How to Tune a Harley-Davidson CV Carburetor for Peak Performance

The Keihin CV (Constant Velocity) carburetor was Harley-Davidson's carb of choice through the Evolution era and into the early Twin Cam years, finally replaced by electronic fuel injection in 2007. It's a well-designed unit — far better than the butterfly carbs it replaced — but it requires correct jetting and setup to perform properly, especially when combined with aftermarket air cleaners or exhaust modifications.

How the CV Carb Works

Unlike a slide carburetor where the throttle directly controls airflow, the CV carb uses a vacuum-operated slide. When you open the throttle plate, engine vacuum lifts the slide (and its attached needle) automatically, proportional to engine demand. This gives smooth, surge-free throttle response across a wide RPM range — which is why Harley stuck with it for so long.

The carb has four distinct fuel circuits that operate at different throttle positions:

• Pilot circuit (0–25% throttle): Idle and off-idle fueling. Controlled by the pilot jet and pilot screw.
• Needle circuit (25–75% throttle): Mid-range fueling. Controlled by needle taper and clip position.
• Main jet (75–100% throttle): Wide-open throttle fueling. Controlled by main jet size.
• Accelerator pump: Delivers a shot of fuel on quick throttle opening to prevent hesitation.

Diagnosing Jetting Problems

Running Lean (Too Little Fuel)

• Popping and spitting on deceleration
• Surging at steady throttle
• Engine runs hot (exhaust glows cherry-red at night)
• Hesitation off idle

Running Rich (Too Much Fuel)

• Black smoke from exhaust
• Fouled spark plugs (black, sooty deposits)
• Fuel smell from exhaust at idle
• Poor fuel economy
• Sluggish throttle response

Pilot Jet and Pilot Screw

The pilot jet controls idle and low-throttle fueling. The stock Harley pilot jet is typically a #42 for carbureted Evolution engines. If you've added a freer-flowing air cleaner, step up to a #45 pilot jet.

The pilot screw (also called the fuel screw) is a mixture screw located under a plug in the bottom of the carb body — Harley caps it from the factory. Drill out the plug to access it. Starting point is 2.5 turns out from lightly seated. Turn out (counterclockwise) for richer idle mixture, in for leaner.

To find optimal pilot screw setting:

1. Warm the engine to full operating temperature
2. Set idle speed to around 900–1000 RPM
3. Slowly turn the pilot screw out until idle smooths and RPM rises slightly
4. Back idle speed down with the idle screw
5. Repeat until further adjustment doesn't improve idle quality

Needle Position

The needle in the CV slide has a series of grooves (typically 5 positions). The clip that holds the needle in the slide determines the effective needle height:

• Top groove (clip up): Needle drops lower, leaner mid-range
• Middle groove: Stock position
• Bottom groove (clip down): Needle raised, richer mid-range

For most Stage 1 setups (open air cleaner, slip-on exhaust), moving the needle clip to the fourth groove from the top (one below middle) is the typical starting point. This richens the mid-range where most modified Harleys run lean.

Main Jet Sizing

Main jet only affects wide-open throttle. For a stock bike with stock air cleaner, the main jet is usually adequate from the factory (#168 or similar). After air cleaner and exhaust upgrades:

• Stage 1 (open air cleaner, slip-ons): step up to #175–#185 main
• Stage 2 (cams added): #185–#200 main, depending on cam selection

Spark plug reading is the definitive way to check main jetting. After a hard wide-open run, immediately kill the engine and check the plug: tan/light brown = correct; white/grey = too lean; black/sooty = too rich.

CV Carb Rebuild

Over time, the CV carb's internal parts wear. Ethanol in Canadian pump fuel attacks the rubber diaphragm, accelerator pump nozzle, and float needle seat. If your carb has been sitting with fuel in it over a Nova Scotia winter, rebuild it before the riding season.

A CV rebuild kit includes: needle valve and seat, diaphragm, float bowl o-ring, accelerator pump parts, and all jets. Costs under $50 in parts. The procedure takes 45–60 minutes with clean hands and a clean workspace.

Carburetor tuning is as much art as science. If you're chasing an elusive miss, surge, or stumble on your carbureted Harley, bring it to our Pictou County shop. We tune these carbs every week and know the symptoms inside out.