Okay boys, I have done this conversion, it was by no means easy on my check book, but spending the extra cash has proven itself profitable in the long run. Let's start at the beginning--About two years ago, I purchased a mint condition '86 350X w/ very very little ride time (the tires still had the little rib in the middle) for $1600. I had it for a few weeks, then one day while it was sitting on my farm, my uncle wasn't paying attention and backed over the front end(the forks/tank/front rim/handle bars/ were all bent to heck, and the fork mounts were twisted nearly off) so reserection of that frame was useless. So after a few weeks, I finally got the engine pulled out, and had it taken down to a engine shop, had it bored over a very little, had the side covers polished, 10:1 compression piston installed, a slightly larger carb installed(don't remember what size exactly) new clutch, and a few other things done to the engine. And then I got the idea of puting the engine in a 250X frame (like what I did with my '87 200X engine) So after another few weeks I found a complete 250X roller for $300. I then took the engine/chassis to a local fabricator and had it installed (I would have done it, if I had the time, but it was in the middle of the harvest) So, now I had an awesome bike built up, and me and a friend were out riding and racing around on some trails, and he decided to jump this creek on his 250R, and he talked me into doing it as well. So I take off towards it, and right as I get to it, I slip it into third, and slip the clutch, so that messed up my whole jump. Next thing I know, I'm flying across the creek, and my front end (directly in front of the engine) slams down on the bank, my front plastic rips in half right in front of the tank, I fall with the rest of the bike down the 8' bank. I tore my front end off! Boy was I mad! We got the bike back to the house, and were looking at it, to see if it were possible to reweld the frame, and notice that there is a lot of rust by one of the tears. The frame had been cracked by the previous owner!!!!!! So once again, I was left with a still great engine, but a junked frame!! So then I decided to go all out. I decided to get a Laeger 250r style chassis, just like the one my '89 250R engine's housed in. Anyway, to make a long story short, I spent several grand on a laeger 250R chassis/a arms/swing arm/steering stem, durablue axle/hubs/sprocket flange, sidewinder sprockets/chain, renthal bars, '88 red R plastic, and a bunch of other "little" things, with an end result of a beautiful $9500 quad! If you have more specific questions, drop me an email, and I'll fill you in on the specifics. This is an absolute great match, I love this bike, I would sell my R before this machine!

350XX, 250R, 200XX, 450S, 300 2WD

 

I currently have a 91 250x chassis with a 350x motor, 250r a-arms, and soon to have a 350x swingarm with 350x shock.

I decided to build a 350rx because I already had all the extra parts to do it. The 250x swapover is really much easier than the 250r.

 

Brad, You are installing a 350X swingarm on the 250X frame. Why not use the 250R swingarm? what is required to install the 350X motor in the 250X frame? Do you have to reshape the gas tank to clear the carb? What frame modifications need to be done? Is there a difference between the 350X swingarm and the 250X?

 

The reason I am going to use the 350x swingarm on the 250x is because of ease of installation. The 350x swingarm can provide good benefits to the 250x chassis. It is about 1.5 inches longer and the shock of course has more adjustments. The 250r swingarm would require either a no-link setup for the shock or tabs welded onto the 250x chassis for the 250r linkage, then you would have to be concerned about having the geometry correct. Otherwise the shock will bind.

Now the only things that will have to be modified to fit the 350x swingarm on are, the top shock bushing on the 350x shock, and shim the front of the swingarm. It is a little narrower than the 250x so shims will have to be installed to align the swingarm.

Also I have installed a 250x spocket hub onto the 350x axle which is wider than the 250x and has 25 splines compared to the 250x which has 24 splines.

I haven't got the swingarm installed yet, but I will take some pictures of the installation and have them posted on my web site as soon as I get time.

