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Chronic MX Tech tip: How to adjust the jetting fuel screw on a four stroke dirt bike

fuel_screw

If you own an older four-stroke, and are not lucky enough to have the latest fuel injector technology, then it’s critical that you familiarize yourself with how to adjust the fuel screw on your dirt bike . It is the key to living a long and happy life with your beloved thumper. If you don’t know anything about your fuel screw, you are in luck. We are about to teach you all you need to know in these ten easy lessons….followed by an informative video.

NUMBER ONE: WHAT DO AIR AND FUEL SCREWS DO?
An air screw (on a two-stroke) and a fuel screw (on a four-stroke) are like the nostrils on a race horse. They can be adjusted to control how the incoming air and fuel are mixed. When the carburetor’s slide is closed to a quarter throttle or less, it doesn’t atomize fuel effectively. For these quarter throttle or less settings, a carburetor uses bypasses to efficiently meter fuel to the engine. These passages channel air into the pilot circuit (where it mixes with gasoline). This circuit controls how the bike runs from idle to the midrange. Both the air screw and fuel screw are tapered needles that effect the mixture of gasoline and air. Both are adjusted with a screwdriver.

NUMBER TWO: WHAT IS THE DIFFERENCE BETWEEN AN AIR SCREW AND A FUEL SCREW?
There are two ways to control the amount of fuel and air metered by the pilot circuit, and they differ by engine type.
Air screw (two-stroke): The externally adjusted air screw controls the amount of air that is fed into the pilot circuit. It is adjusted by turning it with a screwdriver.
Fuel screw (four-stroke): This externally adjusted needle meters how much of the fuel and air that was mixed by the pilot circuit flows into the engine. Since a fuel screw is hidden under the carb, an adjustable fuel screw can make it easier to change.

NUMBER THREE: WHY DO FOUR-STROKES HAVE FUEL SCREWS INSTEAD OF AIR SCREWS?
At low rpm, four-strokes create less engine vacuum than two-strokes. It is the vacuum, created by the engine, that sucks fuel through the pilot and out of the carburetor float bowl. Carburetors with fuel mixture screws flow more fuel and have more finite tuning in low vacuum situations. They are, thus, more applicable to four-stroke engines.

NUMBER FOUR: IS IT RICHER IN OR OUT?
A four-stroke’s fuel mixture screw meters gas after it has been mixed with air. Turning it out lets more fuel into the engine and richens the mixture (just the opposite of the air screw). Screwing the fuel mixture screw in leans the jetting. Do you get the reverse parallel?

NUMBER FIVE: HOW DO YOU ADJUST A FUEL MIXTURE SCREW?
Set the bike to a high idle. Turn the fuel mixture screw in until the engine almost dies. Count the turns out until the engine rpm peaks. If you turn the fuel mixture screw too far out, the exhaust note will become dull and lumpy.

NUMBER SIX: WHAT DOES LUMPY AND DULL SOUND LIKE?
It is the opposite of crisp. When engine rpm peaks, the exhaust note is sharp and crisp, but if you go too far, the exhaust note will become choked and will lose that snappy sound.

NUMBER SEVEN: DOES A FUEL SCREW ADJUSTMENT EVER CHANGE?
Constantly. In morning practice, the air is cool. With more oxygen available in cold air, a fuel screw would probably work best at a quarter turn richer. Remember, richer is a 1/4 turn out on the fuel mixture screw. As the sun breaks, you’ll want to set the air screw back to its original setting.

NUMBER EIGHT: WHAT ELSE WILL CHANGE THE ADJUSTMENT?
Altitude: Elevation changes below 4000 feet have little effect on the pilot circuit.
Humidity: In humid weather, there is a chance you’ll have to lean the fuel screw setting in the afternoon as the climate dries
Rain storms: A fast approaching storm will require a richer setting.
The factory teams use temperature, humidity and histogram meters to help fine tune for each day’s conditions.

NUMBER NINE: CAN YOU FEEL THE FUEL SCREW ON THE TRACK?
If your bike stumbles off idle when the throttle is cracked-especially after you’ve been coasting off throttle-the fuel mixture screw setting is too rich. Turn the fuel mixture screw in. If the setting is too lean, the bike will hesitate, go “waaah” and have an airy feeling right off throttle. A bike that pops also hints at a lean pilot circuit. If your four-stroke pops at the crack of the throttle, suspect the fuel mixture adjustment but also check for an exhaust leak at the head pipe or slip fit.

NUMBER TEN: DOES IT REQUIRE ANYTHING SPECIAL?
Four-stroke owners should invest in a fuel mixture screw screwdriver. Since the fuel mixture screw is hidden under the float bowl, it is very hard to adjust without a very short screwdriver. Your dealer can get you one from Motion Pro. Even better, Zip-Ty racing offers an adjustable fuel screw that can be turned by hand.

