Super high idle?

Discussion in 'Old's Cool' started by btcn, Apr 26, 2011.

  1. btcn

    btcn Long timer

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    Ok, so I have a 1985 Honda Shadow 700 that I just finished restoring.

    Its running GREAT for the most part. I hit 85 MPH in 5th gear just cruising, and it has 6 gears and 6 is OD so its power is good. Its a blast and I'm enjoying it.

    BUT theres one little thing that bothers me. When the engine is warm, it idles VERY high! Somewhere around 3,500 RPM or so, sometimes 4,000! But thats SOMETIMES, other times it idles just a little high at around 2,000 RPM or so.The choke is off as well.

    So whats up with this? I got the idle screw all the way out. When I start it up, it idles nice and smooth at around 800-1000 RPM.

    Also it revs down slowly. If I put in in neutral and rev it, it will rev up real quick, but the RPMS come down VERY slowly.

    Any ideas? I really appreciate ANY help. What should I look at? Air leaks in the intake system?

    BTW heres a pic before the paint job:

    [​IMG]
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  2. freetors

    freetors Long timer

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    Your symptoms describe an air leak to a T. It sounds like there is an air leak between the carb and the engine causing it run way lean
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  3. Lomez

    Lomez Yea...Sunday too

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    +1 on air leak. I'd consider, in order-

    1) too much air compared to fuel being delivered (lean)....for whatever reason (high float, air leak, clogged main jet, etc)

    2) stuck throttle cable or throttle springs very weak and not closing butterflies/gates

    3) stuck choke? I've done this myself.....but then, high-speed performance would probably be affected.
    #3
  4. Skowinski

    Skowinski Eukaryote

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    Not familiar with that bike in particular, but I've seen that type of problem caused by carburetion that is too lean on the pilot circuit.
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  5. btcn

    btcn Long timer

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    Thanks for the responses guys. Yes I was thinking something like an air leak. Maybe check for any tiny leaks in the intake manifolds?

    I think your right it is running a bit lean. I'll also check the throttle cables.

    Also I notice the diaphrams in the carbs go down VERY slowly when I push them up with a finger. Could this be related to the slowness of the engine revving down?

    Since its stock I've been assuming it should be jetted correctly. But maybe not?

    I'll also try and turn the choke on to see if it does anything. Thanks for the tips and I'll let you know what happens. And if anyone has any other ideas I'd appreciate them.
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  6. LuciferMutt

    LuciferMutt Rides slow bike slow

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    That symptom is called "hang idle" and is usually from a big air leak causing a very lean condition.

    Spray some carb cleaner or something around the carbs and boots and intake plumbing while the engine is idling. Listen for changes in engine speed indicating some of the cleaner got sucked into the engine. This will help you narrow down if/where there is an air leak.

    Your comment about the slides being slow to return to position after lifting with a finger-- I'm not sure? Can anybody comment on this?
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  7. Skowinski

    Skowinski Eukaryote

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    The other guys may well be right about an air leak. A bike that old could have intake boots (between carb and cylinder head) that are hardened, cracked, etc... But, I've seen 2 bikes, with stock factory jetting, unmodified, that had hanging idles like you describe. In both cases opening the fuel screws and/or installing larger pilot jets fixed it right up.

    Have you cleaned the carbs, jets, etc?
    #7
  8. btcn

    btcn Long timer

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    Ok, a little update. I just went down and tinkered a little with the carbs.

    First as mentioned I tried the choke. I haven't really used it yet. So I started it up and turned the choke on [as in enriching the fuel mixture like you would when its cold]. The idle soured up, and by the time it got warm it idled SUPER high, at around 6,000+ RPM. Turned the choke off and it went down to it's regular high of 4,000 RPM or so.

    Then I noticed I could see the idle air/fuel mixture screws. It appeared that they have been drilled out as you could see where a screw would be in the hole. On most Japanese bikes in my experience these have a piece of medal welded or screwed in so you can't mess with the mixture.


    So I said what the hell and started turning each one 1/2 a turn one way. I wrote the # of turns and which way down. So I started to go one way, it might have been left and the idle increased even more. So I did it the other weigh, and the idle started to come down slowly. I kept doing it until they were all the way in. The idle is now decent at around 1,300 RPM or so, and the backfiring is almost all gone!

    This must richen the mixture? So I think you guys are right, it must be running very lean. It COULD be jetted wrong. Maybe someone had a different exhaust setup at some point and the stock ones were installed back on and it wasn't jetted back? Just an idea.

    Or more likely it has a big air leak. The intake manifolds do seem pretty hard to me.

    DevilNinjaDog thats a great idea with the carb spray! I'll definitely give it a try!

    Skowinski yes I have cleaned the carbs. Yea I'm definitely getting into the carbs and manifold more. I bet it is the manifold.

    I also have a Suzuki DRZ 250 that does the same thing! Its only about 4 years old and its always done this. I never really payed much attention to it as its a high revving single and I thought it was normal, but now I bet it has a problem or something. It also seems more common on CV type carbs, my other dirt bikes with slide carbs don't do anything like this.

