can it be done ?

Rich Rohrich

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#2
Yes, if you have some basic fabrication skills.
 
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#3
yes ofcorse i do i wouldnt be thinking about it if i couldnt do the work i just worried about the size of the 250 being air cooled it is much bigger and might not fit under the tank and also the carb being at the top of the cylinder not at the botton like the 125 it will be a tight fit if it fits does any one know of any people that have done something like this or a web site that hase imformation i could use?
 
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#4
xr 250 mods?

hi i am looking to try and put a xr 250 motor in my 1990 cr 125 if i can i would like the bike to realy go hard so is there any inexpensive mods that will help the 250 and is there a year were the 250 stood out as a better performer than year befor or after ?
 

Rich Rohrich

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#5
The best web resource I have found for this type of thing is Michael Moore's Eurospares website:

http://www.eurospares.com/other.htm

The two best book resources I've found for bike builders are :

"The Racing Motorcycle - A technical guide for constructors - Volume 1"
by John Bradley (1996)
http://www.eurospares.com/racebook.htm

Tony Foale's book "Motorcycle Handling and Chassis Design"
http://www.eurospares.com/tfoale/book/book.htm


Here's some basic advice I got from a friend of mine who has built a number of special hybrid bikes:

The air boot /and airbox will likely be the biggest challenge.

Work out the best overall countershaft sprocket placement/alignment first -
then do the front and bottom engine mounts. From there you can make a
bracket to fit the swingarm to the rear engine mount. Being a 4 stroke it most
likely wont even be close to aligning with the swingarm pivot bolt.
You may have to rebuild or reform a good portion of the engine
cradle of the frame and part of the upper main "backbone " brace to the
headstay ( perhaps raise it a half inch or inch)





As you can see in the picture below from the Eurospares website, the XR can fit into some fairly unlikely places. What you are trying to accomplish is certainly possible, but until you start measuring things it will be tough to say how difficult it will be.

 
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jmics19067

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#6
A hybrid can be fairly difficult but definately time consuming. The project I am working on I started by Removing the rear motor mount/swingarm collars from the two engines and seeing if the one set from the original will fit into the transplant engine. set the engine in the frame with the swingarm and pivot bolt . check to see if the countershaft sprocket is going to be in the right place , lined up with the rear sprocket , and with the right relation ship for the swingarm pivot.Obviously the sprockets need to be lined up for he chain and you want the front sprocket to be extremely close to the original position or the rear suspension will be effected by chain torque. the closer you get the less chanse you will have a funky rear suspension that you will have to tune out. Then see how close you are and how much has to be modified.

I had to cut out the front of the frame cradle so I can drop the engine far enough for my sprocket to be in close proximity to where it should be and so the exhaust spigot would not be hampered by frame tubes. made a new engine cradle and mounts. had to make new mounts because the ignition system is worlds different.

I cut up a perfectly good fuel tank and modified it for my application.going from a four stroke engine to a two stroke I need room for a pipe. I am very uncomfortable at this moment with anything I know of to make this gas tank safe<haven't found a suitable way to piece back the plastic tank for my feel good factor for when I drop it> so it is only going to be mold for either a fibreglass or hand made metal tank. If anybody knows how I can safely piece together the stock plastic I am all ears.

I use the air box for the frame and half of the rubber air boot that I am hopefully going to be able to use rubber cement to glue to the front half of the air boot that will fit the carburator that part looks like it will be sufficient for my needs.


Currently that is the completion of the project I have yet to make a pipe< be thankfull you are going from 2 stroke to 4 in that regard> So now I have an old big bore 2 stroke engine setting in a frame that I can bolt on modern suspension onto that probably took me a month or two of "when I get a chance to work on it" time and I have yet to make a pipe and silencer combo that will work, fuel tank and an airfilter boot.

It is a fun project but I don't expect this thing to be a better bike than something I can actually go out and buy. good luck and have fun!
 
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#7
thanks guys luckly i was allready thinking about the transplant coz i just seized the 125 today it was a four piont think the jetting went nuts when i put the stock reeds reed block back in
 
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#8
The XR250 engine is no powerhouse. As a matter of fact, I'd put money on the fact that the CR engine (assuming it is a late model) makes at least 10-15 more rear wheel HP. In addition, the XR engine is not the most reliable engine when heavily modified. My dyno experience with XR250's has shown RWHP ranging from 14 all the way to 19HP, depending on conditions. Not exactly good in my book.

I would suggest a big bore kit and possibly a stroker crank for the CR engine. You could even add a flywheel weight to mellow it out some. That way you get the low end power you may be looking for, extra reliability and you will not have to spend a year fabricating.

Chris
 

marcusgunby

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#9
there is no way i would put a 19hp engine in a cr frame-its a waste of a very good chassis with the 30hp std engine.A well tuned cr can make 36hp at the rear wheel.
 
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#10
http://www.off-road.com/dirtbike/jhcrx.html

The pictures doesnt seem to work but they should be out there somewhere.

I've been thinking about it myself when i had my '93cr250 but then i realised the engine is the bikes strong part and there isnt any better engine.
 
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#11
my cr is 1mm over bored with a wisco piston i stripped it down not long ago and found out it had two base gaskits i took it into the shop and was told 2 put it back together with only one now it has just seized because the big end is worn and through the piston into the head and i dont have the money to keep gettin parts for it so thats why i was thinking about the transplant i would rather keep the cr as is but its costing too i no it will cost a bit to do the transplant but after it done its verry little how about the xr 400 or am i just pushing the limits too much i will try and ride a 250 and see how i like it i also think it would be a wast of time and money cutting up a good frame if the xr 250 isnt going 2 meet my needs
 
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#12
my cr is 1mm over bored with a wisco piston i stripped it down not long ago and found out it had two base gaskits i took it into the shop and was told 2 put it back together with only one now it has just seized because the big end is worn and through the piston into the head and i dont have the money to keep gettin parts for it so thats why i was thinking about the transplant i would rather keep the cr as is but its costing too much i no it will cost a bit to do the transplant but after it done its verry little how about the xr 400 or am i just pushing the limits too much i will try and ride a 250 and see how i like it i also think it would be a wast of time and money cutting up a good frame if the xr 250 isnt going 2 meet my needs
 
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#13
I have seen an write up on a xr250 cr conversion but cant find it right now. but did find this a xr200 cr at http://www.4strokes.com/articles/honda/cr200x.asp and http://www.4strokes.com/forums/topic.asp?TOPIC_ID=6700&SearchTerms=xr200 if i find the site that had the 250 I'll let you know. Some people like 4strokes some 2strokes but you cant buy a 4stroke with the suspension of a mx 2storke so some people build their own.
 
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#14
since I cant edit my own post I'll make another? The site Pyromaniac listed is the one I was thinking of to bad the pics don't work because it was just what you are looking for. His phone was listed you might give him a call. Good Luck on what ever you do.
 

MikeT

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#15
Make sure yu cut the back of the CR fender off so you can get proper air flow to cool the fins on the XR cylinder.