89 KDX/ 03 KX Hybrid

Discussion for swapping a KDX motor in a MX frame...
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Mario
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89 KDX/ 03 KX Hybrid

Post by Mario »

I finished my hybrid project earlier this spring and not without a lot of help of this forum. In return I wanted to share my finished product for all to see.
The bike was build for enduro races here in Montana.

2003 KX 125
5' 11" 150lb fast C rider
13t/50t sprokets
.4 front spring
4.6 rear spring, Racetech valves
1989 KDX 200 engine
LA sleeve 240 kit
Wossner piston
RB Head, increased compression and quench re-work
RB Carb (40 pilot 152 Main, 5000ft, 91 octane, Belray H1R 40:1)
BR8ES, Timing at 0 deg.
Boysen Reeds
89 KDX FMF Fatty
2001 KX 250 Pro-Circuit Silencer

212 lbs Ready to ride W/O fuel

Build notes
Weight reduction was a priority. All unused tabs and bosses were removed from the frame. Even removed the light coil from the stator plate. Wiring was striped down, Don't kid yourself all those ounces add up.
I use a non-oring chain and cheap aluminum sprocket, firstly for weight reduction and less parasitic loss, secondly for cost. Aluminum sprocket has lasted almost a season, and cheap bikemaster chain lasts halfway through the season. I remove the chain after every ride and clean in solvent and soak in oil. This is the key to making a cheap chain last.
Kept the stock tank at 2.2 gal, after a 50 mile enduro still had lots of fuel left.
I did not add the KDX hoops, as the pipe serves the same purpose. To me the pipe is a sacrificial item and can be bent back if damaged. Some might disagree. Due to the fact I used the old E-series pipe, it sticks out a lot on the bottom and is prone to getting hit.
I used the E-series Fatty pipe because the motor responded so well to it. Changing from stock 89 pipe to the fatty made the motor come alive. To fit the old pipe on the new frame I sectioned it into 3 parts and rotated each piece to find a combination that fit, then welded back together. I wanted to make sure I did not change the chamber characteristics of the pipe. It gets beat up a lot, I am always bending it back. I drilled a hole in all the rubber mounts and put a bolt through them so they won't separate. Also the Fatty is thinner and lighter then the gnarly significantly so.
Enduro Races require spark arrestor, so I used a pro circuit pipe found on ebay for a 250. Even with an arrestor the bike is not quiet, barely passing sound check. It must be the compression.
I really like the Wossner piston, Nice coating on the skirt, better out of the box (no burrs or swarf like wiescos) cheaper then Wiesco too and seems to be tolerant of the dead cold starts of an ISDE format race.
Instead of a bolt on skid plate I wanted a smaller, lighter version so I took some 22 guage steel, put dimple dies in it and welded it to the frame between the 2 down tubes. This not only serves to protect the case but also adds a strong shear plate to the frame tubes, plus only weighs a few ounces. The jury is still out if this makes a difference in rigidity or feel. If I had bought the steel it would only cost pennies plus it doesn't rattle.
I used the KX 125 kicks starter cause its light. With the increased compression it takes a nice "jab" to start it. Not as hard as a 4 stroke though and in cold start races it is no problem. Worth the trade in weight reduction.
I used the KX air box boot, cut the end off of the KDX boot to make a rubber ring to adapt the smaller carb to the bigger kx boot. Not the best way as it is a pain to get it all together with out the extra rubber squishing out of place. This could be improved.
Front brake line is -3 an braided with steel -3 banjo hose ends. $30 total from summitracing. I like this because the brake line is repairable in the field. Heat shrink on the braided line keeps the stainless braid from wearing into plastic.
E-series motor fits really well in the 03+ frames. Plenty of gas tank/spark plug clearance.

Initial Problems:
The first iteration of the motor had vibration issues. I had the crank bearings changed and balanced and this went away. I also had problems with the cylinder coming loosing and moving around on the crankcase. After a ride the nuts would be loose and the studs fatigue broke upon tightening. Upon inspection I noticed that the E-series cylinders do not have a locating dowel so all that keeps the cylinder from moving is the 4 studs. I initially thought that the motor made too much power and the 4 studs were simply not enough to hold the cylinder in place. When I put the motor back together I stiffened the cylinder head mount,(no spacers) replace all 4 studs, used a OEM base gasket, and re-torqued all 4 studs after initial heat cycle and checked torque very often. No problems yet and the nuts stay tight. All said and done I feel like I am approaching the power limit of this KDX motor. I haven't had any problems yet with the motor failing but the increased power definitely decreases the KDX reliability. The ol'e saying "double the power, square the wear" surely applies here.
I have also had head gasket sealing issues, specifically coolant leaking externally. The big bore kit requires a standard gasket with fire ring that cometic makes. There stamp die is not very accurate and all the holes need to be enlarged to fit over the studs properly. This is done tediously with a round file due to the steel core in the gasket. A light coat of copper spray and clean flat surfaces remedied the problem.

I am super happy with the finished product. The motor has tractor like torque on the bottom and yet the KIPS allows it to pull up top. This engine combination works really well. Riding a modified 200 back to back with the 240 there is no comparison, the 240 has tons more torque. You almost can't stall the bike. The separator plate in the carb also contributes to the crisp power down low. The bike is lighter then any KTM or 250 race bike, really makes a difference in the races when your tired, plus its easier for my tiny stick-arms to handle.

Hope this information is of value to the readers.
What can I say I LOVE my hybrid.
Last edited by Mario on 02:02 am Jul 31 2014, edited 4 times in total.
Mario
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89 KDX/ 03 KX Hybrid

Post by Mario »

Here are some pics on photo bucket, if I can figure out how to post them I will.
http://s1189.photobucket.com/user/mario ... X%20Hybrid
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marrk_us
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Re: 89 KDX/ 03 KX Hybrid

Post by marrk_us »

Nice! I still prefer the E-series engines. Run good pre-mix and the kips won't give you any troubles. I can only imagine how good mine would be if I weighed 150 lbs!!
Mario
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89 KDX/ 03 KX Hybrid

Post by Mario »

Its alittle dirty...not sure I should feel ashamed or not....

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bufftester
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Re: 89 KDX/ 03 KX Hybrid

Post by bufftester »

Nice build, though I'd disagree on the pipe being sacrificial especially since you have to section and weld it to make it work. The pipe does not do the same thing as the hoops regardless of what you've read/been told. Doubt you would notice the extra weight there, for me the peace of mind would be worth it. like the welded glide plate idea, certainly gets rid of the rattle that comes from bolt-ons (you did drill it for oil changes, right?)
Mario
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89 KDX/ 03 KX Hybrid

Post by Mario »

Here is a better picture of the skid plate. Seems to be working as I see some dents. Ya the hoops would protect the W/P better then the pipe would for sure. For me the pipe is easy to make and repair. Its already been bent and bent back multiple times. I happen to work in the fab industry so its easy for me.
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bufftester
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Re: 89 KDX/ 03 KX Hybrid

Post by bufftester »

That looks good, bet it really stiffens up the lower portion of the frame.
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