Installing a 350x motor in a 250x is really easy. No frame modification have to be made as far as cutting is concerned. Although one case modification is required on the left rear of the motor. Spacers are required on the front two motor mounts and the right rear. A top motor mount for the head stay is recommended, and it is really easy to make. In fact the most difficult part of the conversion is the exhaust which will need to be made longer to clear the front frame rail. On mine I had Powroll to make me an exhaust and it bolted right on.

Using a factory carb doesn't require any tank mods. Although I am using a Mikuni flatslide and had to bend the bottom of the tank to clear the cap, but the 250x has a metal tank and is easy to modify.

 

Thanks for your information on the frame. I guess my main concern is will that 2" make that much difference in handling?
Do you run the 250R swing arm? or do you use the stock 250X or 350X? If you are running the r swing arm where on the frame did you mount the linkage bracket so that the suspension will not bind? Do you use the stock 250X gas tank? What about the exhaust system do you have to make modifications or will the 350X exhaust system work?

 

hey guys I installed the 38mm pe carb and i used an IMS gas tank. I heated the gas tank in my oven then when it was hot enough i bent the bottom to clear the carb. I was sick of needing to get gas all the time with the 250x gas tank. i can ride for hours now with out worrying about running out.
PS Brad the 350x shock needs the res. line to be turned around then to have the shock refilled with nitrogen.

 

Thanks Projectx,

I haven't gotten my swingarm bolted on yet (still cleaning it up). I'm glad you told me this so I can be prepared for it.

 

Brad,
How is the swingarm coming? Do you have your frame back from Rocky Ridge yet? I have been doing some reading about the length of the wheel base and how the machine will handle. I do a lot of hare & hound (desert) racing and we encounter miles of whoop sections (one race had 17 miles of straight whoops). From the reading that I have been doing that the shorter wheelbase will give you better handling in the whoops and tight sections. The longer wheelbase will give better cornering and jumping abilities. I thought it would be the opposite that the longer wheelbase would handle the whoops better and the shorter wheelbase will turn better. Is this true? I have been wondering which frame would be better to put my 350X engine in, the 250X frame or the 250R frame? My season consists of 12 desert, 12 motocross, and 3 Grand Prix races. I spend more time in the desert racing since they are longer. I just want to build the best bike for the type of racing that I do. Thanks for any help you can give me.

Travis

 

Anybody ever thought of using a 400EX chassis for the 350X conversion. Every racer I know who ride both the R and 400EX say the 400EX is much smoother and handles better, plus parts are cheap compared to an R. Not only that, but you wouldn't have to modify the frame rails at all like you do on the R frame.

I'm actually working on a YZ400F powered quad, and I was originally going to use a 400EX frame and plastic, etc. I am also using 250R hubs, axle, and brakes. A new 400EX frame and subframe cost's $480 ($600 for most people), and all the plastic with seat is only $250 (or maybe $300-350 for most). Why I say originally, is because now I may be testing a prototype chassis for an aftermarket company, so if that works out I won't need a frame. But even so, my friend is building a Leager's YZ426F, and he even said if he was to do it again he'd just use the stock 400EX frame for the cost savings and they handle very well.

It may seem like doing that would cost more if you already have an R to work with, but you can sell most of the parts you don't need for about the same or more than it will cost to buy new 400EX parts. Even if it costs a little more to build originally, in the long run it would probably be cheaper. Especially if your like me and prefer stock plastic for durability, which is outragious for an R and cheap for a 400EX...

I have had people say they don't beleive the 400EX handles better, they just think the 4-stroke engine must give that impression. But I do know the 400EX's are better handling than an R, because a national racer I race with used to have a WR400F in a 250R frame, now he has it in a 400EX frame, and he said that the 400EX doesn't beat him up near as much as the R did.

Just an idea for anybody else who maybe considering this type of conversion.

Sickman

 

The 400EX definately handels better. My mom has a 400EX and I have a 250R and my best friend has a 350RX, well actually a 450RX and with the 400EX I can ride at full moto pace for alot longer than with the RX or R. Just incase anyone was curious the RX is set up for XC and the power is very smooth,