yz250 in blaster

2stroke4life

2stroke4life

New Member
Jan 9, 2008
210
4
0
northern va
hey yall i picked up a 03 rolling blaster last weekend. and i got a yz250 with bad shocks. so far i have welded all the tabs and the motor is bolted in the blaster frame solid. my next step is making the exhaust work, if i could use the blaster expansion champer itd be the sh*t, does anybody know or have any ideas on doing it, i think my best bet is to cut up my yz pro circuit pipe and make it fit :/. but i hate to dump that pipe if it doesnt work. il post pics soon, im real excited bout the project cus itl be nasty when im done. btw i want to have a clamp on k&n right to the carb, does anybody know size filter itd be for a 2000 yz250 stock carb? thank
 
I just cut out 2 wedge shaped pieces and bent it closed then welded it up when it fit. It was a FMF but that shouldn't matter. I think the big end of the carb is 62mm.

Pipeandmountstext.jpg


More pix: http://s26.photobucket.com/albums/c111/StickerOtis/YZ Otis/

You'll have fun when it's done :)
Good Luck
YZOtis
 
2stroke4life said:
your a badass dude yz otis
Click to expand...

I apologize :)

I remember that when I modded that YZ pipe I had to take the motor out to remove the pipe from the frame. I had to r&r the motor and pipe a few times while grinding, tacking and testing before I finish welded it. Still have to pull the motor to take the pipe off. Couldn't figure out how to make it fit the frame & motor and make it easy to remove. Also had to do some cutting & welding on the silencer pipe.
 
Ive got a 93 Blaster roller that Im doing this swap with this winter. Otis those were some awesome pics and Ill definitely be referencing them!
 
I did the opposite of YZOtis, I cut, rotated, and welded in wedges to get fit.
My goal was to lower the rpm of the powerband for improved torque.
YZOtis's shortening of the pipe will raise the powerband rpm. Value to either approach.

185821_10150157219665803_6323129_n.jpg


Use the YZ pipe, not a Blaster pipe. Entirely different dynamics.

As for fitting it in the frame, I had the same problem, and this is how I solved it.
A 3" header flange makes the pipe come apart for installation in the frame.

199410_10150167877235803_120228_n.jpg


Not a YZ engine in mine but same sort of problems.
 
I guess I dont follow the raising and lowering the powerband by re-fabbing the exhaust. Coming over to quads and jumping into this kind of project is pretty out of my league however, high performance engines fast cars and custom fab are down my ally. This will definitely be a learning thing for me and thanks for the advise on adding a collector! Ill have to assess that "issue" when I get there.
 
crash89 said:
I guess I dont follow the raising and lowering the powerband by re-fabbing the exhaust. <SOME SNIPPED>
Click to expand...

Two strokes are highly dependent on the exhaust to make their power.
The exhaust pipe is a highly tuned resonance chamber that when in tune will triple the power of the engine.
Ruin that tune, and you will cut your power to a fraction of what the stock YZ engine makes.
The length of the pipe is one of the factors in achieving a state of tune (also tapers, volumes and tailpipe/stinger diameter).
Longer = lower rpm, shorter = higher rpm. Of course where the length is added or removed is important too.
RPM is a factor in building horsepower, so shortening the pipe very slightly like YZOtis did has the potential to make slightly more rpm and HP.
My particular engine (2003 KTM250SX) has a reputation for strong but peaky power. Lengthening out the pipe helped smooth that out.

MX engines offer huge hp gains, a typical 250 makes 40-50hp in stock form.
The largest problem with most of them is very limited transmission gear range.
A 300 lb (very light by quad standards) Blaster needs more 1st gear ratio to get moving than a 200 lbs limited traction motorcycle. The overall gear spread on a Blaster transmission is in the range of 6:1, most MX are only about 2.5:1 and even wide geared enduros are only 3.25:1. My KTM Blaster has a too tall 1st gear, and yet top gear is out of rpm at 65mph, even though a change of sprocketing could show you much faster due to this gearing.

