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4Strokes.com Honda Technical: Carburetor Adjustments & Tuning (XR400R/Keihin 36mm)

Though meant for late-90's Honda XR400R with 36mm Keihin carburetor, this info may be helpful for other model carbs

Page Jump Links: Pre-Adjustment Checks | Standard Tuning Recommendations | Adjustments for Altitude & Temperature | Correction Factor Charts | Special Tuning Conditions | Adjustment Procedures | Pilot Screw | Slow Jet | Needle Clip Position | Main Jet | Float Adjustment

Pre-Adjustment Checks

• Air cleaner
• Air leaks
• Float level
• Clogged jets
• Spark plug fouling (improper heat range or other cause)
• Fuel freshness
• Owner modifications (such as exhaust pipe, holes in air box, etc.)

Standard Tuning Recommendations

As delivered, with an exhaust diffuser installed in the exhaust pipe and a noise suppressor installed in the air box, your bike may be jetted rich for your riding needs. The standard settings offer maximum performance for closed course competition -- after removal of the exhaust diffuser and noise suppressor. The standard settings also prevent the possibility of engine damage if an you remove the diffuser and suppressor for closed course competition without making a jetting change. For maximum trail riding performance, adjust as shown below.

  *noise suppressor and exhaust diffuser installed

Adjustments for Altitude & Temperature

All jetting recommendations are based on standard jetting with an un-modified engine. The following conditions can affect the fuel mixture.

Correction Factor Charts

Use the applicable correction factor charts shown below to determine if carburetor adjustments are needed because of changes in temperature and/or altitude. If the correction factor is 0.95 or less, turn pilot screw in ¼ turn. Note: Noise suppressor and exhaust diffuser installed.

Main Jet Correction Factors

Standard Settings (as delivered)

Main Jet Correction Factors

Trail Riding Settings

1. Plot a straight line up from the temp to the horizontal line of altitude.
2. At the intersection of these two lines, plot a horizontal line to the left edge of the chart.
3. Multiply the correction factor times your jet size. Round the resulting number up or down to the nearest jet size. (Honda main jet and slow jet sizes are numbered in increments of 2 or 3.)
4. With Trail Riding Settings (noise suppressor and exhaust diffuser installed), you will find your bike is more sensitive to altitude and temperature changes. For this reason, correction factor charts are given for both the main and slow jets. Also note the range of possible fine tuning of the pilot screw (± ¾ turn).

Slow Jet Correction Factors

Trail Riding Settings

(noise suppressor & exhaust diffuser installed)

Example: At a temp of 35ºC (95ºF) and an altitude of 1,000 m (3,280 ft), recommended carburetion is:

Standard Settings, as delivered:

• Main Jet: 162 x 0.94 = 152.28 = #152
• Pilot Screw = 2 ¼ turns - ¼ turn = 2 turns out

Trail Riding, suppressor & diffuser installed:

• Main Jet: 158 x 0.94 = 148.52 = #148
• Slow Jet: 60 x 0.89 = 53.40 = #52 or #55
• Pilot Screw = 2 ¼ ± ¾ turns out

Special Tuning Conditions

There are some unique atmospheric conditions or situations that may require additional adjustments. It should not be necessary to go more than one jet size richer or leaner to fine tune your bike. If larger jetting changes are necessary, check for air leaks, blocked or restricted exhaust or fuel systems, or a dirty air filter.

Adjustment Procedures

To prevent engine damage, always adjust the main jet before adjusting the jet needle. If there is any doubt, always choose the next richer jet (larger) so there is a margin for safety. You can make the following internal adjustments without removing the carburetor from the engine:

• Main Jet
• Slow Jet
• Needle Clip Position

You may find it easier to remove the carburetor to change the needle clip position. Removal is time consuming, but makes the actual adjustment easier. To change the main jet, the float bowl must be drained. To change the slow jet, the float bowl must be drained and removed. To avoid spilling fuel on a hot engine, remove the float bowl only when the engine is cool.

