Lazy / Slow Return to Idle (or Erratically High Idle)

You might consider Randakk’s GL1000 Carb Rebuild Video

Are you plagued with a bike that runs great from about 2000 RPMs and up, but refuses to idle reliably below that speed? Does it sometimes idle OK, but often “hang-up” at a higher speed and take forever to return to idle? Do you have to put a small load on the engine (by letting out the clutch a bit when stopped and in gear) to get the engine to return to idle speed? Does the idle speed seem overly sensitive to changes in ambient and operating temperature? If so, read on.

Note: Ignition problems such as burned or poorly adjusted points, incorrect timing, “sticky” ignition advance mechanism or fatigued ignition advance springs can create similar symptoms. Always perfect ignition settings before making any carb changes!

For example, a weak battery or faulty charging system will absolutely compromise the ability to support a stable, reliable idle speed!

Assuming you haven’t made any carb rebuilding mistakes (which is all too common), and you’ve eliminated ignition issues, the normal suspects for a lazy, high idle are: Too much ethanol in the fuel causing an over-lean condition. E10 is acceptable. E15 is marginal and not recommended. Non-ethanol fuel is always preferred for these machines. Be aware the “gasoline” sold in America is routinely “out of spec” with regard to the actual vs. advertised ethanol content! Too much ethanol in the fuel causing an over-lean condition. E10 is acceptable. E15 is marginal and not recommended. Non-ethanol fuel is always preferred for these machines. Be aware the “gasoline” sold in America is routinely “out of spec” with regard to the actual vs. advertised ethanol content!

  • Too much ethanol in the fuel causing an over-lean condition. E10 is acceptable. E15 is marginal and not recommended. Non-ethanol fuel is always preferred for these machines. Be aware the “gasoline” sold in America is routinely “out of spec” with regard to the actual vs. advertised ethanol content!
  • Poor / uneven compression values
  • Incorrect valve lash (especially valve clearances that are too “tight”)
  • Improper setting of the curb idle speed. This should be done with an accurate shop-grade tach since the tachs on these bikes are notoriously inaccurate!  Curb idle should be checked and re-set on a fully warmed engine. “Fully warmed” takes 15-20 minutes of running in typical weather.  It is normal for curb idle to increase several hundred RPMs as full operating temperature is achieved.  Caution:  a “good” idle speed when half warmed up will be too high when fully warmed up, so you must accept the inevitable requirement for some “throttle nursing” until the bike is fully warm.
  • Poor carb synch
  • Vacuum leak
  • “False Air”…wry description of extra, unwelcome air entering the carbs inappropriately. Typically, this will occur around the throttle butterfly shafts in worn out carbs or carbs that have been abused by immersion in carb cleaner thereby ruining the felt washers that seal and lubricate these shafts.
  • Missing throttle shaft end plugs (on the front of carbs #1 and #2 ) …location is shown in the picture below. These sealing plugs are important. They should be in place to prevent the entry of “false air.” They were intended by Keihin to be a non-serviceable, permanent installation. The drilled holes behind them are necessary for several reasons that relate to manufacturing requirements and the fact that these carb bodies were intended for many model applications (beyond the GL1000). Again, these holes must be closed off. There is no known source for these plugs as they were never available as a separate part number. Replacements must be robbed from other carbs or fabricated.

    GL1000 Throttle Shaft Plugs

    Location of GL1000 Throttle Shaft End Plugs

  • Improper re-mating of carb synchro links. There should be a washer on either side of the synchro link which joins the throttle butterfly shafts of each carb pair on the left (2/4) and right (1/3) sides. Ditto for the left carb pair to right carb pair balancer.
  • Linkage or throttle cables binding or not enough freeplay
  • Throttle shafts binding
  • Worn or damaged linkage components
  • Loose or damaged rivets on bell-crank
  • Choke not releasing completely
  • Dirty / blocked idle circuits in carb
  • “Sticky” CV slides
  • Overly lean for any number of reasons
  • Aftermarket exhaust
  • Aftermarket air filter
  • Low fuel pressure due to defective fuel pump
  • Incorrect idle air jets (orifice over-sized or missing altogether)
  • Problem with air cutoff valve or its vacuum hoses (this usually causes a back-fire)
  • Mismatched float heights or unequal fuel levels in bowls due to other causes:
    • Floats misaligned
    • Floats “fouling” on adjacent structures
    • Floats binding on pivots
    • Uneven float weights
    • Aftermarket needle/seat assemblies

    See: Floats 101 for more information on correctly setting floats

Poor carb sync is the most common reason for your complaint. The reason for this is that when the carbs aren’t synched well, one or more cylinders have to compensate for the underperforming ones (at idle). This gets the harder working cylinders into the progression of fuel circuits (at idle) to maintain proper curb idle. This creates a problem in coasting situations because there is too much fuel metered out. It only takes a small extra amount of fuel in a low/no-load situation to cause an unnaturally high idle. To make matters worse, this usually activates the centrifugal ignition advancer and you get wild oscillations in idle speed.

However, it’s not well understood that the synch mechanisms incorporate spring-loaded connections in numerous spots. If there is ANY throttle binding, these spring-loaded joints will move and corrupt even perfect synchronization.

