The XR4 FAQ

The XR4 FAQ

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What is an XR400R?
It is a simple, rugged, reliable air cooled Honda with 397cc single cylinder, 4 stroke engine using a SOHC 4 valve RFVC head. The bike has 5 speed gearbox. It has been in production since 1996. Its primary use is as a Trail and Enduro bike. It is often referred to as the XR4.
Does the XR4 vary?
The XR400RT is a 1996 model, V is '97, W is '98. They also vary from country to country, area to area and in the US from state to state. For example A (American) type, ED (European Direct Sales), CM (Canadian) type and U is the Aussie variant.
Which ones do this FAQ refer to?
Mostly UK (ED type) and US models unless otherwise specified.
What differences are there between UK and US models?
Dealing with the T and V models only, it is mostly small differences such as headlamp size and generator output. Other than they are ostensibly the same.
What differences are there between the T and V models?
The V model has differing up rated clutch springs (22401-MM9-000) and a new clutch lifter - basically a 50 thou machining difference (22847-KCY-671). The '97 forks also have a few additional 'O' rings.
Notes:Check you don't have the soft tappets. Poor materials. Dealers rectify. Models between frame Nos JHN2NEO3A*VM100095 to JHN2NEO3A*VM101476. Heavy riders using an air intake baffle/noise suppressor can restrict the intake area with flex from the seat closing off the airbox intake.
What differences are there between the V and W, ED type models?
The kickstart is 3mm meatier on the knuckle with the retaining screw moved to the adjacent side. This was to stop breakages of which a few occurred on earlier bikes. The right side footpeg was also changed to accommodate the new kickstart but '98 kickstarts will fit '96 and '97 models. The forks have differing compression and rebound stacks plus longer slider bushings and the spring rate went up from 0.38Kg to 0.40Kg. The rear shock had compression and rebound curves improved. A small plastic tang appeared on the '98 airbox to stop the filter spring coming adrift - an easy mod though can be done with a small hole and tywrap to get the same effect on earlier bikes. AFAIK the ignition curve was altered with the '98 bikes being retarded slightly, probably for better mid-range. The '98 XR4 handbook states 2 1/4 turns out (same as earlier models) for the pilot screw yet as delivered here they are factory set at 1 turn out. Note on XR4s the pilot screw is an aerated fuel screw and not an air screw (it all depends on which side of the slide a pilot screw is) so in to lean and out to richen. The carb was changed from a PDK 1AA to a PDK 1FA.
How does the W, ED type model carb differ from the earlier PDK 1AA?
Pilot jet #55, was 62. Needle 16012-KCY-681, was 670. Slide 16112-KCY-771, was 670. Pilot Screw 16016-KCY-771, was 670. Jet needle holder 1615-KCY-681, was 670.
Did the US models alter in the same way?

Yes, apart from the carb and air intake which differ from state to state.

Component	96-97 All States	98 XR Calif	98 49 States
Main Jet	162	138	142
Pilot Jet	62	52	52
Main Air Jet	#100	#90	#90
Needle Jet	10 mm	10.5 mm	10.5 mm
Slide	#458	#408	#408
Needle	J6DF	A2EA	A16A
Pilot Screw	2 1/4	1 5/8	1 1/2

Note on some '98 models the airbox restrictor or noise suppressor reduced the available intake area from 63 square centimeters down to 16. The baffle in the exhaust was no longer removable.

