Air cooled

Discussion for swapping a KDX motor in a MX frame...
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KDF200X
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Air cooled

Post by KDF200X » 11:30 pm Jan 23 2019

Hello all
Let me start by apologizing for the run on sentences.
I am currently in the middle of a hybrid build. I'm putting an '86 Kdx200 into an '04 Kx250f.
Currently the engine is in the frame(full engine rebuild year and a half ago); having milled the shifter side case half, faced both swing arm baring sleeves, fabricated new top and front engine mounts (the mount locations were close enough that nothing was relocated only new mounts made), utilized v-force 3 reed cage assembly, a late model Kdx200 intake manifold, Lectron carburetor, air boot replacement consisting of 2.25" I.D. intake hose/flared tube welded to air box-backer plate.

Still to do is; finish the header (nearly done), mount the silencer(new DG)/fabricate and weld mounts to sub frame, fabricate and weld on lower engine mounts, turn support rod for radiator shroud

Back story
I started riding mid summer 2015, my neighbor and friends had ridden for years beforehand. When i got my Kx450f other people in my family started getting bikes. My cousin got the 04 250f. I loved this bike. It made me curious about smaller displacement bikes. I ended up with the 86 kdx200, and loved the power plant(was willing to rebuild after less than 20 of its final hrs.) but the bike was still a $500 bike(most everything is WELL worn). To round this off I ended up buying the 250f from my cousin.
Anyways the 04 is a first year with first year issues(this particular example has gone through at least 3 heads in the last 2 owners), and the 86 is a fresh engine with a well used 33 year old everything. So I'm risking the beautiful handling, balance, and feel of the 250; as well as shedding the sleek gracefulness of the Kdx, in hopes of something better than either.
'86 Kdx200/ '04 Kx250f - hybrid
'06 Kx450f

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Re: Air cooled

Post by KDXGarage » 11:59 pm Jan 23 2019

Good luck on the build. I have read a lot of people saying the 2004 KX250F was bad.

You sound like you have it going well with the build.
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Re: Air cooled

Post by KDF200X » 01:08 pm Jan 24 2019

From the sounds of it the '04-'05 Kx250f can be made to work with a fully built top end(new valves, different profile cams, seats, and I forget the details). However I wasn't going to be the one to put another head on it shortly after purchasing one.(I bought it mid rebuild for a set value, minus the price of parts and labor)

Apart from the engine the bike is ideal. I learned to drift a bike on that 250f. Power was incredibly smooth and predictable. Ergonomics/geometry felt right coming from the 450f. The seat, though narrow, dosn't feel like a foam 2x4. Side note, I am 6'3" ~340lbs my perception of bikes is vastly different from most, think power to weight ratio, and bike to rider weight ratio.(and always a high center of gravity)
'86 Kdx200/ '04 Kx250f - hybrid
'06 Kx450f

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Re: Air cooled

Post by KDF200X » 11:23 pm Feb 03 2019

Hello again
Today we did frinal assembly of the intake, and confirmed the drive train.
The air boot replacement came together perfectly, really tight to the frame and the shock but there's no rub. Chain was cut and tensioned.(I believe it was 117 link with the axle blocks ~2/3 of the way back) The clutch cable was good, throttle cable ran cleanly, the electrical hooked up and run in its final location.(the electrical has been cut back, no odometer - timer button - headlight/break light) Essentially the same electrical set up as the Kxf - series.(if I decide to add a head light I will add an externally charged battery supply due to the 6 volt system)

The header mounts were tack welded but the muffler is not mounted. It was mid 40's today but we had 18" of snow last week. There's now 3" of compressed snow on the back driveway, and almost a foot of heavy wet snow through the back yard and woods. The bike felt soooo balanced n light and slid out smooth and controlled on the corners. The snow on the back woods trails was soft like blow sand and heavy like wet sand. Unpassable for the first lap and doable in upper second gear for the second lap.(louder than $#!+, the neighbor heard it, came over and did some wheelies and kicker pop hops)

