ATV Jetting basics, tuning your ATVs carb for maximum performance.

How do I know my jetting is off ? My ATV bogs down a lot, is jetting to blame ? I’ve added ATV performance parts, what now ?

Jetting doesn’t have to be a mystery, you can tune your own ATV if you understand some jetting basics. A comprehensive installation guide will come with the jetting kit you purchase for your make and model of ATV so instead this guide will focus on jetting basics. Here’s how jetting works…

 The first thing you need to know is that jetting refers to throttle position and not engine rpm. Each jet is effective for a specific throttle position range. The pilot jet or pilot screw as it is sometimes referred to controls the flow of fuel between Idle speed and about one eight of wide open throttle (WOT). The needle jet controls the flow of fuel from the one eight position up to about the three quarter throttle position. Lastly your main jet controls the flow of fuel between the three quarter and wide open throttle positions.

The second thing you need to know is that aftermarket performance parts will change your jetting requirements. Increased engine performance usually means an increased demand for fuel so learning to jet your ATV properly is important.

Other factors that affect jetting are rev limiters, altitude and fouled plugs if your jetting is off. Altitude directly affects engine performance because the higher you go from sea level the thinner the air gets.

Pilot Jet – Pilot Screw: Controls the fuel/air mixture between Idle and 1/4 throttle. Turning the screw IN makes the fuel/air mixture leaner. Turning the screw OUT makes the fuel/air mixture richer. Lean means less gas, rich means more gas.

Every jet is identified by a number and that number relates directly to the size of the opening inside the jet. Again smaller is leaner, larger is richer. When jetting your carb if you find that you need to turn the pilot jet all the way in to improve response then you likely need to switch to a smaller number of screw.

If on the other hand you need to turn it more than 2.5 turns out you likely need a larger numbered screw. When your engine bogs down at the smallest increase in throttle position your pilot jetting is likely too rich.

Needle Jet and its Needle: Controls the  fuel/air mixture from the 1/8th throttle position all the way up to the 3/4 throttle position. The needle within the needle jet is a long tapered pin. As you increase the throttle position the pin pulls out of its jet allowing the fuel/air mixture to become richer. There are several different shapes and diameter of needles/needle jets but the needle jet is often left alone during tuning as it doesn’t affect idle or top end performance. The same rich/lean characteristics apply as with the pilot screw.

Main Jet controls the fuel/air mixture from the 3/4 throttle position up to wide open throttle. Again the main jet is numbered and a larger number indicates a larger hole and a richer mixture. A stock engine will perform well with a stock main jet but once engine performance parts are added its likely that you will need to revisit at least the main jet of your carburetor. Different performance part combinations will create different demands from your carburetor. Anything that increases horsepower will also increase fuel demand.

All the performance parts in the world won’t improve performance if you don’t tune your carb. If you have a rev limiter in place you may need to tune or upgrade that as well because a rev limiter will tend to bog down the engine at full throttle as well. Make sure to follow the instructions included with most jetting kits.

Leave a comment if you have questions or other tips related to ATV carb jetting.

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