250x/CRF 250

[Honda TRX250ex]

could i put a CRF or YZF 250 engine into my 250x frame?
how hard would it be to get the engine mounts right? and would it be faster on a track than a 350x engine

[zeppelin]

i love threads about this, i get to tell alot of the stuff i know from experience. okay so, first you are going to want to choose a motor, since you already have your frame picked out. imo i would get the yfz, it has the best powerband for a quad motor, it has lots of useable low end, the best mid-range out of all of the 250's and it has killer top end power. in compareson the honda's is a little sluggish of the bottom (not really too bad, but it is worse than any of the other bikes, and you'll need low end for your heavier quad), once you get past its low end, or lack of, it starts to get better. it has good mid, but not as snappy as the yzfs, its topend is where it really shines it is really smooth and easy to ride. next the kxf/rmz have the most low end in their class (other than the ktm, but that motor is gunna be hard to come by), then after you get past the bottom end your kinda just left there with no where to go, there int much of a mid range and no top end ( keep in mind this is compareing against the other 250f's if you compare the kfx's top end power to an old school xr it is killer), so you really need to short shift with it which imo isnt really fun. all of these motors will blow any 350x out of the water, aswell well as stock and slightly modded 400's. the frame mounts will definetly be one of the hardest parts, you will need to find out what angle the motor you chose was at while it was still in its dirt bike frame, and line up the counter sproket and try to put it in the same place in the fram as the stock moter for your quad. holy crap this is a long post let know if you have any more Q's about the motors (which is what i really know best because ive never personaly put together a hybrid, just helped friends with theirs.)

[Honda TRX250ex]

lol thanks
so the YZF would be the best choice.
if i put that into a frame how would i make the mounts like from sheet metal?
and would i have to do anything to the gas tank if the motor was to high up
Would i have to make a custom air box

and would it cost a lot of money not including the engine to do it and would it be worth it?

Originally posted by zeppelin
next the kxf/rmz have the most low end in their class (other than the ktm, but that motor is gunna be hard to come by), then after you get past the bottom end your kinda just left there with no where to go, there int much of a mid range and no top end ( keep in mind this is compareing against the other 250f's if you compare the kfx's top end power to an old school xr it is killer), so you really need to short shift with it which imo isnt really fun. Are you on crack cocaine? My KXF's have the best top end out of any bikes i've ridden. It keeps pulling and pulling and pulling. I like the Kawi motors the best out of all the 250f's.

[Honda TRX250ex]

Originally posted by MXFourTwoEight
Are you on crack cocaine? My KXF's have the best top end out of any bikes i've ridden. It keeps pulling and pulling and pulling. I like the Kawi motors the best out of all the 250f's.
i dont really trust RMZ or KXFs not a really big fan of them.

Originally posted by Honda TRX250ex
i dont really trust RMZ or KXFs not a really big fan of them. Why?

[zeppelin]

Originally posted by MXFourTwoEight
Are you on crack cocaine? My KXF's have the best top end out of any bikes i've ridden. It keeps pulling and pulling and pulling. I like the Kawi motors the best out of all the 250f's.

naw im just fine, but what you have is what is commonly known as omg-my-bike-is-so-slow-miosis, it comes from you riding behind everyone else all the time and breathing in there exhaust fumes, making you think that your bike is actualy fast, its most common in cr125 riders, but has been showing up in a few of the kfx guys around here lately :o .but seriously it looks like you race so you probably have some extensive mods to your bike and i was comparing stockers. as for the motor mounts honda trx250ex, call up some local sheet metal shops and see if you can get some 1/8 in aluminum plate. i doubt that you will have clearance problems, but if you do just drain all of the gas out of your tank and beat it with a hammer lol, seriouly, im pretty sure thats all you can do. as for the air box, that was the one of the other really difficult parts, from what i have seen on this site 400ex airboxes seem to be the most popular solution, but since the 250x and 300 have there intake threw the frame you may have to use your stock box and a different air boot, thats just one of those trial and errror things you have to figure out once you get there. and i honestly couldnt tell you how much the whole thing would cost, i think you'll probably find that everyones build cost varies depending on how much your willing to fab up your self, and how much aftermarket you put on it.

[ZeroLogic]

if your going to put a water cooled engine on it you will need a radiator hoses and the works btw

why bother? I mean if it was a 250r frame or someting then hell yea go for it. but in the end youll just gonna dump all your money in putting a 450 motor in a 250x frame and **** while you coulde just buy a stock 450 or a used modded 450

Originally posted by zeppelin
.but seriously it looks like you race so you probably have some extensive mods to your bike and i was comparing stockers Pro Circuit did my motors but I rode it with just a Pro Circuit Ti-4 Lowboy system and some porting for a while and it seemed like it pulled hard(baisicly) stock.

[zeppelin]

i believe you dude, the discriptions that i gave above where just exagerated a little, i was trying to show the extremes so that he would understand better.there is no such thing as a 250f that doesnt make top end power. all of that stuff came from riding a lot of 250f's and really getting to learn there power curves characteristics and when i ride the kfx i awalys tend to short shift it more because it makes so great torque it seems silly to me to rev it out.its the same with the honda, it does have bottom end power but there is just so much more when you rev it out that it would be stupid to chug it around on the track in fourth like you could with the kfx. with that being said porting on kfx's makes a HUGE difference, ive ridden a few ported kfx's and they pull way harder than stock

The porting does help alot,but the Titanium lowboy PC system made a huge difference low end. I don 't use 1st gear at all on the track, but if I'm just crusing in 1st like around the yard and I hit the gas, it almost flips. Its crazy.

[Honda TRX250ex]

ok i was thinking about this one too
a 250r frame with a CRF 450 engine would that work?

[ZeroLogic]

yep

[Hondadudeehhhh]

Originally posted by ZeroLogic
if your going to put a water cooled engine on it you will need a radiator hoses and the works btw

i bet you have never even sat on a kfx250/rmz250 let alone worked on one :rolleyes: . Trust me, i have worked on all of the four stroke 250s (besides 2006 honda) and the kfx/rmz are the most solid out of all of them.


As for making a hybrid like this it would be a dumb idea. And the 250r/crf450 hyrbid, it may fit but it doesnt mean it will work. It would be a tight fit because of the low 2stroke motor the 250r was designed for. Secondly the old stock 250r frame probably would put up with the abuse of a crf motor. They were the best made quad of their time but their time has passed. I don't see any reason to money into a hybrid at this point when buying a production quad is just as cheap/cheaper.

[rollie]

Originally posted by Honda TRX250ex
ok i was thinking about this one too
a 250r frame with a CRF 450 engine would that work?

no, a CRF engine will not fit into a stock R frame, the engine is too tall, you would have to weld higher top frame rails to make it work, a YZF engine works perfect though:cool:

[Honda TRX250ex]

does anyone have with hight and length diminions for a YZF 450 YZF 250 and a CRF 450 and a CRF 250

[kids on quads]

we do the 250 install all the time, we use the rmz motor because being dual cam its 3 inches shorter so it fits the frames better, if you want to see what one looks like go to kidsonquads.com, and press store, there are lots of pictures of one, also my son used one to win all three supercross a class championships in mi, and killed the 450's because the quad only weighed 285 lbs wet