The XR250R Airbox / Carburetion / Exhaust FAQ

Last update: 12/06/98

The XR250R airbox, carburetion, and exhaust can all stand some improvement over what is provided with the stock motorcycle. Here are some answers regarding the most common modifications.....

Q: What is wrong with the stock airbox and airfilter and why should I try to make improvements?
A: The stock airbox and stock airfilter are somewhat restrictive, and after making other modifications such as motor mods or an aftermarket exhaust, your intake system will then become the restriction to better performance. Also, the following modifications are relatively easy and low cost, yet offer significant improvements.

Q: How can I modify the stock airbox / filtration system?

A: First, remove your seat and study the top of the airbox. You will see a black plastic plate with a rectangular "snorkel" mounted in a slightly larger cutout in the top of the airbox. You can carefully pry this snorkel out of the airbox top with a flat blade screwdriver as it is simply glued in (non-California bikes), or drill out the rivets for California bikes. Removing this snorkel will allow a much greater airflow into the airbox, and will allow the entire front face and sides of the airfilter to get outside air, as the snorkel dead-ended into a small section on the front of the air filter element alone.

Second, you can remove the stock airfilter and cage, and cut out the backfire screen. This screen causes a huge restriction in the airflow, and cutting out this screen will help matters a lot. One method that is used is to take a razor knife and physically cut the screen out, then trim off all remaining metal screen fibers until there is nothing left that can come loose and be sucked into your carb / engine (Ouch!). Another method that seems to work is to use a propane torch to soften and melt the adhesive used to hold the screen in so that the entire screen can be pulled out. Whichever method is used, be sure that no metal fibers or pieces remain that could come loose. Do not cut out the stamped metal perforated screen, only the "window screen" like material.

The original purpose of the backfire screen was to prevent a fire in the airbox if the motor backfired. In all of the documentation seen about the XR, this author has never heard of a case of this happening. This may be a liability issue however, so this author accepts no responsibility for this or any other modification listed on this site that is made to these motorcycles.

Third, an aftermarket airfilter element should offer better performance over the stock filter, and filter elements are available from White Brothers, K & N, and Uni. The White Brothers and Unit filters fit over the stock airfilter cage, and the K & N unit can be purchased with their own cage, which does not have the restrictive backfire screen that the stock unit has.

Q: What about the stock carburetor?

A: The stock carburetor is generally thought of to be adequate for most stock and mildly modified motors, but for an all out race motor, it may be a little small (30mm). Mikuni makes carburetors up to 38mm suitable for use with the XR, including their flatslide series, and kits are even available which include a larger carburetor, an XR600 airbox boot, throttle cables, and a quick turn throttle assembly.

Q: What can be done with carburetor tuning?

A: Tuning the stock carburetor is fairly easy, through fuel screw adjustments for the pilot range, and needle clip adjustments and main jet changes for the mid and high ranges of throttle operation. If you make the intake and /or exhaust systems less restrictive through the above mentioned airbox/filter modifications and /or an aftermarket exhaust, you will probably have to go richer on the main jet, and raise the needle one or two clips for best performance. On the later model bikes, the main jet is easily changed without removing the carb, as there is an access cover on the bottom of the float bowl. In order to change the needle on the Keihin carb, the carb must be removed, but it still is not too hard of a job to change the needle clip position.

There has been much written about carb tuning through jetting changes, needle clip position, different needles, fuel screw position, etc., but the main thing to remember is to only make one change at a time, then test that change, before going on to other changes. Using this method will insure that you will know what made the bike run better or worse, and you will learn more through isolating changes.

Q: What can an aftermarket exhaust system do for me and what is available?

A: Many aftermarket exhaust pipes and systems are available for the XR, includinglarge diameter head pipes (on the theory that the stock pipe diameter is too small). Most exhaust systems will require carb re-jetting and tuning for best performance, and will also increase the noise coming out the back end. An aftermarket pipe, with carb rejetting is probably the easiest modification that can be made that offers a big increase in performance.

Following is a listing of most of the known pipes available for the XR250R, and a description for each system:

DG Performance has their RCM series (reverse cone megaphone), which is a slip on system with steel headpipes and an aluminum cannister.   It is              available with or without their spark arrestor.

White Brothers makes several types of exhausts for the XR250R, including their elliptical disc "E-Series" which is an elliptical cannister (more volume          than round in general), with Supertrapp type discs at the rear for tuneability.  This cannister type is available in the Meg-series which has a outward             tapering entry into the cannister, or their S-bend series, which simply maintains the diameter of the straight pipe into the cannister inlet.   White              Brothers also offers their "SS Works" header pipe to replace the stock headpipe with a larger diameter pipe, on the theory  that the ID of the stock              headpipes is too small.

White Brothers also sells a "Megalloy series" which is a slip-on reverse cone megaphone design with an aluminum can, or their "Mega 4" (by FMF)               racing megaphone, which is nickel plated, and will work as shipped or can take a Supertrapp disc pack.

Supertrapp offers their tuneable disc system on aslip-on pipe with an aluminum cannister.  There are two versions, their "Race series" or their "EAR"           series which is quieter.  Supertrapp now offers their internal disc series as well, which is a slip-on system with an angled tip, in a T6063 aluminum              cannister (most companies use 6061-t6), in a racing or quiet version.

FMF offers their "Performax" series system which uses a nickel plated steel slip on pipe with an oval aluminum cannister.  It has an elliptical                Supertrapp-type disc system, and comes with a quiet core that you can choose to insert or leave off. It comes with all necessary clamps and hanger              bracket for installation.

Cobra sells a system that is a reverse cone megaphone design with an aluminum cannister, and is available in their "4-stroke" exhaust or their "ISDE           Pro" (quieter) series.

Pro Circuit has a reverse cone megaphone series that works with the Supertrapp disc pack or can use the Krisman exhaust insert.  It has a plated              finish.

IMS Krisman sells a spark arrestor insert to replace the OEM insert on the stock pipe.   It is claimed to increase horsepower.

Summary: If you want some simple modifications that you can do without motor dissassembly, try a pipe, some airbox/filter mods, and rejetting / tuning of the carb. Once you decide to modify the motor, go for a displacement increase before you get crazy with cams, porting, etc.

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