    I have seen a lot of bike do this, particularly older Japanese bikes, but I've never minded it until now, as I want that V-Twin sound like when it starts cold!:lol3

    Thanks for the help, and yea can someone who knows comment on the sticking carb slides? I'll look into it more and keep taking in any ideas. I'll keep you updated.
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  9. LuciferMutt

    LuciferMutt Rides slow bike slow

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    Hmmm. Turning the mixture screws IN is supposed to LEAN the mixture (can anybody confirm this?). Typically to set them, you turn them all the way in, then back out 1.5-2 turns depending on the bike.
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  10. Skowinski

    Skowinski Eukaryote

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    Yep, on 4-stroke carburetors they are almost always (to my knowledge) called fuel screws - because they adjust the amount of fuel to the pilot circuit. Opening them allows more fuel and a richer pilot circuit jetting. On my KTM 2-stroke carburetors they are called air screws, and do the opposite. Not sure why the difference :scratch, that's for someone smarter than me to answer. :D
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  11. btcn

    btcn Long timer

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    Thats strange! I wander why leaning it even more would lower the idle.

    I wander if it is blocking so much fuel that it has to idle lower?

    Because turning on the choke makes it rev almost to redline, and this is just adding more fuel.

    When it idles high it will chug along at about 15 MPH in first gear without any throttle input.

    I am now pretty sure its an air leak. Probably in the intake manifold.

    I'll probably go ahead and look for it and just order new intake manifolds and see what happens from there.

    I will also take a look at the jetting.
    #11
  12. achtung3

    achtung3 Long timer

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    Also check the fuel filter.
    I was experiencing high idle on my 82 seca and found out that it was the extra inline fuel filter that I installed to prevent dirt into the carb, removed and problem solved.
    It was restricting fuel flow to the carbs.

    Good luck
    #12
  13. racer 07

    racer 07 Lock & Load

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    If the mixture screw is on the air cleaner side of the carb it is most likely an air screw. Adjust out =more air=lean Turn in= less air =rich. If the mixture screw is on the intake side of the carb it is the reverse. The mixture is controled by the size of the idle(pilot)Jet and the idle air bleed. The mixture screw in this case controls the volume of fuel/air mixture. So adjust out =rich adjust in =lean. There may be some exceptions but this is a good rule of thumb. Hope this help. Racer07
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  14. ignatz72

    ignatz72 call me iggy

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    +1

    If on the opposite side, it's called a fuel mixture screw. Most people interchange the lingo.

    But I just replied to racer 07's 666th post, so evil could have me backwards... :D
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  15. btcn

    btcn Long timer

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    These are three great ideas.

    So I took it for a little ride and it overall runs MUCH better with the screws turned the way they are. It lacks a little bit of low end power [below 2,500 RPM], but it runs cooler and idles pretty nicely at about 1,000-1,500 RPM. The hang idle is also a WAY better, it still hangs a little in the high revs, but not enough to be a huge deal and doesn't do it in gear anymore.

    achtung3 thats interesting. I never even thought about that. I do actually have an inline filter for the same reason you did! Wander if this could have to do with anything? I'll actually go and take it off right now just to see if it has any affect! Although it does have an electric fuel pump I'll do it just so I can eliminate it or not.


    achtung3 and ignatz72 that sounds more like what this screw is doing! If it is restricting the air it would make much more sense.

    Do you guys mean closer to the air intake side of the carb or closer to the intake manifold side? I'll go take a pic of it right now and post it and see if you can tell from that.

    Thanks for the ideas!
    #15
  16. anotherguy

    anotherguy unsympathetic

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    In addition to the already mentioned check the float level. If low the pilot circuit will be lean.
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  17. btcn

    btcn Long timer

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    I'll take a look at that today as well.

    I'm gonna go check these things out today.
    #17
  18. batoutoflahonda

    batoutoflahonda Long timer

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    I've seen a lot of people do that on all sorts of carbs. Usually they are masking a cold starting issue. Basically what they did is take it out of the idle circuit, and it was running on the edge of the idle and primary circuit. That's why it would do it some times, and slowly idle down. If you google how the jets/mixture components are all related, you'll see how one circuit works as another comes on line.

    Basically, if one cranks the idle air/mix screws to an overly rich or overly lean point, or even right on the edge of it, it will kick the idle up enough for there to be sufficient air flow to draw off primary circuit. As the bike warms up, the volumetric efficiency improves so it requires less fuel to get the same amount of power, the idle goes up. If you close the throttle, it runs off that extra fuel, until the vacuum on the carb slowly lessons and the amount of fuel getting drawn in slowly decreases, hence the slow to idle down. It also causes that popping back fire (actually after fire if comes out the exhaust, back fire if it pops out of the carb).

    So try not to get too hung up on rich/lean when it comes to mix screws. Think of it more in terms of idle speed. Save the mixture head aches for jetting.

    Sounds like you pretty much fixed it though.
    #18
  19. btcn

    btcn Long timer

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    Ok, I just got done working on the carbs.

    First, turns out that one of the diaphrams was stuck! Therefor, it was almost running on 1 cylinder when you rev it!

    MAN did it make a difference in power! There was NO power below 2,500 RPM before, but now there is a lot like a V Twin should have! Its a different bike now!

    One was sticking partly and one was fully stuck!

    It feels like double the power now! No more backfiring either!

    It idles a little better too. I believe i leaned out the mixture as the screw is near the intake manifolds. But as batoutoflahonda says it seems to have helped or fixed the problem.

    The hang idle is all gone too!

    Now it idles fine, right around 1,000 RPM warm. Is this normal? I'd LIKE to have it just a little lower at around 700 or 800 RPM, but it runs fine now.

    I also couldn't find any leaks in the intake manifolds. I think the carbs were just screwed up.

    Thanks for the help. I'll update this if anything happens.
    #19
  20. scottyb

    scottyb Been here awhile

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    i knew you would figure it out
    #20