KDX Hop-Up Guide

By: Canadian Dave

The KDX is one of the most versatile bikes on the market today. In stock form it is a light easy to ride stealthy quite woods weapon that anyone can enjoy riding. One of the greatest advantages of the KDX, unlike many other bikes, is that it’s able to grow with the rider’s ability for only pennies a smile.

I think the most sensible way to approach any hop up is in stages so that’s the way I’ve laid out this section. I have set it up so that, in my opinion, if you follow the steps in order you’ll be assured the biggest bang for your buck at each stage.

ENGINE MODIFICATION – GETTING THE MOST OUT OF 200/220 cc

1995 - 2003 KDX200/ 1997 to 2003 KDX220

STAGE 1

If you’re running a stock bike but are looking for more bottom end grunt you can retain  the stock 47 tooth rear sprocket and exchange the stock 13 tooth front sprocket for a 12 tooth sprocket. This is the least expensive choice.  If you like the results the next time you're due for a chain and sprockets I'd  retain the stock 13 tooth front sprocket and install a 50 tooth rear sprocket.  A 13/50 combination will require a 110 link chain.  This modification will allow you to better use the power available in stock form by reducing the gaps between 3rd and 4th  gear and allow you to run in 2nd or 3rd gear rather than shifting between 1st and 2nd in tight conditions.  People riding in heavy load conditions such as hilly areas, sandy conditions, mud etc will find this modification particularly useful. The stock 47 tooth sprocket works well with stage 2 or higher modification so put it in a safe place if you plan to continue customizing your engine’s performance to best meet your needs.

STAGE 2

Step One:    Replacing the stock expansion chamber with either a FMF or Pro Circuit expansion chamber is the single biggest bang for the buck modification you can make. To say it really wakes up a KDX is, well, an understatement.

If you are looking for more bottom end to mid range power then I would suggest the FMF Torque pipe. Model K-G35

If a stronger mid range to top end is what you’re after then either the FMF Rev pipe (model K-G30) or the Pro Circuit pipe (model PK95200DP) are excellent choices.

Some tips on choosing the right pipe for you riding conditions - 

If you are riding in tight trees, mountainous areas or in high load conditions such as sand or mud and find you spend much of your time in forth gear or lower then I would suggest the torque pipe. The strong bottom end pull offered by the torque pipe is to your advantage and you may find the more abrupt mid range hit of the rev pipe requires more clutch work in the tight trees and the rear wheel is more likely to break loose and spin in slick/rocky areas where the bike is searching for traction.

A rev pipe is a good choice for those who spend more of their time in third to sixth gear and ride in fast open areas and are looking for a strong mid range hit and better top end power.

When selecting a pipe it is important to remember where you would like the most power focused. A torque pipe will steal a little top end power in trade for improved bottom end performance. The reverse is true for a rev pipe. A torque pipe will allow the KDX to pull hard off the bottom with a less abrupt mid range hit than the rev pipe. The rev pipe gives up some bottom end power and has a more abrupt, authoritative midrange hit and more top end power.  Either pipe will make more power everywhere when compared to the stock pipe.

Its important to note that because of differences in port timing the KDX200 and 220 don't react the same to each pipe.  Both the FMF pipes and the Pro Circuit pipe were originally designed to be used on the KDX200.  Installing a Torque pipe on a KDX200 will improve performance over the stock pipe from idle to wide open throttle.  However installing a torque pipe on a KDX220 will improve performance from idle to about 7500  rpm  then fall flat just like the stock pipe with no meaningful increase in over rev or higher rpm performance.  If you're looking to improve your 220's power delivery from the bottom to top end then a rev pipe is a must.  You'll often see the FMF KG-30 advertised as a rev pipe for the 200 and as a torque pipe for the 220 for this very reason.  The torque pipe is still a viable choice for 220 owners who focus the majority of their riding in tight technical conditions or for those who want to maximize bottom to mid performance and are willing to give up some upper mid to top end performance in trade. 

These are only intended to be guidelines. An experienced aggressive technical woods rider that tends to ride the KDX more like a 125  might prefer the stronger midrange hit and improved top end performance of the rev pipe. It is a very personalized decision but if you follow these general guidelines you should be able to find the most advantages pipe for your riding conditions and style.