A related and common cause is blocked idle circuits. Again, one or more cylinders have to compensate for the one(s) not contributing at idle. You then have to screw the curb idle screw into the point that the progression circuits are activated. Havoc ensues.

The idle circuitry involves ALL of these components working correctly:

  • #35 idle fuel jet (under the rubber plug between the primary and secondary main fuel jet towers). This is a VERY small orifice and easily compromised by debris.
  • #110 or #115 idle air jet (external – in the brass elbow) This should be modified to fix the infamous “off idle” glitch.
  • 3 idle fuel transfer ports located under the “puck.”
  • Idle mixture screw properly adjusted (setting varies by year …consult your manual).
  • Idle discharge port jets (aka pilot fuel nozzle …the small brass tubes about 1/2″ long that enters the main venturi near the throttle butterflies). I am referring to these.
  • Myriad fuel and air passages in the plenum and individual carb bodies.
  • Air cutoff valve (prevents backfire on overun).
  • Float bowl gasket in good condition (really!).
  • Float bowl true and not damaged or warped.
  • Clear fuel inlet screens (under each float valve).
  • Proper float settings.
  • Well-functioning float valves.
  • Curb idle setting screw.
  • Clean, filtered, fresh fuel.
  • Proper fuel pump pressure.
  • Clean air filter.
  • OEM muffler (aftermarket mufflers can disrupt idle performance if back pressure is significantly more or less than OEM muffler).
  • Fully disengaged choke linkage / butterflies.

Even though you may have performed a carb sync., it doesn’t mean you necessarily got a good one. GL1000s are very sensitive to carb sync. Ditto for float height!

Unfortunately, these variables are all inter-related. Everything has to be on-spec for proper performance. The idle circuits in a GL1000 are ridiculously small, so slight problems are greatly magnified.

To summarize, the solutions for a lazy / slow return to idle (or erratically high idle) are:

  • Eliminate vacuum leaks
  • Perfect ignition
  • Perfect idle circuits – usually requires a thorough carb overhaul, but this is worth trying this in situ carb cleaning method first.
  • Perfect float settings
  • Perfect synchronization

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15 thoughts on “Lazy / Slow Return to Idle (or Erratically High Idle)

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  1. Howdy! Im a young guy who is just starting in the world of GL1000s. I have a 76GL1000 that will fire right up when peacock is off and Idle at about 3000 rpms. This stops however when I turn the petcock on to where the cylinders begin to flood. I have done a complete rebuild in the carbs setting the brand new floats to spec as well as the adjustment screws. Any advice?

  2. I’ve got a 75 wing that I can’t get to idle quite right. It sounds clunky at idle. I’ve gone through the carbs and made sure the idle passages are clear, set float height, changed ignition coils, replaced points, adjusted set timing, replaced timing belts, adjusted valves, polished slides, new battery… but still sounds clunky at idle and occasionally surges to 2500 rpm before it falls back to 1000 rpm. Any tips?

  3. New fuel pump fixed the problem for me. I had eliminated vaccuum leaks, had carbs synched, adjusted idle speed, but still had problems. Then I replaced the fuel pump and the problem was gone.

  4. Hey, hi im from Bartlett, il and I ordered a rebuild kit and filter for my GL 1500. I wasnt aware there were 2 different sized fuel filters and you sent me both sizes! Unheard of in this day and age and shocked the heck outta me! Plus a hand written note addressed to me man im impressed. Told my wife and all my gearhead friends. Very cool and appreciated i want you to know. Speaks volumes to your character and integrity which is lacking today. I havent had time to
    look at bike yet, been busy with other things being a family man but promise to make good on your thoughtfulness and either purchase it or return it.

    Thanx so much, Gary

  5. Have been struggling with slow return to idle a long time while everything listed above seemed OK. Finally slighlty lowered idle speed and the problem was solved. Any explanation?

    1. Explanations include:

      1. Curb simply idle set too high …perhaps using an inaccurate tachometer (like the one on the bike!)

      2. Very poor sync

  6. I have been using your carb kits for the GL1000 for a long time and have restored several GW’s. I have run across a problem for the first time since I have been repairing GW’s. I went to Honda school in Gardena, Ca. back in 1975 for the GL1000 when it first came out. After all these years I have found a problem I have never seen before. After I rebuild a set of carbs (many times over the years) I give them a test by shooting air into the venture to make sure the slides “lift” properly. This set of 1977 carbs have two carbs that the slides don’t lift. If I take out the screws that hold the vac-piston cover on, the air will lift the cover. You probably already know why. Please tell me so I can finish my rebuild. Thanks for your excellent products, they’re great, Jack

    1. These possibilities come to mind:

      1. Leak in cap “buttons”
      2. Dirty inner “bores” in caps
      3. Dirty “piston” aspect of slides
      4. Physical damage to piston and/or caps …like a slightly bent piston aspect that is causing “binding.”

      Hope this helps.

  7. last year when I got my 1979 GL1000 running , I was trying to sync the carbs and found that one of the carbs had double the vacuum than the other carbs I was able to sync the other 3 but that one is still double the vac. got any idea what could be wrong before I disassemble and rebuild all 4 –I did disassemble and clean them before I tried to sync them ( this bike sat of 20 years in a barn before I got it) .

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