How to Bypass a CDI Box on an ATV – Easy Guide

Last Updated on November 2, 2023 by Jhonni Jets

All-terrain vehicles (ATVs) are popular for recreational riding and utility purposes. Many newer ATV models come equipped with a CDI (capacitor discharge ignition) box to control the ignition timing and improve performance. However, some riders want to bypass or remove the CDI box to simplify wiring or get a raw power delivery.

This guide will walk through the steps for bypassing a CDI box on an ATV. We’ll cover:

• How a CDI box works and why you might want to bypass it
• The tools and supplies needed
• Wiring diagrams for connecting ignition directly
• Setting static ignition timing
• Rejetting the carburetor
• Pros and cons of bypassing CDI

While removing the CDI box can provide some benefits, it also requires adjustments to maximize performance and prevent engine damage.

Understanding CDI Box Operation

First, let’s look at what a CDI box does:

• Controls ignition timing based on RPM signal
• Provides high voltage to ignition coil
• Smooths power delivery for easier starting
• Allows adjusting ignition curve for performance

The CDI takes input from the flywheel pickup coil to determine engine speed. It uses this information to adjust spark timing for smooth operation across the RPM range. Removing the CDI means the timing will be fixed, not variable.

On most ATVs, the CDI box is located under the seat or by the front left fender. It has wiring connecting it to the ignition coil, pickup coil, and kill switch.

Reasons for Bypassing the CDI Box

Here are some common reasons riders choose to bypass their ATV’s CDI box:

• Simplify wiring and eliminate potential failing component
• Remove rev limiter effect for higher top speed
• Achieve fuller power band and torque
• Address issue of damaged/malfunctioning CDI
• Gain more tuning and jetting flexibility

Bypassing essentially converts the ignition to a more basic, raw configuration. This can be beneficial for some riders but also has downsides to consider.

Tools and Supplies Needed

To properly bypass and remove the CDI box, you’ll need the following tools:

• Wiring cutters/strippers
• Needle nose pliers
• Voltmeter
• Soldering iron and solder
• Zip ties for securing wires
• Timing light

Supplies:

• Magneto puller tool
• Replacement spark plugs
• Carburetor jet kit
• Dielectric grease for connectors

Having the right tools will make the wiring and timing adjustment steps much easier when bypassing the CDI.

Wiring Diagrams for Bypassing CDI Box

The key steps for bypassing will be:

1. Disconnect CDI box from wiring harness
2. Connect ignition coil directly to pickup coil
3. Ground kill switch wire

Refer to your ATV’s service manual for the exact wiring schematic and connector types. But in general, you’ll be splicing the pickup coil and kill switch wires to the ignition coil directly.

Here is a typical wiring diagram for a basic CDI-less ignition system:

[Simple wiring diagram depicting pickup coil splice to ignition coil, grounded kill switch]

The pickup coil provides the timing signal, the ignition coil produces spark, and the kill switch shuts off ignition when needed.

Setting Static Ignition Timing

Without the CDI box, the ignition timing will be fixed and not adjust with RPM. Timing is adjusted by rotating the magneto on the flywheel. Here are the steps for setting static timing:

1. Remove magneto cover and spark plug cap.
2. Attach timing light to spark plug and ground it.
3. Rotate magneto counter-clockwise to advance, clockwise to retard.
4. Loosen magneto bolts and rotate while watching timing light at idle RPM.
5. Set timing between 12-15° before top dead center.
6. Tighten magneto bolts and recheck timing with revving engine.

Getting the timing in the ideal range will provide maximum engine performance. Too much advance can cause pre-ignition issues.

Rejetting the Carburetor

Removing the CDI box changes the ignition timing curve and power characteristics. You’ll need to rejet the carburetor for proper air/fuel mix.

Follow these steps for rejetting:

• Install needles, pilot jets, and main jets from carb jet kit
• Adjust air/fuel screw increments of 1/4 turns
• Use plug reading charts to identify optimal color
• Perform plug chops at high and low RPMs
• Change jets based on readings to lean/richen mixture

Proper jetting prevents burning or seizing from too lean a mix. Expect to do trial testing to find the ideal jetting.

Pros and Cons of Bypassing CDI Box

Here are some key benefits and drawbacks to weigh:

Pros

• Simpler, less wiring
• No rev limiter for more power
• Increased tuning flexibility
• Eliminates potential CDI failure

Cons

• Loss of variable ignition timing
• Harder cold starting
• More tuning necessary
• Potential engine damage if not adjusted properly

Overall, bypassing the CDI box can provide a small performance gain and simplicity for riders. But it requires more knowledge and adjustments to avoid decreased reliability or damage.

Closing Thoughts

Bypassing or removing the CDI box on your ATV can provide some benefits but also has risks. Make sure to properly rewire the ignition, set static timing, and rejet the carburetor if choosing to bypass.

Fully consider whether the small power gain is worth losing variable timing and easier starting. Apply the steps in this guide carefully or consider consulting a professional mechanic before bypassing the CDI on your ATV.