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  #1  
Old 10-24-2015, 01:07 PM
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Default My Gen5 CR250AF build

     

Hi, I am a new member and this will be my very first post here on atm!

After reading/drooling over doggers315's various threads, I decided it was time to stop day dreaming and start my own project.
I have always been fascinated by the 500, but the tracks around here are small and twisty. My current racebike, a 2009 CRF450R is a bit to much and heavy for the type of tracks I normaly ride at. So a 500 would be WAY too much I think.
So the CR250 engine sounds like the best choice of power.
On the chassi side I went for a gen5 450 frame. I know alot of people don't like the gen 5 chassi, but I love it. My previous bike was a 2005 CRF450R (gen4) wich i liked alot, but when I traded it for my current bike , a gen5 450, the 2005 felt like a tractor. Once I lowered the rear with a ProCircuit link and revalved the suspension I felt it was really good. And after reading that the gen5 chassi is even better with a 2 stroke engine I was sure.

To the build part:
I live in Norway, and it is very rare to find complete rollers. Even blown up/worn out bikes are expensive. So most of the parts for this build will come from ebay. I don't have unlimited funds for this project, so I plan to borrow some parts from my 450 while I hunt for the final pieces, like suspension, brakes, wheels and swingarm.
So far I have got a frame, subframe, airbox, gas tank, and radiators.
The radiators I bought from a guy on ebay that had used them on a simular build, so they will fit rigth on.

I plan to use the OEM ''Y'' by cutting it out and move it up to get clearence for the pipe. I have ordered frame rail extension from loudmouthmx.com that will fill the gap.
Rest of the engine mounts and fueltank adapter I will get from conversionparts500.com.

When it comes to the airboot/airbox mod I am not quite sure. Conversionparts500 sells a airboot for a 500 conversion that will fit the oem gen5 CRF airbox and a carb on the other end. I have e-mailed them and asked if this airboot will fit with 250 engine, but they could not confirm this becaus they had not tried that combo yet.
But if the carb location is about the same on the 500 and the 250 it will fit. So if anyone have any experiance with carb location on 500 vs 250 that can confirm this, I would be thankful.
If this airboot fits, I dont have to mod my airbox.

The next on my shopping list is an engine. I plan to buy a used 2000 or 2001 bottom and overhaul it with possible a new crank. The cylinder will be a new 2001, if I don't get a super deal on a used good top end.
I might get a 97,98 or 99 bottom end if I can't get hold off a 00 or 01.

This bike will be mostly stock to begin with.

Here a pic of the frame with the ''Y'' removed.
Dok1.docx
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  #2  
Old 10-25-2015, 09:38 AM
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Sounds like the start of a cool project.

You are encountering some of the same problems I did by building some
thing no one else has done. In your case, I can't think of anyone that has
used a CRF450 frame with a CR250 engine.

The problem with the airbox/boot position is the carb on a CR250 is lower
and farther away from the box than the carb on a CR500. So no, the two
are not the same. Best bet is to purchase the faceplate from CP and the
longest airboot you can find (97-99). Once you have the engine mounted,
you can mock up the airbox and boot and arrive at the optimum postion for
the faceplate. That takes all the guess work out of the equation, and is how
I did mine.

As for the engine mounting, that 450F frame will come with more room around
the Right side case cover which should drastically reduce your workload. As
for engine mounts, you'll probably have to fab up your own because I don't
think CP or anyone else sells them, especially the fronts.

A decent 2000 or 2001 CR250 donor engine shouldn't be too hard to find on
EBay. I have built some nice engines by buying just a complete bottom end
assembly off of EBay, dismantling it and loading up new cases with the used
parts. You can save a ton of money doing it that way and still have a stout,
reliable engine. Same goes for the top end. By the used cylinder to get the
cylinder head and power valve components. Then purchase a new cylinder
from Honda and load it with the used parts. One thing to consider, if you
plan to go wild with porting some time in the future, purchase a 2000
cylinder. It has much more porting potential than a 2001.

Feel free to pm me with questions as you go, I might be able to keep you
from making some of the mistakes I did.

Have fun and keep us posted.
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  #3  
Old 10-25-2015, 07:30 PM
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Yes dogger has set the bar high for us. Sounds like a nice build, personally I would give the gen5 250f chassis a look over. That being said the 450 suspension has more potential with the KYB components.

I have a spare mostly new 2001 CR250 motor kicking around and a couple used 2000 units.....

keep us posted on the build!
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  #4  
Old 10-26-2015, 04:26 AM
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Dogger:

Thanks for the info on the carb situation. Yes I guess I have to go the down the same route that you have. But I can't find anyone making a adaptor plate for the gen5 airbox, so I have to make my own from scratch or try to adapt one from a gen4 airbox (like the one you used). But I have to investigate further if it is possible to use the gen4 adapter plate. Did you use airfiltercage and airfilter from a CR250 or CRF250?

CP500 does have the lower engine mounts for a gen5 with 500 engine, but not the front mounts. I was going to use the CP500 lower mounts for the front mounts too. Don't you think this will work?

I think that down the road I will do some porting to the cylinder or maybe a 265 big bore kite. I am not sure.
I don't want a monster engine with a massive hit, but a easy to ride with a wide powerband.
I was going the same path that you where thinking about the carb. With a Kawasaki or Yamaha PWK carb With TPS and powerjet, but after reading about your 36/39 intelajet setup I am going for the same setup.


Seanmx57:

Yes he has set the standard for sure!!. This build will not come close his ''factory'' machines.
I agree on the KYB. That was one of the resons I went for the 450. I have worked alot on the KYB suspension on my 450.

