Harley-Davidson Motorcycles are synonymous with American biker culture.
And because HD manufactures parts for bikes made in the 50s, there might be just as many carbureted Harleys on the road as fuel-injected models.
Both carburetor and fuel-injected Harleys develop idling issues just like any other moto; this article explains the most common reasons why a Harley-Davidson motorcycle won’t idle.
1. Faulty Idle Air Control Valves
Plenty of Harley-Davidsons equip an idle air control valve that governs the amount of air the throttle plate lets slip through the throttle body.
The amount of air allowed through affects your HD’s idle speed, so if your HD’s idle air control valve wears or goes out of adjustment and too little or too much air gets in, your Harley will idle at inconsistent RPMs and eventually may fail to idle altogether.
The most common reasons why Harley’s idle air control valves go faulty are:
- Improper idle speed.
- Wear from heavy use.
- Failure to conduct routine inspections and tune-ups.
That said, there are other situations, such as the one exemplified in the following conversation between two real-life HD riders:
“I have an ‘04 [Harley-Davidson] Road Glide… I have a problem with the bike idling too low at all temperatures. I pulled out the idle air control valve, and it is not cycling with the ignition turned on or when the bike is running.”
“Check the fuses under the right and left side covers. Use a multimeter, not the eyeball method.
Also, check the wires to the IAC connector; there is a history of the wires separating inside their insulation due to being tied strapped to the frame too tight.”
- The idle air control valve is responsible for sustaining the bike’s idle against variables in battery current, air quality, and shifts in climate.
- If a faulty idle air control valve is why your Harley won’t idle, you’ll notice it happens more at cold starts and when running extra electrical accessories.
2. Faulty Carburetor
Most Harley-Davidson motorcycles built before 2007 were fuel-injected.
Although Harley fitted bikes with fuel injection as far back as 1995, the traditionalism of the brand kept carbs on many of their bikes for another eight years.
Carburetors require more upkeep than fuel injection systems do. Your HD’s fuel supply is compromised if your carb is corroded, clogged, or damaged. An improper air-fuel ratio can make it so your Harley-Davidson motorcycle won’t idle.
“I have a 96 [Harley-Davidson] XLH 883 [Sportster], and my bike won’t stay running unless it’s choked. I took it all apart and cleaned everything in it, the jets and all. It still won’t run. When it’s idling, it sputters and blows out black smoke.”
“Check the vacuum line to the carb to make sure it is connected. Check the carb and manifold seals for a good seal. If you get the bike to start with the choke, adjust your idle speed screw to keep the cycle running without the choke. Adjust the idle mixture screw for smooth idle and readjust the idle speed screw to the correct rpm at idle – 950-1100 rpm.”
If a clogged or corroded carburetor is why your HD cruiser won’t idle, you may notice that your Harley only idles like normal with the choke pulled out. The good news is that a carb rebuild is easy at home with the right tools and even easier for an HD-literate mechanic.
“I have a 2002 [Harley-Davidson Sportster] 1200 custom that just got handed down. My father kept it in the garage for the winter. I went to start. It won’t stay idling unless you hold the choke out or give it some throttle. Also, when you ride, there is a hesitation where the bike kind of jerks like it loses fuel supply forms a split second.
“The carb probably needs a good cleaning. If you have to leave the choke pulled out for it to run, it’s either an intake leak or a clogged jet. With the bike running, spray some wd40 around the intake seals and listen for changes in the RPMs.”
3. Blocked Fuel Injectors
While fuel-injected Harleys are easier to keep up with, FI (fuel injection) systems still require inspection and occasional services to keep your fuel flowing and your bike idling as it should.
Suppose your Harley-Davidson’s fuel injectors are blocked. In that case, your combustion chamber won’t get the proper amount of fuel required for combustion, and you’ll notice it’s idle running rougher and rougher and coughing until, eventually, your HD won’t idle at all.
- An ECU Electronic Computer Unit regulates the injection system on modern Harley.
- When your HD is idling correctly, the ECU communicates with sensors monitoring the various components in your spark, air, and fuel systems.
- Based on the data collected by these sensors, your ECU adjusts the fuel injection and ignition as needed to keep your bike running at ideal RPMs.
If your Harley sits for an extended period, its fuel breaks down and gets contaminated by moisture.
If you run bad fuel through your HD’s injector systems, the injectors can get blocked by the mineral deposits, and your Harley-Davidson cruiser won’t idle.
Related: Harley Davidson Won’t Shift From 2nd To 3rd Gear? (Answered)
4. Faulty Spark Plugs
Damaged or worn-out spark plugs don’t generate the ignition required to combust your Harley’s fuel mixture. The erratic spark generation caused by old spark plugs can cause your Harley-Davidson motorcycle’s RPMS to flutter until it eventually won’t idle.
- The spark plugs are the small screw-in components responsible for commencing your HAlrey’s ignition process.
- Without evenly firing spark plugs generating consistent surges, the air-fuel mix will burn unevenly inside the combustion chamber, causing fluctuations in your HD’s idle speed.
- All spark plugs incur damage from receiving repetitive blasts of electricity, and all spark plugs burn out eventually.
“2001 [Harley-Davidson] FLSTFI 1450cc w/ 25,000 miles. I was riding home from work when the engine started running rough, coughing and sputtering. I tried nursing it home, but it would not idle without sputtering. I’ve owned this bike since new, and this is the first time it’s ever given me any trouble.
