Yz250 conversion?

Pony · Post by **Pony** » July 24th, 2012, 7:09 pm

What's up guys! Havent posted in a while but still lurk quite often. I got an RD400 and have kinda been messing around with that for a bit and have been doing a little dirt bikin'. Coming off a separated and dislocated shoulder injury right now. Anyways glad to see this place is still awesome.

My 05 yz250 is cutting out and running like crap. I don't feel safe jumping it and I snagged a low hour 99 and it has proven to be a sweet fmx bike and good enough on the track and all the other play riding I do. I was planning on selling the 05 because money is tight and I could use it. But I got to thinking about my box of 500 parts and I think I really want to step up to the ultimate dirt bike.

I set the cases in the frame and I was surprised at the fit of some of the mounts. The bottom mount fit perfect but the the motor was too far back for the swingarm pivot was too far back. When I tried to line up the swingarm pivot the motor would not fit into the cradle because of the front mount. I figure it is a possibility to fit the motor in with new mounts but I wouldn't be able to tell without cutting up the frame. Plus the clearence between the head and the frame would be super tight. I'm thinking that the only way to make it work would be a new cradle.

Has anyone done a yz250 2t conversion? If it was not a nightmare an I had a little direction I would go for it. I love my yz.

Would it be better to just sell the yz and get a gen 3 cr? I am not trying to be lazy but I have enough other work in progress bikes that satisfy my need to tinker and I just want a af 500. The yz already has the suspension set up and I have a lot of spare yz parts.

Thanks guys, just looking for some insight.

NightBiker07 · Post by **NightBiker07** » July 24th, 2012, 8:19 pm

Well, if you already have the bike set up for your riding preferences, do the YZ. Most bikes require the cradle to be replaced for a 100% OEM looking job anyway. You MAY be able to jack that frame open a little bit, but I dont think that much would be safe.

If you have the tools and skills, and like how the YZ handles, go for it! It looks like it fits in there pretty good already!

Post by **AlisoBob** » July 25th, 2012, 8:11 am

I remember a few RM250 conversions, but not a YZ.

Go for it!!

http://www.youtube.com/user/500bigbore · Post by **Roostius_Maximus** » July 25th, 2012, 9:26 am

I'd posted pics of a 2006 or 7 with white plastics and a cr5 mill, looked good.

Pony · Post by **Pony** » July 25th, 2012, 10:45 am

Roostius_Maximus wrote:I'd posted pics of a 2006 or 7 with white plastics and a cr5 mill, looked good.

Do you have a link to these pics? I have searched both sites and have not been able to come up with much.

http://www.youtube.com/user/500bigbore · Post by **Roostius_Maximus** » July 25th, 2012, 10:47 am

I looks don my skydrive and can't find them either, I'll get them found and loaded again.

cokesdog · Post by **cokesdog** » July 25th, 2012, 10:02 pm

if i new how to post a picture i would show something

Pony · Post by **Pony** » July 26th, 2012, 6:55 am

Upload them on photobucket, or you can email them to me and I'll post them up. tsh336@hotmail.com

Pony · Post by **Pony** » July 26th, 2012, 11:30 am

After finaglin' the cases in this frame I am having a hard time figuring out exactly how I would attack this. I have been looking at othe conversions and 4t cradles seem to have more room and it is more moving the "Y" than anything. When bolted together at the swingarm pivot it seems to be lined up perfect although you wouldnt be able to tell until the bike had a countershaft sprocket on it. This is very encouraging knowing that chain alignment wouldnt be an issue. But the cradle just doesnt seem big enough. I am curious if you cut out the "Y" and the front of the cradle you would not be able to follow the line of the front down tube and would look and probably hold up like shit.

This has to be an issue in some other builds, I just have never done one of these before. I guess I need to look at some 125 builds, seems like they may face a similar problem.

Any of you guys have any opinions on how to open up the cradle and keep it clean?

Pony · Post by **Pony** » July 26th, 2012, 5:54 pm

I know I am posting faster than ya'll are seeing this but did some more moving around and believe that I have it in there the best way, but it is tight. I am not sure be might even have to have removable mounts on one side but I cant tell about that yet.

