How To Clean A Dirt Bike Carburetor And Unclog The Pilot Jet

Clogged Pilot Jet Symptoms

The pilot jet, which KTM calls the idling jet, runs the engine from idle to about quarter throttle. When it blocks, the problems all sit at the bottom of the throttle:

• Starts only on choke, dies without it. The choke on carbs like the Mikuni TMX opens its own fuel drilling, so it supplies the fuel the blocked idle circuit cannot.
• No idle. KTM's troubleshooting table lists a blocked idling jet as the first cause when the engine has no idle speed.
• Turns over but will not start after sitting, even with fresh fuel in the tank, because the bowl is still full of stale fuel.
• Stumbles at small openings but pulls fine once the throttle is well open, because the main circuit is far larger and rarely blocks.
• The idle air screw does nothing. KTM uses a screw that has no effect as the sign of a wrongly sized idling jet. A partly blocked jet behaves the same way.

Why Old Fuel Clogs The Pilot Jet

The pilot jet is the smallest calibrated hole in the carb. On the 2022 KTM 250 SX the Mikuni TMX 38 runs a size 35 idling jet next to a 450 main jet, so the idle circuit is a tiny fraction of the main circuit. Three things happen to fuel that sits in the float bowl:

1. It evaporates. KTM's manual says the most flammable parts of fuel evaporate when a bike stands for a long time, leaving fuel that is hard to start on. Refill the bowl with fresh fuel and it starts right away.
2. It oxidizes into gum. Chevron's fuel technical review explains that gasoline reacts with oxygen to form gums, which leave deposits and can plug filters. As fuel in a jet dries, the gum stays behind on the metal. In a hole that small, a thin film of varnish is enough to choke it.
3. Ethanol pulls in water. E10 can hold 6,000 to 7,000 ppm of water at 21 °C (70 °F), compared with about 150 ppm for plain gasoline. When it cools or takes on more water, it separates into a gasoline layer on top and an ethanol and water layer underneath. Up to 90 percent of the ethanol can move into that water layer, and an engine will not run on it. KTM's manual is blunt about the result: water in the float chamber causes malfunction.

Ethanol does not "eat" a modern carb. KTM and Honda both approve fuel with up to 10 percent ethanol, and Chevron notes that modern fuel system elastomers are chosen for oxygenated fuel. The real enemy is fuel that sits, ages and picks up water.

Watch The Pilot Jet Clog

Press each step to age the fuel in the bowl. The readout shows how much flow is left through the pilot circuit and what that does to the idle. Watch the throttle bar: the pilot circuit is the one in charge right where the bike stops idling.

Now break it yourself. Drag the varnish slider and then open the throttle. The idle dies long before the jet is fully closed, while larger openings keep pulling because the needle and main circuits take over.

Diagnose It Before You Pull The Carb

KTM's no-start list starts with the simple things, and so should you. Most of these take two minutes.

1. Check the starting basics. Fuel tap on, tank breather hose not kinked or blocked, correct cold-start procedure with the choke pulled and the throttle closed.
2. Drain the float bowl into a clear jar. Tap off, rag under the carb, remove the drain screw plug and let it empty. Look for water droplets, a separate cloudy layer at the bottom, dirt, or fuel that is dark and smells like varnish.
3. Refill with fresh fuel and try again. If it starts and idles, the problem was stale fuel and you are done.
4. Still only runs on choke? The pilot circuit is blocked. Plan to remove the carb.
5. Check the spark plug while you are there. A sooty or wet plug is on the same KTM no-start list, and a session of cranking on choke often fouls one. See our fouled spark plug guide.

For a wider checklist that covers spark and compression too, use our dirt bike won't start guide.

Tools And Supplies

• Screwdrivers that fit the jets and bowl screws without chewing the slots, and the sockets for your carb clamps
• Spray carb cleaner and a small container to soak parts
• Compressed air with a fine nozzle
• A clear jar for the drained fuel, rags, nitrile gloves and eye protection
• A new float bowl gasket or O-ring if the old one is torn or flattened
• Fresh fuel, and premix at your manual's ratio for a two-stroke (our premix calculator does the math)

Safety from KTM's manual: fuel is highly flammable and poisonous. Work with a cold engine, away from flames or sparks, with good ventilation, and wipe up spills straight away. Carb cleaner is also flammable and harsh on skin and eyes. If the carb has damaged threads, a stuck jet or a cracked body, take it to a shop.

