don't thank me, thank the guys that are responding with suggestions! I'll be sure to document the rest of my debug process here for future reference.
     
  14. Apr 12, 2019 #14

    dannymax

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    If your carbs are still off check the throttle linkage for excessive play and look closely at the sync screws for stripped or worn threads or possibly a non oem screw. Very rarely worn bolts can jump threads causing strange issues.
     
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  15. Apr 12, 2019 #15

    sdt354

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    Right on Dann-o. As you know, a stripped sync screw nearly drove me crazy a while back.
     
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  16. Apr 16, 2019 #16

    c0zgrove

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    Ok I’m at a total loss now guys. I’ve now replaced all the vboost boots as well as went ahead and put new plugs in. I made sure to put the short screws in the right place when installing the intake. I went through and cleaned every electrical connection I could get to with contact cleaner.

    The bike idles a lot better now BUT the phantom high idle still happens. It will do it even with the throttle cables detached so I know that’s not binding up. I’ve sprayed carb cleaner everywhere to try and find another vacuum leak but no luck. I’m out of ideas and ready to roll this thing off a cliff!
     
    Last edited: Apr 16, 2019
  17. Apr 16, 2019 #17

    c0zgrove

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    I tried to get a video of this issue but every time I turned the camera on the bike acted normal. Here’s the only video I got of the bike acting up but it’s not quite where the idle drifts up to 3-4k rpm and stays.

     
  18. Apr 17, 2019 at 11:00 AM #18

    Edward

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    Stand the bike up straight and see if it does the same thing. I'm trying to eliminate float adjustment as a cause. I quick check of the timing advance curve in my manual shows the RPM to be right at the point the ignition module begins to control the spark. 2000 RPM is the lower range and then the timing rises on a much more gentle curve after the engine hits 2800 - 3000 RPM. My gut feeling is that this is a carb problem.

    I had a great deal of trouble getting my "push-pull" throttle cable to perform properly after I reinstalled my carbs. I know you have stated you did this but...

    This kind of malfunction can be caused by improper synchronization. Have you synchronized the carbs with a 4 carb synchronizer? I see you have done this.

    Check the enrichers to make sure they are not becoming stuck and all are released. Examine the enricher forks carefully. Probably not the cause since the enrichers aren't involved with a mere throttle blip.but I'm just brainstorming here.

    check the needle holders and make sure the nipples are in their holes on top of the slides.

    I'll read the thread again and see if I can pick up anything I've missed.
     
    Last edited: Apr 17, 2019 at 11:15 AM
  19. Apr 17, 2019 at 11:08 AM #19

    Edward

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    Yes, yes, this is critical to proper function. Stand the bike on its center stand as opposed to its kick stand and see if there is any change. Move the head from lock to lock while it is running.
     
  20. Apr 17, 2019 at 11:32 AM #20

    c0zgrove

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    It definitely does it straight up, I've been working on it on the center stand and was also sitting on the bike with it upright when I made that video. I checked/adjusted the floats when I had the carbs off to change the vboost boots.

    I synced the carbs with a 4 gauge synchronizer and I made sure none of the carb plungers were getting stuck because I also saw that mentioned somewhere else. I always check those needle nipples as well after seeing some youtube video of a vmax idling issue cause by one coming out.
     

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