All these pictures are with a bolt through the bottom mount. According to my calipers I would need to make a cut behind the bottom mount and the motor needs to come forward .65 of an inch to get the swingarm pivot in. Cut and relocate from mounts and it is bolted in.

Set up like this, exhaust looks like it would be perfect. Only problem I could see is possibly pipe springs being tight.

Motor is in there tight. I belive the motor mount on the head would need to come off and become smaller. Maybe fill the bolt hole grind it smaller and drill a new hole.

Bottom mount lines up, relocate front mounts. The whole cradle needs to come forward .65 of an inch. Maybe make a cut above the Y (not all the way through) cut the cradle behind the bottom mount and push the frame forward. that would close the cut above the Y up and then you could weld it shut. What do ya'll think?

This looks to be a problem to me. If you cut the cradle behind the bottom mount the rails angle back into the frame. I dont know how you would properly re-attach the frame and it be as strong as before. Do you fab guys think this would work?

Regular spark plug works and it looks like the radiators are already plumbed to work, just block off one of the head spigots.

The YZ250 carb would not fit. Im hoping that with the motor over 1/2 an inch forward a PWK would fit. I belive the YZ carb might be taller because of the electrical. I dont know though.

The tank does not fit though. It sit on top of the motor. Needs to come down maybe half an inch.

I think I have an idea though. If I heated up the inside where I wanted it to stretch, and the warmed it a little on the outside I might be able to pry the tank enough to sit around the motor. Just make some of the lower part of the tank a little bow-legged.

Ok guys I have some decent pics uploaded, shared a little bit of what I think are going to be some of the major hurdles. Ill be honest, this thing looks pretty damn do able. I am just worried about getting the motor in there without pulling it apart. It was tight assembling it in there and I didnt have a crank or piston to clear.

I know ya'll would LOVE for me to jump into this because as I have looked around I dont think a YZ conversion has really be documented. I am not about to break out the sawzall but I wanna figure out if I think I am going to take this on. I was planning on selling this bike to finance a ring for my girlfriend. If this looks like a fairly straight forward conversion though this might be my last money pit before I spend a year recovering from wedding expenses.

How would ya'll get the motor forward and keep it as clean and factory as possible?

other · Post by **other** » July 27th, 2012, 1:09 am

Looks to be about as tight as the equivelent Gen 4 Honda AF conversion. Certainly there ain't much room on those either ! If it was me doing it, and accepting that the cradle needed modding, I wouldn't concern myself with the lower mount, I would set the whole thing up on the swing arm bolt and chain alignment, and then the other mounts are really easy to fabricate or move . . . just my 2c worth. . . .

Post by **AlisoBob** » July 27th, 2012, 8:05 am

1. Set bolt for swing arm pivot. adjust for chain alignment

2. Check pipe fitment ( especially pipe outlet position. Clock motor as needed)

3. Check airbox fitment. Close is ok. Pipe fitment is more important.

4. Mod / Replace cradle as needed.

Pony · Post by **Pony** » July 27th, 2012, 8:43 am

AlisoBob wrote:1. Set bolt for swing arm pivot. adjust for chain alignment

2. Check pipe fitment ( especially pipe outlet position. Clock motor as needed)

3. Check airbox fitment. Close is ok. Pipe fitment is more important.

4. Mod / Replace cradle as needed.

Sweet. I did come up with another set back later last night. The motor will have to come forward to bolt the swingarm pivot. Like I said before it will come forward a little more 1/2 an inch. You can see how much I am off currently.

I pulled the carb off my 500 and it does not look good. I pulled my blown shock without the spring on it and it looked even worse. It does not look like the carb would clear the frame enough or the shock spring even when it is moved 1/2 an inch forward. I do not want to get into modifying the backbone of the frame, that just doesnt seem like a good idea to me, and way over my head.

Another question I have run into is how would I move the cradle forward? The backbone frame is not as tubular as the perimeter frame on the honda looking at this pic could you see any way to move it forward enough to solve the clearence problems in the back without having it "hump" out in the front. I am not a fabricator is why I ask. I love how clean and factory the Honda conversions are and if this looks like some cbr motor crammed into a hill climber Im not into it.