When I get the chassi/engine setup working well, I plan to do some mods to the suspension. Maybe RaceTech valves, DLC coating on the lower legs and MX Tech Huck valves. But that is way in the future for sure.
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Old 10-26-2015, 09:55 AM
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Interesting that CP doesn't market a faceplate for the gen 5s. I don't know
how you're fixed for equipment, but if you have the ability to fab up a face
plate from scratch, that would probably work best. Use 1/8" 6061 Aluminum
plate and cut it with a band saw or similar. I'm not sure if a gen 4 faceplate
can be adapted since I haven't done any gen 5s. The reason I recommend
fabbing one from scratch is this will allow you to cut the airboot opening and
the mounting holes in the best location for your specific needs.

I did a post on what went into my airbox/boot combo a while back. I used a
2005 airbox, 97 airboot, 2000 filter base plate, CP faceplate and a Twin Air
base plate retainer. I use 97-01 air filters. The airbox/boot was a big concern
early on in my build. Sean and I spent a lot of time hashing out ideas including
the possibility of having to have a custom airboot cast or using carb spacers.
In the end, the combo I listed above fit perfectly.

The problem with the CP engine mounts is they are designed for the CR500
engine which sits differently in the frame compared to the CR250. You may
be able to get by with the lower mounts since you aren't changing out the
cradle, but the front mounts will have to be custom. The front mounts on
the 250 engine will be further away from the rails compared to the 500. I
fabbed up two piece mounts for my build. I used 6061 angle stock to provide
offset and a larger weld footprint for the base and then 1/4" plate That I cut
using OE mounts as a pattern for the shape, only bigger. I mocked everything
up several times to make all the mods to get the engine right where I wanted,
then welded the mounts together and then to the frame. I have some pictures
of the process in my build post.

Like I said, this has the potential to be an interesting project, but it will be a
handful with all the problems you'll need to work out. I think I spent close
to a year just working out the details before I even started on the actual
fabrication.

You can thank Sean for that super cool taper bore carb set up, he's the one
that turned me on to the idea. Works way better and is much less complicated
than the "electric" PWK.
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Old 10-26-2015, 04:02 PM
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Quote:
Originally Posted by dogger315 View Post
I did a post on what went into my airbox/boot combo a while back. I used a
2005 airbox, 97 airboot, 2000 filter base plate, CP faceplate and a Twin Air
base plate retainer. I use 97-01 air filters. The airbox/boot was a big concern
early on in my build. Sean and I spent a lot of time hashing out ideas including
the possibility of having to have a custom airboot cast or using carb spacers.
In the end, the combo I listed above fit perfectly...
Good info, thank you. Any idea how much longer the 97-99 air boot is versus the 00-01?
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Old 10-27-2015, 08:37 AM
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I don't know the lengths off hand but the 97 and 98 used a short body PJ
vs the longer body PWK used on the 99-00, so the 97-98 will be the
longest followed by the 99 and the 01. The 00 is the shortest.
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Old 10-28-2015, 03:07 PM
Faded's Avatar
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Thanks for the reply. That makes sense, but when you go to the parts fiche it shows 98-99 being the same, and you can't even get a 97 anymore. Before you responded I jumped the gun and ordered a 98-99 so we'll see when it gets here. I've located a used 97, but it's an hour away and I'm not sure when I'll have time to go get it.


1997 - 17253-KZ3-B00 - OBSOLETE
1998 - 17253-KZ3-J00
1999 - 17253-KZ3-J00

***I should mention that my set-up is an 01 engine, 02 frame, 02 air box, 00 cage/boot, 04 KX250 Carb (the e-PWK) with TPS/Power Jet delete. With this I'm about 12mm short.***

Sorry to threadjack, sounds like it will be a great build and I'm glad I'm subscribed so I can follow it. Good luck.

Last edited by Faded; 10-28-2015 at 03:13 PM.
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Old 10-28-2015, 05:46 PM
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I know Sean (seanmx57), took the time to measue a bunch of different year
airboots and listed the results on another website. If I remember correctly,
the 97 was the longest. I know the 97 boot is NLA, but the 98 boot is real
close in length. That "e" PWK has an appreciably shorter body than the
standard PWK AS. IMO, if you stick with the "e" you're going to have to
space the boot or the carb. If you opt for a carb spacer (between the cyl
intake and the reed block), you'll gain some low end power at the expense
of some top. Spacing the boot is a PITB due to mounting and sealing issues.
Most of the mods to address this that I've seen use a machined phenolic (or
similar material), spacer 5-10mm in depth. The problem is the filter base has
to be modified by removing the mounting studs and drilling holes to mount
through bolts to accomodate the increased depth. You will also lose the seal
between the airboot lips and the faceplate using a spacer unless you go high
tech and machine a spacer that incorporates a faceplate style flange.

For the sake of expediency, I would ditch the "e" in favor of a conventional PWK,
follow that with a VForce or Rad Valve for the extra spacing, and a 97-98 airboot.
This combination should bolt right up without any drama.

As for thread jacking, IMO this is useful information for the OP or anyone else
following this thread.
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  #10  
Old 10-28-2015, 09:10 PM
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I compared a gen 1 CR250 boot to a gen 2 CR250 boot to a steely CR500 boot, 89+. I never didn't any comparisons between 97, 98 and 99 boots. I figured they were the same as the motor and subframe didn't change that I know of. The gen 1 is the longest. The #'s are somewhere on CR500riders.com in the AF section.
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