How to Adjust Idle Honda TRX400 (TRX 400)

Question

I need the correct procedures to adjust engine idle speed on my Honda TRX400EX. I want to tune the carburetor because the engine runs too slowly while out of gear.

My four-wheeler started running rough about 2 months ago and has progressively gotten worse. It went from running slightly irregular to idling erratic and bumpy. When it runs slowly in natural it feels like it’s going to stall.

I want to alter the idle speed so that it runs a little faster while disconnected from a load. How to adjust idle speed on carburetor and where is the idle adjustment screw located on the carb?

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Jeremy Manoska 5 months 2020-12-07T16:57:53+00:00 1 Answer 106 views 0

Answer ( 1 )

  1. To properly adjust the idle speed you need to first perform a tune-up on the vehicle. It might be a faulty component that is causing it to idle rough or run too slowly. You should clean or replace the air filter element if it is dirty. Replace and gap the spark plugs to factory specification.

    Spark Plug Gap, Type – 1995-2003 TRX400

    SPARK PLUG GAP: 0.8 – 0.9 mm (0.03 – 0.04 in)
    STANDARD SPARK PLUG TYPE: NGK DPR7EA-9 or DENSO X22EPR-U9
    EXTENDED HIGH SPEED RIDING PLUG TYPE: NGK DPR8EA-9 or DENSO X24EPR-U9
    COLD WEATHER PLUG TYPE: NGK DPR6EA-9 or DENSO X20EPR-U9 (Below 5°C/41° F)

    Spark Plug Gap, Type – 2004-2007 TRX400

    SPARK PLUG GAP: 0.8 – 0.9 mm (0.03 – 0.04 in)
    STANDARD SPARK PLUG TYPE: NGK DPR8EA-9 or DENSO X24EPR-U9
    COLD WEATHER PLUG TYPE: NGK DPR7EA-9 OR DENSO X22EPR-U9 (Below 5°C/41° F)

    Be certain the valve clearance is within factory spec. Valve clearance should be 0.15 (0.006) for both intake and exhaust (1995-2007) models. Check cylinder compression and change the engine oil and filter.

    Engine Idle Speed Adjustment

     

    1. Before you modify idle speed perform engine maintenance as outline in your owner’s manual.

    2. You must run the engine or drive it for 10 minutes or until it reaches normal operating temperatures. You cannot accurately balance the idle speed until the engine is properly warmed.

    3. Connect a tachometer, you can buy one here https://amzn.to/37OFNX4. Follow installation instructions. Basically you just connect the alligator clamp to the spark plug wire.

    How do I adjust Idle to tick over correctly using tachometer?

    4. Place the all-terrain vehicle on a level surface and put it in neutral position.

    5. Slowly adjust the idle by turning the throttle stop knob in either direction until you have achieved the desired rpm.

    Adjusting Idle TRX400 1996 1997 1998 1999 2000 2001 2002 2003

    Throttle Stop Adjusting Knob Location

    6. IDLE SPEED RPM: 1400 ± 100 rpm for 1995-2007 models.

    LCD display on some tachometers lag by 1 or 2 seconds. So be certain to make slight adjustments and pause before adjusting it any further.

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