Check for codes. Check for intake leaks, [and if the spark] plugs look normal when you take them out.”
Inspecting your spark plugs during routine service intervals will catch a worn spark plug early before it hurts your Harley’s idling.
5. Worn Ignition Coil, Coil Packs, or Spark Plug Wires
Like every other part of your Harley, your spark plugs are powered by a dynamic process that includes ignition coils and packs to transfer voltage.
Furthermore, your Harley-Davidson’s spark plugs hook onto spark plug wires that relay current to the plugs.
As we explained above, the constant power surge puts wear on the spark plugs and zaps the coils and wires the plugs are integrated into.
Storing or riding your Harley in corrosive conditions accelerates the wear of your ignition coils and wiring.
If not caught in time, the wear worsens until it sheds so much current that the surge can’t clear the spark plug cap, and your Harley-Davidson motorcycle won’t idle.
- If a work ignition coil, pack, or spark plug wire is why your HD won’t idle, you’ll experience a dip in engine performance beforehand, possibly including frequent misfires.
- Since these components all wear at different rates, the spark production and your HD’s ignition will err, causing rough idling and fluctuating RPMs.
- Eventually, the idle gets so inconsistent that your Harley won’t idle.
6. Clogged Air Filter
Your Harley-Davidson’s air intake system starts at the air box responsible for sucking air into the bike’s manifold network. If left unchecked, a clogged air filter can be enough to cause a Harley-Davidson motorcycle not to idle.
- Your Harley’s airbox includes a filter to prevent dirt, debris, and moisture from entering your bike’s engine.
- Most HDs are equipped with paper air filters that need to be replaced according to the service intervals outlined in its manual.
- An unkept air filter is quickly saturated with contaminations, preventing the appropriate amount of air from entering your Harley’s engine.
- Other models use reusable filters that can be cleaned by hand and reused.
Regardless, impaired air intake capacity allows extra fuel to enter the space left in your combustion chamber by the lack of air, causing the engine to run rich on fuel, overheat, and backfire, hurting your Harley’s idle.
7. Air Intake Leak
A leak in your Harley-Davidson’s air intake system allows air to escape, creating an air-fuel mixture rich in fuel and short on air. If left unchecked, air leaks can become severe enough to make it so your Harley won’t idle.
Depending on where the vacuum leak develops in your Harley’s intake system, it could be letting air escape from the ignition and idling systems, where the air is required for healthy operation.
Furthermore, an internal leak could be letting air into places it doesn’t belong, allowing it to interfere with other systems’ functioning.
Here are some of the most common places vacuum leaks form on Harley-Davidson motorcycles:
- Faulty Air Intake Boots
- Faulty carburetor gaskets
- Exhaust headers
- Air filters
- Broken or worn seals in your air, fuel, or ignition systems.
Related: Harley Davidson Won’t Start? (Solved & Explained)
8. Clogged Fuel Filter
The fuel filter on multiple Harley-Davidson models is in the bike’s fuel tank, which sucks in fuel, removing moisture, rust, or mineral contaminants from your Harley’s fuel supply.
- The fuel travels through the filter and into the injection system, which shoots it into the chamber to mix with the air and combust.
- A clogged fuel filter hinders fuel flow from your HD’s fuel tank into the engine, permitting extra air to enter the combustion chamber and fill the space for the lack of fuel.
Excess air in the chamber causes your Harley’s engine to run rough, backfire and misfire and hurts your HD’s idle speed until your bike might not idle at all.
9. Cam Chain Out of Sync
If your Harley-Davidsons’ valve timing falls out of sync due to a faulty cam chain, your engine compression is reduced, and your Harley won’t idle properly.
- To keep engine time, older Harley-Davidson motorcycles used cam gears, while newer models equipped a part called a cam chain.
- Like any mechanical chain, Cam chains must be adjusted occasionally, as the material they’re constructed from stretches over time and expends/retracts according to different temperatures.
- Furthermore, the cam chain eventually wears out from everyday use, losing its ability to retract to its spec tension.
If you ride your motorcycle with a worn-out cam chain, the engine timing gets thrown off, affecting your Harley’s performance and causing idle problems.
On the other hand, your HD’s engine and idle timing can get thrown off via incorrectly aligning or installing your new cam chain when replacing the worn chain.
Incorrect engine timing causes frequent stall outs and engine failures, leading to a Harley-Davidson that won’t idle.
10. Faulty Ignition Timing
Some Harley-Davidson motorcycles have their ignition sequence timing, and the bike’s ECU adjusts air-fuel flow automatically.
Some of these Harleys even stock monitor the crank and cam chain position via sensors, temporarily delaying or advancing your HD’s ignition timing to compensate for angle shifts.
Suppose either the position sensor or the HD ECU fails or incurs damage from moisture, blown fuses, or wire fraying. In that case, the units can relay incorrect data to one another, causing the ignition timing to go out of sync while compensating for a change in the cam’s position that never actually took place.
A faulty ECU reading results in poor idling, dips in engine performance, frequent stall outs, starting problems, and a Harley-Davidson motorcycle that won’t idle.
Faulty sensors typically need to be replaced, although in many cases resoldering the wiring may do the trick, at least as a temporary fix.
In some cases, ECUs need to have their software updated by a Harley dealership technician, while other times, the unit needs to be replaced.