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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
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.

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:
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.
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.
Needle Clip Position
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.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.
Float Adjustment
Sometimes it may be necessary to remove the carburetor and the throttle cables to adjust the float height.
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.Other Related 4Strokes.com Technical Articles:
4Strokes.com Technical: Honda Related