1. Warm up the engine for 10-15 minutes.
2. Ride a practice lap with the standard jetting and a new spark plug. Note engine acceleration and response in relation to the throttle opening.
3. Verify carburetion by removing the spark plug and reading its firing end.
4. Consider engine response, altitude and temperature conditions (from tables above). Then decide what carburetor settings or replacement jets may be necessary.
5. Turn the fuel valve OFF.
6. Place an approved gasoline container under the drain tube.
7. Using a flat screwdriver, turn the drain screw counterclockwise to drain the float bowl.

Pilot Screw

Simple, use the pilot screw to adjust the idle mixture. Turn the pilot screw in to lean out the idle mixture or turn the pilot screw out to richen or fatten up the idle mixture.

Slow Jet

Honda carburetor jet sizes are numbered in increments of 2 or 3. When changing the slow jet size, increase or decrease it gradually until the desired jetting is obtained. Use only genuine Honda jets because Honda jet size numbers do not correspond with other carburetor manufacturer's jet size numbers. It is not necessary to remove the carburetor to change the slow jet.

1 - Choke Lever

2 - Float Bowl

3 - Baffle Plate

4 - Main Jet

5 - Baffle Plate Notch

6 - Slow Jet

1. Push the choke lever (1) up out of the way (ON position).
2. Using a short Phillips screwdriver, remove the three screws securing the float bowl (2).
3. Carefully lower the float bowl. Check that the baffle plate (3) surrounding the main jet (4) is not disturbed during removal of the float bowl.
4. Note the location of the baffle plate notch (5) in relation to the overflow pipe. The baffle plate must be reinstalled in this position.
5. Remove the baffle plate.
6. Using a flat screwdriver, carefully remove the slow jet (6).
7. Install the new slow jet.
8. Making sure the notch in the baffle plate is located to fit around the overflow pipe, install the baffle plate.
9. Install the carburetor float bowl.
10. Lower the choke lever to its OFF position.

Needle Clip Position

1. It is not necessary to remove the carburetor to change the needle clip position.
2. Remove the seat.
3. Remove the fuel tank.
4. Remove the sub-frame.
5. Loosen the front insulator mounting clamp. From the right side, leaving the cables attached, slide the carb out of the insulator.
6. Using a Phillips screwdriver, remove the two screws and the carburetor top.
7. Using a Phillips screwdriver and taking care that the lock washer doesn't fall into the carburetor, remove the screw and lock washer on the slide actuating arm.
8. Using a Phillips screwdriver, remove the two screws that connect the slide actuating arm to the slide.
9. Leaving the screws in place in the connector piece, lift the actuating arm and connector piece out of the carburetor body. Pull it up and flip it rearward out of the way.
10. Lift the slide out of the carburetor body.
11. Slide the needle up and remove it from the slide.
12. Change the clip position (raise clip to lower needle, lower clip to raise needle).
13. Reinstall the parts in the reverse order of removal.

Main Jet

Honda carburetor jet sizes are numbered in increments of 2 or 3. When changing the main jet size, increase or decrease it gradually until the desired jetting is obtained. Use only genuine Honda jets because Honda jet size numbers do not correspond with other carburetor manufacturer's jet size numbers. It is not necessary to remove the carburetor to change the main jet.

1. Remove the float bowl plug (17mm wrench).
2. Using a socket wrench or main jet tool (6mm), remove the main jet.
3. Insert and tighten the new main jet.
4. Check the 0-ring of the float bowl plug and reinstall it.

Float Adjustment

Sometimes it may be necessary to remove the carburetor and the throttle cables to adjust the float height.

1. Hold the carburetor in a position where the float arm tang will just close the float valve without compressing the spring-loaded pin in the end of the float valve needle. To avoid possible damage, do not press the float against the float valve.
2. Float Height: 14.5 mm (0.57 in)
3. Position the float height gauge against the carburetor body. If the gauge has been set to the specified float height, it should just touch the float without causing the float to move.
4. If the float height is not correct, carefully bend the float arm tang toward or away from the float valve until the specified float height is obtained. Replace a float that is damaged or extremely misaligned. Remember that vent tube routing is critical. Any change in the stock routing of the tubes may pinch the tubes or create low pressure.

Other Related 4Strokes.com Technical Articles:

• Carburetor Jetting (Mag Reprint)
• Carburetor Overview
• How to Read Your Spark Plug

4Strokes.com Technical: Honda Related