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.

  

  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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