What about Aussie models then?
I know little about these save that visually they are easy to identify. They have a 40T rear sprocket instead of a 45 and a chainguard. I also understand the engine is restricted in the inlet tract. Note. UK buyers beware when purchasing from Europe as some models can be grey imports from Australia and will take some work to de-restrict.
Are UK and US '98 models that much better than '96 or '97 models?
Not really providing you do the work on the carb/exhaust to get better carburation and tweak the suspension to your liking.
So, how do I set my carb/exhaust up on my '96 or '97 bike?
The bikes were delivered jetted correctly for no air box noise suppressor or exhaust insert baffle fitted and for sea level and 68F. Work from this baseline. The standard main jet should be a 162, the pilot a 62. The needle should be a J6DF with the clip in the middle position. To improve matters with these components you need to lean off the needle as a stutter can be felt on constant cruise at about 55 mph. To do this raise the needle clip one step thereby dropping the needle. A better solution is to purchase an A16A HRC dual taper needle (16012-NKK-000) and fit it in the middle position. A #408 slide also improves matters, altering the throttle valve cutaway. The standard #458 slide can be made into a #408 slide by machining 20 thou from the base. Additionally the bike may feel as though it bogs over small bumps. To cure this you need to limit the down travel of the float from the standard 25mm to between 18 and 21mm. To do this take the carb off, remove the float bowl (paying attention to the position of the white nylon main jet baffle) and keeping the carb right way up, measure vertically from the edge of the carb body on the centreline of the main jet to the metal float arm. Carefully adjust the small metal tang by the float pivot pillars which the float hinge pin goes through. Bend this with a pair of small pliers till the desired measurement of 19.5mm is achieved. Invert the carb and check the float height (measuring in a similar manner) is 14.5mm, which is the standard setting.
Notes: When adjusting or changing the needle, note the location of the nylon friction washer when removing the slide operating arm and pin. Make sure when installing the needle in the slide that the link arm (the little arm that attaches to the slide) is positioned so that the attachment plate does not block the hole drilled in the slide that is adjacent to the hole for the needle.
Didn't Honda issue a bulletin on jetting for '96 bikes?
- John.Rushworth.com/XR4FAQ/carb_set.htm - Carb Tuning Information.
- John.Rushworth.com/XR4FAQ/main_cor.htm - Main Jet Correction factors for standard settings as delivered.
- John.Rushworth.com/XR4FAQ/trail.htm - Main Jet Correction factors with air box baffle and exhaust diffuser installed.
- John.Rushworth.com/XR4FAQ/trailslo.htm - Slow Jet Correction factors with air box baffle and exhaust diffuser installed.
Any tips to make all this easier?
Ignore the little red handbook procedure for removing/working on the carb as it can involve removing the whole rear sub frame and delving into the wiring harness. Instead support the bike on a suitable stand under the engine and after removing the seat, tank, side panels, exhaust silencer, air box to carb clip, sub frame bottom mounting bolts - and then leaving the top one in - simply hinge the sub frame up complete with the air box and mudguard in place and then retain the sub frame with a bungee. Removing the top shock mounting bolt and swinging the shock to one side also helps.
How do I adjust my carb and correct for changes in altitude/temperature and my riding style?

Make sure: Carb clean, vent tubes OK and not trapped, plug set, Vvs set (Adjust to max permitted tolerance for bedding in as heat like the 600 can cause the Vv seats to shrink into head so tightening up Vv clearances during early life of the engine). Setting the Vvs at the max tolerance marginally helps the bottom/mid-range torque too. Make sure timing is OK, air filter clean/oiled, fresh clean fuel. No air leaks on carb/exhaust.

Order of fettling carb:

Float height/downtravel
Main jet
Idle system. Pilot jet and Pilot screw
Needle taper
Check Main jet
Air slide
Idle system. Pilot jet and Pilot screw
Needle groove
Repeat and repeat till happy.....

Here is what bits do what:

Idle to 1/4 throttle	The top (usually a parallel diameter portion) of the needle goes from little to full effect
Idle to nearly 1/2 throttle	The pilot screw goes from maximum to minimal effect
Idle to 1/4 throttle	The pilot jet gives maximum effect, although it continues to flow throughout the throttle range reducing to minimal effect at WOT (Wide Open Throttle)
Just before 1/8th throttle	The throttle valve cutaway goes from minimal and then to maximum effect at 1/4 and then back to minimal effect just after 1/2
The needle clip step	It gives minimal effect at 1/4 raising to maximum effect just before 1/2 and tailing off toward 3/4
The needle jet taper or tapers	Commence a minimum effect at 1/4 raising to maximum effect just before 1/2 and then tailing of at 7/8
The main jet	Takes minimum effect after 1/2 crossing the tailing off point of the needle taper at 7/8 and then rising to maximum effect at WOT

To work out correction factors for Altitude and Temperature you can use this formula in lieu of my spreadsheet:

CF = The sum of (1.0778-0.00111 times T = 0.000000000238554 times A times A - 0.000010777 times A) Where T is temperature in Farenheit and A is Altitude in feet.