Everything in life is a compromise. An MX powered Blaster is no exception.
Tall 1st gear makes technical trails and rock climbing difficult.
Limited top speed makes safety sense, but is annoying to have more sluggish quads inch by you on long flats.
Hillclimbs, wheelies and acceleration are its forte. Hugely fun.

Steve
 
Wow, great info. Thank you for explaining that!

Now when talking gear ratios you brought up, thats revolutions of the front and rear sprockets correct? You said 6:1 so that would be 6 revs of the front sprocket for 1 rev of the rear. Or are we talking gears in the transmission itself vs the rear tire? Wouldnt tire diameter play a bit of the role too? Dirt bikes have quite a larger tire than quads so its no wonder why they have significantly taller gears? Or am I going in a completely different direction with my thinking!? haha
 
crash89 said:
Wow, great info. Thank you for explaining that!

Now when talking gear ratios you brought up, thats revolutions of the front and rear sprockets correct? You said 6:1 so that would be 6 revs of the front sprocket for 1 rev of the rear. Or are we talking gears in the transmission itself vs the rear tire? Wouldnt tire diameter play a bit of the role too? Dirt bikes have quite a larger tire than quads so its no wonder why they have significantly taller gears? Or am I going in a completely different direction with my thinking!? haha
Click to expand...

<GRIN> You are going in a completely different direction with your thinking! :)

The 6:1 refers to the gear spread of all the gears in the transmission by itself.
I cannot remember the exact numbers, but a quick look up reveals the Blaster first and 6th gear ratios are:
1st = 3.1
6th = 0.9

The KTM 250 is
1st = 15/29 = 1.9
5th = 23/21 = 0.9
It is just a coincidence that both these have final ratios of 0.9, most vary.
The KTM enduro wide spread gearbox only gets a slightly taller 5th = 23/18 = 0.8.

So I might have been a bit off on my initial numbers (aged memory!) but you get the picture.
Much larger gear spread on the Blaster than any MX bike.
The MX bike engine has a bit more rpm range in its disposal too, but it doesn't make up for the Blaster gearing advantage.

Final drive differences can usually be made up with sprockets if not too extreme.
Most MX bikes like to run 14/50 sort of sprockets so the Blaster 14/40 combo not so far off.
I am running 12/44 to get a top speed that I would get with 14/50ish on a motorcycle.

In between the transmission gear ratio and the engine there is also a primary reduction ratio, adding to your math from engine to wheel.

Steve
 
Last edited:
Finally got around to starting this. Yesterday got a pretty good amount done. Tore the quad down, cut off and ground down previous motor mounts and got the rear swing arm trimmed to fit the yz engine. I bolted the swing arm and engine in so I can start fabbing up some mounts.

[URL=http://s805.photobucket.com/user/oscarson66/media/yzswap_zps0d519530.jpg.html][/URL]

[URL=http://s805.photobucket.com/user/oscarson66/media/yzswap2_zpsbbdb6b94.jpg.html][/URL]

[URL=http://s805.photobucket.com/user/oscarson66/media/yzswap3_zps06134ab4.jpg.html][/URL]
 
looks like both ends need rotated down, cut away as needed, you'll still be way further ahead by using the pipe designed for that engine, even if an added wedge or 2 is needed to make it work.
i'd make 2 cuts as best shows above, get the fisrt section bolted to the mounted engine, and the rear section attached to a mounted silencer, then fill in as needed to connect the center section to both front and rear sections
 
OH. MY. GOD.. This thing is a friggin monster! I did end up cutting the pipe into 3 pieces and making it fit. I can remove the pipe with the engine still in the frame. I just have to take the steering linkages off and turn the bars all the way one side to get enough movement and it'll slip right out. I ditched the thumb throttle and used the bikes twist throttle. It took me a little thinking to figure out how I wanted to run the radiators. Im big into function over form, so as long as they worked the way they're meant to I dont care about looks. Ended up just bending the front fender tabs and bolting them to those. So happy with this thing! Ill get more pics when my phone decides to send them.
[URL=http://s805.photobucket.com/user/oscarson66/media/yzswap6_zps943b1c8e.jpg.html][/URL]
 
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