How to Troubleshoot Honda 400EX Exhaust and Jetting Issues?

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On a Honda 400EX, deceleration popping usually traces to the pilot circuit or an exhaust leak — not the main jet.

The 2006 TRX400EX runs a 397cc RFVC air-cooled, dry-sump single, fed by a 38mm piston-valve carburetor with an accelerator pump. 400EX exhaust and jetting problems almost always sit in one of four places: the throttle range where the symptom appears, the airbox setup, the exhaust, or your elevation.

Which Fuel Circuit Owns The Symptom?

The throttle position where the trouble shows up names the circuit. Idle to 1/4 throttle belongs to the pilot jet and fuel-mixture screw, 1/4 to 3/4 to the jet needle and clip, and 3/4 to full to the main jet. Deceleration popping looks like a main-jet problem and almost never is — start at the pilot circuit, then check for leaks.

Symptom Or Reading Circuit Or Cause First Move
Pops on deceleration Pilot circuit or exhaust leak Set the pilot screw, then check for leaks
Rough from idle to 1/4 throttle Pilot jet / fuel-mixture screw Adjust the pilot circuit
Stumble from 1/4 to 3/4 throttle Jet needle and clip Move the clip one groove
Weak from 3/4 to full throttle Main jet Change the main jet one size
Riding above 6000 feet Thin air, rich mixture Drop the main jet one size
Airbox lid removed More airflow, lean mixture Go richer on the main
Plug dark or black Rich mixture Lean it out one step
Plug gray or white Lean mixture Richen it one step

Jetting And Exhaust Fixes: What Decides The Numbers

An aftermarket exhaust, a lidless airbox, and high altitude all push richer. A 2004 400EX reference lists the stock carburetor as a Keihin QB10A with a 38mm throttle bore, a #148 main jet, a #38 pilot jet, the needle clip in the 3rd groove, the pilot screw 2 1/4 turns out, an 18.5 mm float level, and a 1400 RPM idle.

Dynojet’s Q107 kit covers 1999–2008 Honda 400EX models and splits Stage One by exhaust and elevation. With the stock exhaust it calls for a DJ142 main below 3000 feet, DJ138 from 3000 to 6000 feet, and DJ134 above 6000 feet. Add an aftermarket pipe and each number steps up once: DJ146, DJ142, DJ138. The same sheet sets the fuel-mixture screw at 3.5 turns out below 5000 feet and 3 turns above.

Stage Two assumes the airbox lid comes off, and the mains jump hard: DJ165 below 3000 feet, DJ160 from 3000 to 6000, DJ155 above 6000 with a stock exhaust; DJ170, DJ165, DJ160 with an aftermarket exhaust.

Jetting access takes minutes: remove the top carb plate, the slide lift arm screw, and the needle plate screws; flip the linkage forward and lift out the slide; pull the stock main jet; install the adapter and supplied main jet; set the mixture screw to spec. The full sequence sits in Dynojet’s Q107 jet kit instructions.

Exhaust work usually forces the rejet. If the pipe is undecided, this 400ex exhaust system comparison covers the tested options before you order jets to match.

The Order That Stops You Chasing Jets

Change one variable, ride, then decide the next move — skipping the test ride turns every following adjustment into a guess.

  • Match the symptom to its throttle range first.
  • Check the header, slip joint, and muffler connection for leaks. Leaks cause decel popping even when jetting is close.
  • Turn the fuel petcock off before you pull the carburetor.
  • If it runs well at high throttle but stumbles with the lid on, open the airbox or drop one main-jet size. With the lid off and a lean reading, go richer on the main.
  • Read the plug after a hard pull: black means rich, gray or white means lean, tan or brown is right.

Dynojet’s settings apply only to 1999–2008 Honda 400EX machines — do not carry them over to another ATV.

FAQs

Why does a 400EX pop when you let off the throttle?

Deceleration popping comes from the pilot circuit and from leaks, not the main jet. When the throttle closes, the engine still pulls a little fuel through the pilot jet, and a lean pilot setting or an air leak at the header, slip joint, or muffler lets unburned fuel ignite in the pipe. Fix the leak and set the pilot screw before touching mains.

Do you have to rejet after adding an aftermarket exhaust?

Usually yes. A freer-flowing pipe moves more air, which leans the mixture, so the main jet typically steps up one size. Dynojet’s Stage One list runs a DJ146 below 3000 feet with an aftermarket exhaust versus a DJ142 with the stock pipe. Elevation and whether the airbox lid stays on decide the exact number.

How can you tell if a 400EX is rich or lean?

Read the spark plug after a hard pull and a quick shutdown. Dark or black means rich; gray or white means lean; tan or brown is right. Throttle position narrows it further: idle-to-quarter symptoms point at the pilot circuit, mid-throttle at the needle, and wide-open at the main jet.

References & Sources

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