Step Two: Now that exhaust gasses are flowing better you need to turn your attention to the intake side. Step Two is a freebee. Remove or modify the restrictive air box lid. See: The Air Box Lid, Free Frisbee Included, at the end of this section.

Step Three: Installing a set of Boyesen reeds will improve bottom end throttle response and help extend top end power. I am talking about the reeds alone not the RAD Valve. I would recommend the Power Reeds (product number 607). They are less expensive and last longer than the Pro Series reeds. See Boyesen reeds vs. RAD Valves at the end of this section.  A Delta Force reed valve is also available for the KDX and fits 1989 to 2003 KDX200s.  KDX owner report improved throttle response over the entire rev range with a small increase in over all power.  Most that have tried both the Boyesen RAD Valve and Delta Force Reed Valve on the KDX prefer the Delta Force.

Step Four:  If you have a KDX200 go to Stage Three. If you have a 1997 to 2003 KDX 220 this is your stop. The stock 220 comes from the factory with a 33mm carburetor. The smaller bore increase the velocity of air flowing through the carburetor with the intention of increasing bottom end power and throttle response. Unfortunately its at the expense of upper rpm power. Installing a 1988 or newer 35mm carburetor from a KDX200 or boring the stock 33mm carburetor to 36mm will increase mid range to top end power, with only the slightest loss of bottom end power/ throttle response.  As an added bonus the transition between bottom end and mid range power is actually smoother with the larger carburetor. Jetting for the enlarged 36mm carburetor is the same as the 200.

STAGE THREE

Step One: RB Designs is an Oregon based machine shop that is performing some very impressive modifications to motorcycle carbs. They bore the KDX carb to 36mm, install a venturi divider in the exit side of the carb, modify the jet screen, clean up the pilot circuit's bore, resize the throttle valve's cutaway and rejet the carb.  The results are outstanding and its become one of my favorite mods.  The off idle carburetion is excellent with clean crisp throttle response.  You can easily lug the bike in a gear higher that you could normally and chug your way through the slickest conditions by almost eliminating unwanted rear wheel spend then when things oven up a little off you go.  The bottom end becomes very 4-stroke like.

STAGE FOUR

Step One: Cylinder porting. A number of companies offer cylinder porting to redirect and increase the engine's power. What they’re basically doing is changing the exhaust port timing and reshaping the transfer ports to meet the owner's individual needs.

Step Two:  Head machining. The squish band can be adjusted to redirect the engine's power. The head can also be turned down to increase compression. Modifying the cylinder head may  require the owner to switch to race gas so be sure to let you tuner know what your requirements are.   For a  better understanding of the  advantages of cylinder porting and head modifications check out what Eric Gorr has to say. 

A tuner will often  port the cylinder and machine the head to achieve  well balanced package.

Tuners with proven experience modifying KDX cylinders and heads include Fredette Racing Products and  Eric Gorr’s Forward Motion Racing.  Both are very customer oriented shops and are happy to answer your questions over the phone. 

STAGE FIVE

Step One: Replacing the stock silencer will result in a modest increase in performance and a meaningful reduction in weight. I would recommend a woods friendly spark-arresting silencer. They’re quieter than the free flow silencers and are "Forestry Legal" for those riding on public land. If you need to run a spark-arrester look at the FMF Turbine Core II (model SA-7-202) or the Pro Circuit Spark Arrester (model SK95200DSO) among others. If a free flow silencer is more to your liking consider the FMF Power Core II (model 7-202) or the Pro Circuit (model SK95200D-304) among others.  Many people have commented that they are surprised to see installing an aftermarket silencer all the way down at Stage Five, its a very popular mod.  The stock silencer performs quite well and the average rider wouldn't likely report a meaningful difference in performance after installing and after-market silencer.  Its just not a very big bang for your buck performance modification.  If your goal is to keep the bike as quiet as possible you won't find a better silencer than what comes stock, its an acoustical black hole.