Multiply the standard jet size by the CF (Correction factor) to get the new main jet size. Round the resulting number up or down to the nearest jet size. (Honda main and slow jet sizes are numbered in increments of 2 or 3).

If CF is 0.95 or below, turn the pilot screw in 1/4 turn.

John.Rushworth.com/XR4FAQ/cf_xr400.wks - MS Works Ver 4.0 Correction Factor Spreadsheet.
John.Rushworth.com/XR4FAQ/cf_xr400.xls - MS Excel Correction Factor Spreadsheet.

Note jetting also depends on *how* you ride so what works for one does not necessarily work for another i.e. do you feed the throttle or whack it open, do you ride on full bore a lot, poodle about, riding style etc.

The following conditions can also affect the air/fuel mixture.

Condition	Mixture will be:	Adjust to:
Cold temp	Lean	Richer
Warm temp	Rich	Leaner
Dry air	Lean	Richer
High Humidity	Rich	Leaner
High Altitude	Rich	Leaner

There are sometimes unique atmospheric conditions or situations that may require additional adjustments.

It should not be neccessary to go more than one jet size richer or leaner to *fine tune* the bike.

Condition	Main Jet Adjustment
Long straights	Next Larger
Uphill sections	Next Larger
Sand	Next Larger
Mud	Next Larger
High humidty	Next smaller
Raining	Next smaller
Hotter than 113 F (45 C)	Next smaller

Notes: Dynos are not "ideal". I used a combination i.e. dyno, CO meter, plug chops and "seat of the pants feel", riding between sea level and 9,000'. Being able to go up to a height where you can feel that the jetting is affected helps you understand what is happening. The HRC needle does make the transition between the circuits better on '97 and '97 bikes. A recommended Insert for Trailriding is either a Thumper insert or St/St Vortip. The Vortip does not need re-packing. With either insert I would ditch the Torx fasteners and replace them with St/St M5 cap head screws with plain and spring washers.

What about '98 UK and US models?
The UK one should be fine as delivered save for any CF changes for altitude/temperature and riding style.

For US models my friends at Baja Designs recommend:
98 XR 400, California Model: The California model XR 400 runs well with the stock jetting as long as the airbox restrictor and stock muffler remain in place. If more power is desired, and the airbox restrictor is removed and muffler changed, the bike will be dangerously lean. The airbox restrictor reduces the available intake area from 63 square centimeters down to 16. The baffle in the muffler is no longer removable, but the entire spark arrestor assembly can be changed ($34) and a new less restrictive baffle (Vortip- $69.00) installed.

To rejet for sea level conditions, an A16 dual taper needle (3rd clip position) ($15) is required as well as a 58-60 pilot jet and a 162 main. The fuel screw should be adjusted from 2.0 to 2.5 turns out.

98 XR 400, 49 State: As with the California edition of this bike, this bike has an intake restrictor and an non-removable baffle. It does have the A16 needle stock, so if more power is desired, removing the restrictor and modifying the muffler along with a jetting change are what is required. Jetting should be a 58 pilot and a 162 main. Approx. 2-1/4 turns on the fuel screw.
Is there another carb for the XR4?
Edelbrock produce the QwikSilver Carb.
- John.Rushworth.com/XR4FAQ/qwiksilver/ XR400R QwikSilver Off-Road/Dual-Purpose Carburetor
What's the best air/fuel ratio?

The best theoretical mixture ratio of air and fuel is 14.7:1. In this case you will, theoretically, get no measured CO. Real world bikes are jetted somewhere around that ratio. 12:1 would give a max power mixture whilst say 18:1 would give you the best fuel economy. Those are the extremes. It's all a question of what you want and what the manufacturer intends you to have in the first place.