The gearing on both the Kdx and Kxf ran 48 tooth rear sprockets and there is a 50 on there right now. The gears feel so short, but it was awfully slick out. Going to wait for traction till I decide whether to change it or not.(the knobby on the rear has ~1/3 of its tread left)

The clutch worked well, I forget what brand.($50 eBay special, springs-plates-frictions) The clutch cable is a modified kx450f cable.(had a used one around and it had the length the others didn't) The new pads and rotors worked well despite no wear/break in and being packed with snow.(EBC sinterd pads and driven 'blade' rotors) the carburetor seemed ok but it hasn't been tweeted yet and the weather is a ways off from what I specified when ordering the Lectron.

All in all things are looking very good at the moment.
'86 Kdx200/ '04 Kx250f - hybrid
'06 Kx450f

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Re: Air cooled

Post by KDXGarage » 01:08 am Feb 04 2019

Thanks for the report!

Get a spark arrestor. That must have been super loud.
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Re: Air cooled

Post by KDF200X » 11:55 am Feb 04 2019

Very loud, I have the muffler (DG straight through) but no spark arrestor. I'm thinking about grinding the exposed welds off the end off the muffler and welding a short piece of aluminum tube to the end. So I can use a slip over and clamp on(set screwed) spark arrestor.(ORV licenses are great, but if you're not abiding the rules you're still violating the law. Please if you're not riding leagaly, please do, the last thing we as a dirt riding community need is a reputation that makes it easy for the public to write us off as a bunch of unresoposible hoodlums, regardless of how close to the truth it is) :yawinkle:
'86 Kdx200/ '04 Kx250f - hybrid
'06 Kx450f

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Re: Air cooled

Post by KDXGarage » 02:20 pm Feb 04 2019

You have the exhaust pipe, but no silencer or spark arrestor, correct?
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Re: Air cooled

Post by KDF200X » 11:07 pm Feb 04 2019

Header is fabricated and mounted, the silencer is here but not mounted(no mounting points yet), and I have not purchased a spark arrestor yet.
'86 Kdx200/ '04 Kx250f - hybrid
'06 Kx450f

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Re: Air cooled

Post by KDF200X » 09:21 pm Apr 15 2019

Hello again.
I have now got the exhaust squared away, it's ready to ride.
Cosmetically the only remaining concern is the Kickstarter. At the moment it rests on the side of the frame where the very top of the boot settles. This is to avoid the expansion chamber which runs between the cylinder and the frame/Kickstarter.
'86 Kdx200/ '04 Kx250f - hybrid
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Re: Air cooled

Post by KDXGarage » 10:29 pm Apr 15 2019

Congratulations!
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Re: Air cooled

Post by KDF200X » 11:07 pm Apr 16 2019

Hay all,
I got the chance to ride the bike today. The first thing that I noticed was that the real power of this thing is loooow in the rpm's.(new tire Atrax-TG4 100-90x19) It then pulls smoothly through the mid and high rpm's. I am waiting for a day in the low 70's to dial in the carburator fully.

With the rear sprocket up two teeth(19" rear wheel) it feels like the gears are incredibly short, first is almost unusable.(~8mph screaming the engine) The top speed feels similar to the '86(?low to mid 60's?), however the ergonomics are vastly different.

The balance was awesome.(coming from the kx450f the weight felt noticably lower to the ground) We got a ready to ride weight of 220lbs.(weighed on works pallet scale) It is clearly in need of the suspension being resprung, although I am hesitant due to the redistribution of weight and how it may effect the spring rates.(I resprung the 450 two years ago and it has proved to be great in the woods using stock valves and 7wt oil instead of 5wt)

This bike is very nice as is and I think with minor adjustments can be made spectacular; carburator tweek, sprocket swap, bark busters (I tagged both pinkie fingers on trees today)

That's it for my preliminary assessment. I will try to keep this thread informed and up to date. Ride safe everyone.
'86 Kdx200/ '04 Kx250f - hybrid
'06 Kx450f

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Re: Air cooled

Post by KDXGarage » 08:28 am Apr 17 2019

Congratulations.