STAGE SIX

Step One: Cylinder boring. The cylinder can be bored to a larger size, replated and an oversized piston installed. Two popular sizes include a 225cc and a 240cc kit. If you’re interested in finding out more about 225cc and 240cc kits I would recommend contacting Eric Gorr’s Forward Motion Racing. They’re a one-stop shop and offer everything from boring to cylinder repair, replating and over sized pistons. He has an excellent reputation and is perhaps the best shop in the US for your cylinder repair and replating needs. I have heard nothing but positive feedback from his customers. Check out Eric Gorr’s website.

For jetting recommendations click here.

1989 - 1994 KDX200

STAGE 1

If you’re running a stock bike but are looking for more bottom end grunt you can retain 1 - the stock 47 tooth rear sprocket and exchange the stock 13 tooth front sprocket for a 12 tooth sprocket. This is the least expensive choice.  If you like the results the next time your due for a chain and sprockets I'd 2 - retain the stock 13 tooth front sprocket and install a 50 tooth rear sprocket.  A 13/50 combination will require a 110 link chain.  This modification will allow you to better use the power available in stock form by reducing the gaps between 4th, 5th and 6th gear, with only a slight drop in top speed. People riding in heavy load conditions such as hilly areas, sandy conditions, mud etc will find this modification particularly useful. The stock 47 tooth sprocket works well with stage 2 or higher modification so put it in a safe place if you plan to continue customizing your engine’s performance to best meet your needs.

STAGE 2

Replacing the stock expansion chamber with either a FMF or Pro Circuit expansion chamber is the single biggest bang for the buck modification you can make. If you have access to a mig or tig welder you might also consider modifying your stock expansion chamber.  You can find detailed instructions here.  To say it really wakes up a KDX is, well, an understatement.  

Step One : Both FMF( model K-14 ) and Pro Circuit ( model PK89200DP ) offer a MX pipe that boosts power across the rpm range.

Step Two: Now that exhaust gasses are flowing better you need to turn your attention to the intake side. Step Two is a freebee. Remove or modify the restrictive air box lid. See: The Air Box Lid, Free Frisbee Included, at the end of this section.

Step Three: Installing a set of Boyesen reeds will improve bottom end throttle response and help extend top end power. I am talking about the reeds alone not the RAD Valve. I would recommend the Power Reeds ( product number 607 ). They’re less expensive and last longer than the Pro Series reeds. See Boyesen reeds vs. RAD Valves at the end of this section.

STAGE THREE

Step One: RB Designs is an Oregon based machine shop that is performing some very impressive modifications to motorcycle carbs. They bore the KDX carb to 36mm, install a venturi divider in the exit side of the carb, modify the jet screen, clean up the pilot circuit's bore, resize the throttle valve's cutaway and rejet the carb.  The results are outstanding and its become one of my favorite mods.  The off idle carburetion is excellent with clean crisp throttle response.  You can easily lug the bike in a gear higher that you could normally and chug your way through the slickest conditions by almost eliminating unwanted rear wheel spend then when things oven up a little off you go.  The bottom end becomes very 4-stroke like.

STAGE FOUR

Step One: Cylinder porting. A number of companies offer cylinder porting to redirect and increase the engine's power. What they’re basically doing is changing the exhaust port timing and reshaping the transfer ports to meet the owner's individual needs.

Step Two:  Head machining. The squish band can be adjusted to redirect the engine's power. The head can also be machined down to increase compression. Modifying the cylinder head may  require the owner to switch to race gas so be sure to let you tuner know what your requirements are.   For a  better understanding of the  advantages of cylinder porting and head modifications check out what Eric Gorr has to say. 

A tuner will often  port the cylinder and machine the head to achieve  well balanced package.

Tuners with proven experience modifying KDX cylinders and heads include Fredette Racing Products and  Eric Gorr’s Forward Motion Racing.  Both are very customer oriented shops and are happy to answer your questions over the phone. 

STAGE FIVE

Step One: Cylinder boring. The cylinder can be bored to larger size, replated and an oversized piston installed. Two popular sizes include a 225cc and a 240cc kit. If you are interested in finding out more about 225cc and 240cc kits I would recommend contacting Eric Gorr’s Forward Motion Racing. They’re a one-stop shop and offer everything from boring to cylinder repair, replating and over sized pistons. He has an excellent reputation and is perhaps the best shop in the US for your cylinder repair and replating needs. I have heard nothing but positive feedback from his customers. Check out Eric Gorr’s website.