How To Clean A Dirt Bike Carburetor, Step By Step

1. Drain the bowl and remove the carb. Turn the fuel tap off, drain the float bowl, and remove the carburetor following your service manual. Take a photo of the throttle cable routing and every hose before you unplug them.
2. Remove the float bowl. Turn the carb upside down over a clean rag so nothing falls inside. Look at the bottom of the bowl. Brown gum, white crust or a water film confirm the diagnosis.
3. Unscrew the pilot jet. On the Mikuni TMX 38 it sits next to the needle jet and main jet. Use a screwdriver that fills the slot. On the 2022 KTM 250 SX the stock size is 35.
4. Hold it up to the light. A clean jet shows a round, clear hole straight through. A varnished jet looks dull, partly closed or completely dark.
5. Soak and blow it through. Soak the jet in carb cleaner, then blow through it with compressed air from both ends. Repeat until you see a clean, round hole.
6. Do not push wire or drill bits through the jet. The size of the hole is the calibration. A scratched or enlarged jet will never meter the same again, so replace a jet that will not come clean rather than reaming it.
7. Clear the passages too. Spray carb cleaner into the pilot jet's well, the idle air screw passage and the choke passage, then blow them out with air. KTM groups the jets, air screw and choke together as carburetor components to check and set.
8. Check the float needle and seat. KTM lists a dirty or worn float needle as a cause of the carb overflowing and the engine not revving out. Wipe the needle tip and replace it if it has a groove worn into it.
9. Clean the bowl and reassemble. Wipe the bowl clean, fit a good gasket, and refit the stock jets unless you are rejetting on purpose. Snug the jets properly, because loose carburetor jets are on KTM's fault list too.
10. Refit the carb and check the throttle. KTM specifies 2 to 3 mm (0.08 to 0.12 in) of throttle cable play at the grip, and the idle must not change when you turn the bars from lock to lock.

If the bike sat for a long time and the bowl had heavy gum or water, also flush the fuel tank and tap, check the fuel filter if your bike has one, and run fresh fuel through before you refit the carb. Otherwise the old fuel just refills the bowl you cleaned.

Reset The Idle After Cleaning

KTM's procedure for the carbureted 250 SX works on most carbs with an idle air screw:

1. Screw the idle air screw all the way in, gently, then back it out to the basic setting. On the 2022 250 SX that is 1.5 turns.
2. Warm the engine for at least 5 minutes.
3. With the choke off, set the idle speed screw to 1,400 to 1,500 rpm.
4. Turn the air screw slowly clockwise until the idle starts to drop, note the spot, then turn it slowly counterclockwise until the idle drops again.
5. Set it halfway between, at the highest idle. If the idle rises a lot, bring the idle speed back down and repeat.

KTM adds that if you can turn the air screw all the way without any change in engine speed, the idling jet is the wrong size. After a cleaning, it is just as likely that the pilot jet or its passage is still partly blocked, so check that before you buy a different jet. Redo the idle after big changes in air temperature or altitude, and see our jetting guide for why.

How To Stop It Happening Again

A stabilizer slows oxidation, but it is not a cure for water. In a Practical Sailor test of five fuel additives, none prevented ethanol phase separation. Draining the bowl and keeping fuel fresh matters more than any bottle. For two-strokes, mix premix in small batches; the Yamaha YZ manuals warn against using premix that is more than a few hours old.

Going into the off-season? Our winterizing guide covers fuel, battery and storage in one pass.

Mistakes To Avoid

• Rejetting to fix a clogged jet. A blocked pilot jet makes the bike lean and lumpy at small openings, and a bigger jet just hides the problem. KTM's tuning chapter assumes clean parts before any jet change. Clean first.
• Reaming the jet with wire. It changes the calibration for good.
• Cleaning the carb and keeping the old fuel. The stale fuel in the tank refills the bowl and starts the process again.
• Assuming non-ethanol fuel never goes bad. An NREL study summarized by the Renewable Fuels Association found ethanol-free fuel degraded to the same degree in storage. Ethanol adds the water problem, but all gasoline ages.

Track Carb Work With Your Engine Hours

MotoMind logs each carb clean with notes, parts and cost, so you can see when you last pulled it and which jets are in it. It tracks maintenance by engine hours and by date. Date-based tasks, like a monthly check on a bike that sits, can send push reminders; hour-based tasks show as upcoming, due or overdue in the app.

FAQ

Why does my dirt bike only run with the choke on?

The most common cause is a blocked pilot (idling) jet. The choke opens its own fuel drilling, so it supplies the fuel the blocked idle circuit cannot. Drain the float bowl, try fresh fuel, and if it still needs choke, clean the pilot jet.

Can I clean a carburetor without removing it?

Draining the bowl and refilling with fresh fuel fixes stale fuel. A varnished pilot jet usually needs the carb off so you can remove the jet, soak it and blow it through with compressed air.

Can I use a wire to clean a pilot jet?

Avoid it. The hole size is the jet’s calibration, and wire can scratch or enlarge it. Soak it in carb cleaner and use compressed air, and replace a jet that will not come clean.

How often should I clean my dirt bike carb?

Whenever the idle goes bad after the bike sits, and at the manual’s interval. KTM’s schedule for its 2022 250 SX motocross bike checks and adjusts the carburetor components every 40 operating hours or 12 months, and KTM says to drain the float bowl if the bike sits for more than a week.

Does ethanol fuel ruin dirt bike carburetors?

KTM and Honda approve up to 10 percent ethanol. The problem is storage. E10 holds far more water than plain gas and can separate into a water and ethanol layer that the engine will not run on. Drain the bowl and keep fuel fresh.

What size is the pilot jet on a KTM 250 SX?

The 2022 KTM 250 SX with the Mikuni TMX 38 uses a size 35 idling jet as standard, with 30, 32.5, 37.5 and 40 listed as options. The 2023 and later 250 SX uses fuel injection and has no pilot jet.

Do fuel stabilizers stop phase separation?

No. Stabilizers slow oxidation, but a Practical Sailor test of five additives found none prevented ethanol phase separation. Keep water out and drain the bowl for storage.