Unless I am missing something, looks like I am better off trading this bike for a gen3. What do ya'll think?

homenf · Post by **homenf** » July 27th, 2012, 9:30 am

Bummer.
Seems weird it's that tight at both ends of the engine. If you did mess with moving the cradle to fit, how would the pipe fit be with the front forks bottomed out?
I had to add a fair pocket to my pipe (gen 3) for the front tire to clear and my cradle ran in the same line as stock.

cryzsurfer · Post by **cryzsurfer** » July 30th, 2012, 3:59 am

When I stuck a YZ250 smoker motor in my 04 CRF 250 chassis it was like that and wouldn't clear the spring and much worse than the 500 motor. I got a brand new Rad Valve and cut it adding a little more angle and it cleared it nicely. My 500 motor had an epic failure in the kick mechanism that resulted in busting out the center and primary cases. It's just sitting till I forgive it.
Craigus

scooter5002 · Post by **scooter5002** » July 30th, 2012, 7:56 pm

[quote="cryzsurfer"]When I stuck a YZ250 smoker motor in my 04 CRF 250 chassis it was like that and wouldn't clear the spring and much worse than the 500 motor. I got a brand new Rad Valve and cut it adding a little more angle and it cleared it nicely. My 500 motor had an epic failure in the kick mechanism that resulted in busting out the center and primary cases. It's just sitting till I forgive it.

Did the little tab that bolts to the case that holds the kick start mechanism break off and shitcan everything? Roosty had mine apart almost 2 months ago and showed me the hairline crack (Easy boys, not THAT hairline crack) they develop. Stress fracture I guess. Mine was definitely wounded and probably have broken soon. If I ever actually RIDE the fuckin thing. Adam knows his stuff. That's good info to have, that $5 part failure can cause a massive expensive failure like that.

Doppler · Post by **Doppler** » August 8th, 2012, 8:34 pm

Looks like some of the build will be tough to figure out, but you can do it!! At least I hope you can I'd love to see a YZ conversion.
I wonder if you can get an intake that has an angle to it pushing it to the left?

other · Post by **other** » August 8th, 2012, 9:13 pm

Doppler wrote:Looks like some of the build will be tough to figure out, but you can do it!! At least I hope you can I'd love to see a YZ conversion.
I wonder if you can get an intake that has an angle to it pushing it to the left?

If you can't buy one, you can always make one. My first engine conversion back in 1985 (a DT engine into a YZ125 chassis) had a fabricated aluminium "boot" to suit the required shape, with a rubber tube connector at the carburettor end. I think I saw one that Roosty did similar on this forum somewhere as well . . . ? Sealed brilliantly, and was still working without a problem when I finally got rid of that bike in 1994 ! Just sayin' it's an easy option.

2strokeforever · Post by **2strokeforever** » August 8th, 2012, 10:21 pm

3. Check airbox fitment. Close is ok. Pipe fitment is more important.

i think its easier to weld the pipe (or silencer) than try and screw around with an airboot that dosent fit

id try and trade the YZ for a blown up 4 stroke, i think most 4 strokes have big enough engines to fit the 500 in without issues

honda and ktm chassis work without too much work, the 2010 yammy 250 f handles real good, if you want i can hold my engine jig up to 250f and say what i think would need to happen to make it work

hoofarted · Post by **hoofarted** » August 9th, 2012, 7:16 am

Regarding what 2Stroke said:

http://bannedcr500riders.com/board/view ... 5622#75622

http://www.youtube.com/user/500bigbore · Post by **Roostius_Maximus** » August 9th, 2012, 8:37 am

sweet, i couldnt find where i hosted those pics

cryzsurfer · Post by **cryzsurfer** » August 13th, 2012, 4:04 am

I'd have to look at it closely as I just stuck it off to the side I was so bummed. I have all of the parts to fixit I just can't stomach it still since that fateful day when it broke.
Craigus

BannedCR500Riders.com