Be sure to Like us on facebook at http://www.facebook.com/ChronicMotoX to get our weekly tech tips. Meanwhile, here is a handy video from a seemingly trusty “Matt the Mechanic” to help you master fuel screw adjustment on your four stroke.

Comments (4)

  • Mark Brown

    I have a 2005 540 KTM exc with a non tps Carb ( has hot start) . 45 pilot 180 main needle 3rd pos. Heres my question : The fuel screw is turned 1/2 turn out to idle any more and it blubbers.. I put a 42 pilot in and same thing. Never had a KTM rfs act like this. I bought the bike from a guy who said he never rode it ODO says 60 hrs. ( not that that’s relevant) What could cause the pilot circuit to run rich?

    Reply
    • Chronic MX

      Hi Mark. Thanks for your question. It honestly could be quite a number of issues. First off, maybe it is not the jetting after all? You did the right thing by trying to go down on the pilot and adjusting the fuel screw… but the “blubbering” may not necessarily be the jetting. Perfect jetting is critical, but so is valve adjustment, top end condition, and the type fuel used. So first off, if it were our bike… we’d be sure all else is up to par before ever trying to dial in the jetting. 1. Be sure your valves are adjusted. Chances are at 60 hrs the guy never adjusted the valves yet… which could have a tendency to cause the symptom you described. A fresh top end at 60 hrs is a must too!! 2. Next, check the entire “airflow” for any leaks at the exhaust port, head gasket, and air boot intake and manifold. 3. Be sure fresh premium fuel is used… if the bike sat around and the guy never used it… the gas is probably no good and needs to be drained. Chances are you probably already did that…but just saying. These are all preliminary steps before getting into the carb. Once those things are done (valves, fresh top end, check for leaks, and new fuel) you can then begin the jetting process. Make sure the carb is clean though…. you stated that the guy said the bike sat around. Jetting isn’t going to do any good if the carb is gummed up. Hardened fuel deposits from the bike sitting around and not being used is a common problem and will definitely throw off jetting and could easily cause that blubber and inaccurate pilot adjustment. So with a “clean slate” and all the above done…you can now dissect the jetting. If the bike has no aftermarket parts… we suggest setting the jetting back to the exact factory specifications in the manual. Start there…set it back to specs… based on geography location etc. If there are mods…its another story… but if no mods are on the bike…starting at the factory specs will make your life easier. That’s a tricky bike to jet to begin with, so be sure all else is done above before ever starting the jetting process. Once the jetting process begins… your goal will be to adjust the pilot size up or down until the bike gets its best idle at approx. 1 to 1 1/2 turns out. Moving from a 45 to 42 really isn’t as big a jump as you think…so keep adjusting until you are able to get the best idle at 1 to 1 1/2 turns out. Anything below 1 or above 2 1/2 requires a change in the pilot size. When you clean the carb prior to this process, be sure to inspect the needle too. You may want to contact JD jetting. 253-939-7114. Ask for Dean. He is the master at jetting any bike and can definitely recommend the best jetting and discuss your issue over the phone. However, like us, he will suggest starting with a “clean slate” before starting the process. JD sells jetting kits that are typically much improved over stock. His kit may be the way to go… and will come with new hardware (jets, needles, etc) that wont have “gum” enameled on them. The kit will have specifications and instructions that are dead on. That’s our two cents… its going to be a game of trial and error for you… and you may have to invest a few hundred bucks to get that beast dialed in… but starting with a clean slate will help you narrow it down to the jetting. We have seen many owners fumble with jetting for weeks… only to discover it was something else all along. You may find that after doing the preliminary steps, the problem wasn’t the jetting. Also, an aftermarket fuel screw might be a good idea. They are inexpensive and it sounds like at 60 hrs, that fuel screw may be “tired”. Start there and please circle back and let us know how you are progressing! Ride on. Cheers, Chronic MX

      Reply
  • Mark Brown

    Thank you, Valves are good, compression is great. Down to a 38 pilot and still can’t turn fuel screw out past 1/3 turn when hot. Is there any other circuits in play for the idle system? I have the 1/8 – full throttle working perfect now. This bike is a 540 and has an unstock carb with a hot start and no TPS. Think its off an MX Thumper?

    Reply
    • Chronic MX

      Ok. My gut says that you must have a bad fuel screw. Make sure all 3 components are on the tip of the fuel screw… a washer, O ring, and spring must be present on the fuel screw. If you are going that far down on the pilot and it still makes no difference…then a bad fuel screw is suspect. Sometimes riders mess with the fuel screw and lose the tiny critical parts. If it has no washer or O ring or spring… that could definitely cause your issue. The only other inkling is that you have an air leak somewhere. Ride on! Chronic MX

      Reply

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