ATV Carburetor Adjustment and Tuning
ATV Carburetor Adjustment involves tuning your ATV’s carb for maximum performance. How do I know my jetting is off? My ATV bogs down a lot, is jetting to blame? I have performance parts, what now? These are all common questions so lets get you up to speed on ATV carburetor adjustment.
You can tune your own ATV jets and avoid common jetting mistakes if you understand some jetting basics, explained below. 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 explaining the basic principles of how ATV jets work. How jetting works…
#1 – The first thing you need to remember is that ATV jetting refers to throttle position, 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.
#2 – 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 ATV carburetor adjustment techniques 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
The pilot jet, also referred to as pilot screw, controls carburetor 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 and Needle Jet
The 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
The main jet controls the fuel/air mixture from 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. Make sure to follow the instructions included with most jetting kits.
Steps to tune your ATV Carb In The Field
Now that you, hopefully, understand a bit more about the basics of how ATV carburetor parts work these are the steps to take if you’re in a jam on the trails. Blockage, damage, fuel contamination, mud in the air intake system and other conditions can cause your ATV carburetor to be unable to deliver the right amount of fuel to your ATV engine. You always want to tune your ATV carburetor at home or in the shop but that’s not always possible. If you’re in a jam, this is what you can do.
- #1 – Visually inspect your ATV. Check for any signs of a fuel leak. With the engine off grasp the spark plug wire and make sure it’s securely attached to the engine. Look for signs of obvious damage or dirt in the air intake. Make sure the battery is properly attached.
- #2 – If you have a phone with signal let someone know that you are on the trail and that your ATV is not working properly, tell them where you are. Safety first.
- #3 – If you have the owner’s manual for your ATV follow the carburetor tuning instructions carefully. Every ATV is slightly different.
- #4 – Still stuck? No owner’s manual? No screwdriver? OK. Visually inspect the ATVs carburetor and locate the air/fuel adjustment screw. This is not attaching the carburetor to the engine, it’s going to turn freely if you adjust it.
- #5 – Using a key, or edge of a driver’s license or anything small enough to fit in the carb adjustment screw, carefully turn the screw clockwise(inward) as far as it will go. Do not turn it outwards or it will eventually fall out. Inward to start, all the way in but don’t force it too tightly.
- #6 – With the screw turned all the way in the ATV will be in the lean position, very little fuel will pass. From there turn the screw one quarter turn out and try to start the engine. Optimal settings will typically be 1.5 to 3 turns out on engines in good working order.
- #7 – If it won’t start or sputters and stops turn the screw another quarter turn outwards and try again. You will repeat this process until the ATV, hopefully, starts and runs without stalling.
- #8 – Don’t worry if the engine doesn’t sound well at this point. When it’s running without stalling your air to fuel mixture will be close to what the engine needs to run. You can continue to adjust it a bit until the engine sounds better and runs without hesitation. If you turn it too much you will eventually flood the engine with too much fuel and this will make it bog down and be hard to start as well. A flooded condition will require time for the fuel to evaporate.
- #9 – Get your ATV to the garage or a shop as soon as possible to prevent engine damage. An ATV engine that runs too lean will run hotter and you risk burning a piston or damaging rings. If it runs too rich you risk valve damage and backfiring.
- TIP: Always keep an owner’s manual, a small flashlight and a set of basic tools with your ATV when out on the trails.
- TIP: When planning a trip with your ATV check to see if there is a major elevation change between where you normally ride and where you are going. You may need an ATV carburetor adjustment if there is a change in altitude to account for the change in air pressure. Air pressure decreases at higher altitude(adjust inward to give more air) and it increases at lower altitude(adjust outward to give more fuel).
Carburetor Jet Size Chart
Carburetor jets control the amount of fuel which is mixed with air inside the throttle body of your carburetor. This carburetor jet size chart shows you how effective each jet is at specific throttle levels. A Pilot jet is effective at idle speed up to 1/4 throttle(low-range). A needle jet is effective from 1/4 throttle to 3/4 throttle(mid-range). A main jet is effective from 3/4 throttle up to wide open throttle(top-end).
When diagnosing a carburetor or tuning problem look at the jet responsible for the range in which the problem is occurring.
ALWAYS follow the manufacturer’s specifications whenever possible. Jet size is usually stamped onto the jet itself, typically in milimeters(mm), ranging from small to large. Changing a carb jet for a smaller sized jet will reduce fuel flow. Likewise, installing a larger jet will increase fuel flow and may help a performance engine run more smoothly. Note: an engine that runs rich is less likely to sustain heat damage than and engine that runs lean.
I hope that this carburetor tuning guide helps you understand how jets work so that you can best dial in your ATV for maximum performance(and fun!).
General ATV Carburetor Cleaning Instructions
- Put on some safety glasses and prepare a clean workspace
- Remove carburetor from ATV and clean exterior with a brush and soapy water
- Remove float bowl taking care not to damage it’s gasket
- Remove float pin and float assembly and check for wear and damage
- Remove needle jet and inspect for blockage and damage
- Remove Pilot and main jet and inspect for blockage and damage
- Important: Turn the fuel air mixture screw all the way in and count the number of turns until it is fully tightened. You will need this information to adjust the mixture upon re-assembly.
- Remove the fuel air mixture screw
- Remove the throttle adjustment screw
- Thoroughly clean all components and inspect for obstructions and damage. Note: compressed air is extremely helpful in clearing obstructions.
- Replace any damaged components, seals and gaskets
- Re-assemble the ATV carburetor in reverse order
- Tip: lay parts out in the order the were removed to make assembly easier
- Adjust the fuel to air ratio by turning the mixture screw the number of turns you recorded earlier
- Re-install the carburetor to your ATV and start the engine. Additional adjusting may be required.
Conclusion
Carburetor jets become clogged naturally over time and a periodic carburetor cleaning is required. Always consult with your ATV owner’s manual for specific maintenance instructions. Always check the fuel lines, filter and gas tank for contaminants and clean as required at the same time to avoid further clogging.
The majority of carburetor problems are a result of blockage and worn out parts. Troubleshooting a clean ATV carburetor is also much easier than troubleshooting a dirty carb. Typical tools required include a flat head screw-driver, compressed air, an 8mm wrench and carburetor cleaner. Note: Some gaskets and plastic components will swell if exposed to carb cleaner so remove these prior to use.