%co	A/F	%co	A/F	%co	A/F	%co	A/F	%co	A/F
0.02 0.04 0.06 0.08 1.00	14.53 14.41 14.22 14.16 14.10	1.2 1.4 1.6 1.8 2.0	14.03 13.97 13.89 13.81 13.76	2.2 2.4 2.6 2.8 3.0	13.68 13.58 13.53 13.44 13.37	3.2 3.4 3.6 3.8 4.0	13.30 13.23 13.14 13.07 12.99	4.2 4.4 4.6 4.8 5.0	12.92 12.85 12.79 12.69 12.63

%co	A/F	%co	A/F	%co	A/F	%co	A/F	%co	A/F
5.2 5.4 5.6 5.8 6.0	12.53 12.45 12.39 12.32 12.24	6.2 6.4 6.6 6.8 7.0	12.17 12.09 12.01 11.95 11.88	7.2 7.4 7.6 7.8 8.0	11.81 11.75 11.68 11.60 11.53	8.2 8.4 8.6 8.8 9.0	11.45 11.39 11.31 11.24 11.15	9.2 9.4 9.6 9.8 10	11.07 11.00 10.93 10.85 10.78

In the UK fuel falls into these categories:
Super Unleaded 98 Octane
4 star leaded 97 Octane
Unleaded 92 Octane

The US I believe is:
Super Unleaded 92 Octane
Mid grade unleaded 89 Octane
Regular unleaded 87 Octane

The differences are to do with how octane is measured. In the US the octane number you see at the pumps is the average of two octane numbers; the Research Octane Number (RON) and the Motor Octane Number (MON). Note Unleaded fuels as a rule tend to have a higher specific gravity, which with all else being equal makes for richer jetting. The man to talk to about fuel is: Rich Rohrich, Applied Fluid Dynamics, rrohrich@interaccess.com

What are the basic specs of the XR4?

1997 Honda XR400RV Specifications
Displacement	397cc
Bore & stroke	85 x 70mm
Compression Ratio	9.3:1
Carburation	36mm Keihin piston-valve
Ignition	Solid-state CD w/electronic advance
Final drive	O-ring, 15/45
Suspension
Front	43mm Showa cartridge leading-axle fork w/18-way compression and 12-way rebound damping adj.
Rear	Pro-Link single Showa shock w/spring preload, 16-way compression and 16-way rebound damping adj.
Brakes
Front	Single disc, twin-piston caliper
Rear	Single disc, single piston caliper
Wheelbase	56.1 in.
Seat height	36.6 in.
Ground clearance	12.2 in.
Dry weight (claimed)	256.8 lbs.
Fuel capacity	2.5 US gal., 0.4 US gal on reserve

What's the difference between a US gallon and a UK one?
1 UK = 4.546 litres. 1 US = 3.785 litres.
Any other useful conversion factors and formulae?
Deg F -4 5 14 23 32 41 50 59 68 77 86 95 104 113 122

Deg C -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45 50
- John.Rushworth.com/XR4FAQ/temperature.wks - MS Works Ver 4.0 Temperature Conversion Spreadsheet.
- John.Rushworth.com/XR4FAQ/temperature.xls - MS Excel Temperature Conversion Spreadsheet.
1 metre = 3.28 ft, 1 mile = 1.609 km, 100 mph = 160.9 km/h, 1 horsepower = 550 foot-pounds/second, 745.7 Watts = 1 horsepower, Volts times Amps = Watts, Volts = Amps (current) times Ohms (resistance), 1kg = 2.2lbs.
Any other useless information?
The weight of various components. Forks with oil = 10lb each leg, Top Yoke = 2lb, Bottom Yoke with bearings and nuts = 3lb, Frame = 22lb, Engine = 84lb, Swingarm, shock, bearings and linkages = 20lb, Carb, Cables and Throttle = 3lb, Air Box and Sub Frame = 9lb.
What manuals are available for the XR4?
Honda workshop manual. Photocopies in UK are approx 15 UK pounds instead of paying mega bucks for the glossy printed version. Part Nos 62 KCY00C for 96T model. I've not checked for later versions.
Honda XR400R 96/97 setup competition manual, which is quite inexpensive. Reorder part No is S0937 available only as an American publication. Contact Baja Designs. I've not checked for later versions.
How fast does it go?
Here is a gearing calculator for the XR400 and my article on gearing.
Are there any pictures of your own bike?
Yes! Here she is.
Is this all there is to this FAQ?
No, it's work in progress and there is loads more as and when I get around to it.

Deg F	-4	5	14	23	32	41	50	59	68	77	86	95	104	113	122
Deg C	-20	-15	-10	-5	0	5	10	15	20	25	30	35	40	45	50

Version last updated Monday 9th November 1998. John Rushworth.

John.Rushworth.com