Somebody give me a MAP!

The carbureted dirt bike crowd, mainly the 2-stroke, might have one major issue against their ride. J-E-T-S. Jets! Jets! Jets! Oh wait, wrong sport.

Fuel-injected bikes feature an ECU or fuel tuner that allows you to adjust fuel delivery and sometimes ignition timing across different engine conditions. This digital equivalent of rejetting a carb lets you "remap" the fuel and air mixture at the touch of a button to address altitude, temperature and even humidity that affect engine performance.

Need to Rejet Your Carb?
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On a carbureted bike, you can't remap, you have to re-jet. Two strokes, which comprise the majority of carbureted bikes have a pilot jet, main jet and a needle jet. (A carbureted 4-stroke also might include a leak jet which works with an accelerator pump. For bikes with a choke circuit, an optional starter or choke jets help with cold starts.)

Types of Carburetor Jets

The type of Jets in a Carbureted Dirt Bike include:

Pilot Jet - also known as the slow jet - controls low end and initial throttle response, i.e. idle to about twist of the throttle. You need the pilot jet to get started, when idling and riding at low speeds.

Needle Jet - controls mid-range power or to throttle twist.

Main Jet - top end power or full throttle or pinned to win.

A two-stroke with appropriate jet settings for riding dunes at sea level won't function well when riding Thunder Valley MX. Air density drops in higher elevations which means less oxygen for combustion. In other words, less air (oxygen) means less fuel needed. Without proper jet adjustments, this results in a higher fuel to air ratio because the jets deliver the same amount of fuel with less air. Higher fuel to air leads to a sluggish throttle response, bogging and fouled spark plugs from all the burnt fuel.

Alternatively, higher air to fuel ratio happens if you take the dirt bike jetted for riding at Thunder Valley and ride at sea level thereby creating a lean condition. A lean condition causes hard starting, a hanging idle (RPMs rev for several seconds before dropping), bogging, backfiring, inconsistent throttle, engine overheating and the tell-tale sign of white or light gray spark plug.

Whether you have a rich or lean condition, it needs fixing. A fuel injected bike automatically adjusts the fuel to air mixture thanks in part to the MAP, but on a carbureted bike you need to manually adjust the jets or swap out the existing jets for those appropriately sized to the environment.

Follow the general guidelines below to determine the best jetting on your dirt bike:

  • Stick with stock setup for riding up to 3,000 feet in elevation
  • Drop one to two sizes on main jet for 3,000 to 6000 feet
  • Drop two to three sizes on main jet and check clip position for 6000 to 9000 feet
  • Drop three to four sizes and adjust pilot jet for riding elevations 9000 feet or higher

Like with all changes, make small adjustments and work from there.

What!? Clip position? Sounds complicated. Forget it. Gimme a fuel-injected bike! Don't get your throttle all twisted up in knots. The clip has five positions and controls how high or low the needle sits inside the jet affecting the fuel flow. A clip position of 1 (or on top) delivers less fuel and a clip position of 5 (or on the bottom) allows more fuel. So simply by adjusting this clip you can adjust for lean and rich circumstances.

You might only need to adjust the needle depending on the severity of altitude and conditions. Temperature and humidity also contribute to lean and rich fuel to air ratios.

Before rejetting your dirt bike get the following:

How to Jet Your Carb for Altitude

1. Check Current Jetting

Figure out the size of the main jet and pilot jet, and determine needle clip position. Write everything down before making changes. A main jet has a size like #175 which determines the diameter of the hole. The bigger the number the more fuel passes through. Main jet sizes typically range from #120 to #200. A pilot jet uses sizing between from #35 to #55. Adjust needle position with the clips.

2. Drop the Main Jet

When adjusting for altitude adjust the main jet. So, when riding higher altitudes, you want a smaller numbered main jet. Remember, the main jet affects performance when rolling the throttle between to turn. Therefore, if you have a main jet size of 165 and you need to jet for 8000 feet in elevation, try a 160 or 158 jet.

3. Adjust the Needle

If the main jet change doesn't produce the desire outcome, adjust the needle. For example, if the clip position shows 3, move to position 2 which lowers the needle into the jet, allowing less fuel through and leans out the fuel to air mixture.

4. Tweak the Pilot Jet and Air Screw

If idle runs hot and/or low throttle response bogs or otherwise feels off, try a leaner pilot jet and adjust the air screw which sits on the side of the carburetor which affects air to fuel ratio at idle and low throttle. Turn out (loosen) for a leaner mix or turn in (tighten) to get a higher fuel mix. An air screw setting between 1 and 2.5 turns from a fully seated position usually works well at the sea level to 3000-foot level.

You can tell if you got the jetting correct if the experienced symptoms disappear. Expect:

  • Crisp throttle response
  • No bogging with proper power from idle to full throttle roll
  • Brown or tan color on spark plug

If you see the signs of a rich setting (black) or lean setting (white) on the spark plug, adjust as necessary.

Fine tuning the jetting on your dirt bike might go smoothly the first time or require multiple rejetting and adjustments. You may discover at low idle things work great but then go awry at high speeds. Time for adjustments which then fixes the top end speed and mess things up at idle - thus the need for patience.

T-W-E-A-K. Tweak! Tweak! Tweak!

Now you know why most flock to the 4-stroke fuel injected dirt bike.

(Hopefully) helpful tips:

  • If you plan on trail riding in the mountains get a "jet start" before leaving. Change your jetting before leaving.
  • Bring extra jets and the necessary tools especially if your ride day includes varying elevation ranges and / or you expect wide changes in temperature.