STAGE SIX

Step One: Replacing the stock silencer will result in a modest increase in performance and a meaningful reduction in weight. I would recommend a woods friendly spark-arresting silencer. They’re quieter than the free flow silencers and are "Forestry Legal" for those riding on public land. If you need to run a spark-arrester look at the FMF Turbine Core Spark Arrestor ( model SA-752 ) or the Pro Circuit Spark Arrester (model SK892000DSR) among others. If a free flow silencer is more to your liking consider the FMF ISDE Quiet Silencer (model 7-52) or the Pro Circuit (model SK89200D-304) among others.  Many people have commented that they are surprised to see installing an aftermarket silencer all the way down at Stage Five, its a very popular mod.  The stock silencer performs quite well and the average rider wouldn't likely report a meaningful difference in performance after installing and after-market silencer.  Its just not a very big bang for your buck performance modification.  

For jetting recommendations click here.

For pre-1989 recommendations check out what Jeff Fredette has to say here.

THE AIR BOX LID, FREE FRISBEE INCLUDED

The engine can’t breath properly with the stock air box lid in place. It is just too restrictive. You have two choices. You can either pull the air box lid and Frisbee it into the back corner of the garage or modify the stock air box lid for better flow.

REMOVING THE AIR BOX LID

Removing or modifying the air box lid will increase the level of noise emitted from the engine's intake. If you would like to keep noise levels to a minimum then modifying rather than removing the air box lid is your best bet.

1989 to 1994 KDX200

Removing the air box lid will require some 1989 to 1994 KDX owners to relocate their voltage regulator. 

1995 to 2003 KDX200/ 1997 to 2003 KDX220

Kawasaki seems to be in denial over the wide spread modification of the 1995 to 2001 KDX models and has chosen to mount the CDI to the air box lid. Removing the air box lid will require these owners to relocate their CDI. Both Fredette Racing Products and A-Loop make CDI relocation kits.

MODIFYING THE AIR BOX LID

Modifying the stock air box lid involves removing the snorkel and drilling a number of inch / 1 cm or larger holes in the flat section of the lid just  behind the snorkel opening.

When I originally removed ( then later replaced and modified) my air box lid I was concerned that the bike would become prone to drowning out at river crossings. It seems my concern was unfounded. To date I haven’t experienced any problems. 

Boyesen RAD Valves vs. Boyesen Power/ Pro Series Reeds, It’s Your Money.

I’ve used the stock reed valve with both the factory and Boyesen Power reeds and the replacement Boyesen RAD Valve in my 1989 KDX200 and again in my 1998 KDX220. The same RAD Valve will fit both generation of KDX.  After some playing around I have come to the following conclusion; when installed in the KDX the RAD Valve does not offer a worth while performance increase over the Power or Pro Series Reeds for the average rider.

Boyesen says, "RAD Valves are designed to correct flow problems associated with today's angled carburetors. The angled position of the carburetor, relative to the cylinder, causes an uneven charge distribution and a loss of flow velocity to the intake ports of the engine.  I believe that this is a true statement but expect the results differ depending on the design of the intake system.

If you’re looking to get the best possible performance increase for the money then the RAD Valve just doesn’t make sense. Installing Boyesen reeds will increase the intake velocity at low and high rpm by better controlling the incoming charge of fuel and air. Its nothing earth shattering, but you can feel the results when you ride the bike and you can see the result when rejetting after installing the reeds. A KDX220 with an over-bored carburetor, FMF Rev pipe, performance silencer and Boyesen Power Reeds required exactly the same jetting as the RAD Valve equipped engine. To the average rider the two will perform almost identically. So do you spend the $160 on a RAD Valve or $30 on Power or Pro Series reeds?

I would recommend skipping the RAD Valve, and install the Power or Pro Series reads. If you’re still looking for a performance increase spend the money you saved on something else that will result in a detectable increase in engine performance. Best of all you won't be asking yourself " where did that $150 go? "

For what its worth, that’s my opinion.

David

If you have any comments or suggestions smack the e-mail icon and send them my way or visit the JustKDX Forum.

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