Do you have any pictures?
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Re: Air cooled

Post by KDXGarage » 10:42 am Apr 18 2019

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Re: Air cooled

Post by KDF200X » 02:28 pm Apr 22 2019

Hello again
I was able to get out and do some serious riding with the bike this weekend.
Saturday we got out to Cedar Creek Trail head. The wether was ideal, thought the soil/sand was dry on top and quite dusty in places. We started out with the North loop(10 miles), it being one of our favorites. It proved to be better than usual(the woops weren't too deep/wide). The bike was very light and directable. The strange thing is getting into the habit of shifting again,(it's a ⅔ gear trail on the 450) using second through fifth.
Next we rode the South East loop, where conditions were sandy and the oncoming traffic was frequent. The thing has an amazing ability to hop into and around the walls of the trail in a corner.(old established orv motorcycle/quad trail~4'wide by ~1½' deep) Though the suspension is highly inadequate, it is very predictable.
To finish off the day we made a series of bad directional decisions and hit everything we had done earlier in the day .... while traveling in the opposite direction. All in all it was a good day. I wasn't leading the pack for long but man it was fun while it lasted.(leader of the day burned himself out romping woops going fast... no Sunday riding for him)

Sunday
That morning a buddy of mine said that if I was ready to go he'd be by in 25 minutes to pick me up and go riding. I jumped on it.
We went to Horseshoe, not a trail I'm familiar with but it sure was tight. This was first to third gear $#!+, in woops 10"-18" deep, in tails that are just wider than the bars. We didn't do a full loop. Maaaby 20 miles total, including our retreat.(probably 12 miles, although there were those clear stretches parallel to the road...?.) Not a terrible time, bringing up the tail end for the warm-up of the much more tallented.(again paced myself and didn't burn out my legs)

My takeaway is the bike seems solid at the moment, still in need of suspension(soon hopefully). I'm going to wait until winter overhaul to refinish the frame.(powder coat)
'86 Kdx200/ '04 Kx250f - hybrid
'06 Kx450f

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Re: Air cooled

Post by Molly's 70 » 04:20 pm Apr 22 2019

I really like it! :bravo:
If you lend someone $20, and never see that person again, it was probably worth it.

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Re: Air cooled

Post by KDXGarage » 10:42 pm Apr 22 2019

Thanks for the ride report!

For now, turn the fork compression adjusters all the way soft.
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Re: Air cooled

Post by KDF200X » 08:30 am Apr 24 2019

The ride feels to soft already. I have ordered the fork seals and bushings already, need to order springs for the forks and the shock yet.
The suspension will be right on this thing, hopefully within a month.(group trip planned over Memorial day weekend to Michigan's Upper Peninsula)
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Re: Air cooled

Post by KDXGarage » 08:55 am Apr 24 2019

What is your weight in riding gear?? If you think it is too soft with MX valving, it sounds like you have two problems, just one you don't know yet.
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Re: Air cooled

Post by KDF200X » 09:47 am Apr 25 2019

Not sure on ready to ride weight, ready to work I'm 330lbs.(big burly man/no longer a potato)
I'm not very knowledgeable about the suspension aspect of the bikes. Suspension never fees right for me on any bike.(go figure) The 450 I keep referencing is Kawasaki's motocross bike from two years later, and I have adapted to the reworked suspension on that. That bikes forks were sprung for my weight and substituted a heavier fork oil during there first rebuild.(very close if not just right) The valving was untouched and responds well to aggressive riding. The rear was resprung(unsure of oil type), sag set and has been solid since.
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Re: Air cooled

Post by KDXGarage » 08:25 pm Apr 25 2019

Usually MX valving beats people's wrists and forearms to death.
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