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YZ125 motor / GP Roadrace cylinder mod

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Found a YZ125 motor that had

been used in a shifter kart, for

dirt cheap, and have a cylinder /

top end off of a GP roadrace bike.

Will take quite a bit of fabrication

and mods, build a custom pipe

...but the power gain should be

well worth it.

The GP bikes make 50 to 56+ hp,

but this motor will get the power

curve shifted for more low end.

Combustion inserts are available

from 12:1, up to 18.5:1.

58mm cyls are also available.

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Edited by yamaha7442
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What kind of jug is that? It has an electronic PV setup must of had a different ignition system cdi etc gp road race bikes also go boom fast too.

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A few comments. I race a crg shifter with a kart specific modified 99 cr125 to run in the "stock moto" class. First and foremost 125cc kart engines are ported to provide max power at anywhere from 12,500 rpm up to 14,000 rpms depending on the brand of engine. Kart engines do not make low or mid power. All kart engine builders even go as far as removing and plugging the exhaust valves for more peak hp. So imo you will be very dissapointed if you're thinking a road race built engine will work in a mx bike. I't wont.

Edited by Polar_Bus
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What kind of jug is that? It has an electronic PV setup must of had a different ignition system cdi etc gp road race bikes also go boom fast too.

Aprilia

...their system can program when, how fast

the PV's open ...its awesome.

This cyl can also run Pneumatic, or even

hook it to the yamaha governor

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How are you planning on using this motor? Off Road? If so, you will have a nearly impossible time trying to keep it on the powerband. Just my thoughts.

The cyl has a double blade PV, that works

great for broad mid.

Pipe design, ign, and exh port height

will determine the powerband

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A few comments. I race a crg shifter with a kart specific modified 99 cr125 to run in the "stock moto" class. First and foremost 125cc kart engines are ported to provide max power at anywhere from 12,500 rpm up to 14,000 rpms depending on the brand of engine. Kart engines do not make low or mid power. All kart engine builders even go as far as removing and plugging the exhaust valves for more peak hp. So imo you will be very dissapointed if you're thinking a road race built engine will work in a mx bike. I't wont.

Haha

...no offense, but this isnt my first kick

at a modded motor.

Saying it wont work is like saying Yamaha

could never have built a 4 stroke mx bike.

How did they do it ? Didnt they use one cylinder

off of a GP bike ? Airflow is everything.

People laughed at Audi trying to run a diesel

at Lemans. Now Mazda has the Skyactiv diesel

They both had fuels made for the motors,

Mazda using chicken fat biodiesel.

You've most likely heard of Swedetech,

how much hp do their full race Honda's

make these days ?

Edited by yamaha7442

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Here's a dyno comparison of a couple

Kart KF 125 cc motors, and the orange

line is a direct drive set of cases with

the GP 125 cc cyl, no PV, and a basic

ign, running on VP 113, and a roadrace

pipe.

Obviously adapting this cyl to a YZ 125

bottom end will have some setbacks,

and power losses, but even if it

ended up with 40 to 45 hp, it would

be am improvement.

...just tryin to keep the 2 smoke world

goin' :)

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Sure, with enough money, anything can be done! Will it work better at MX than a showroom model YZ125. Probably not.

 

Again, this is only my opinion. Which doesn't mean much!!!

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Haha

...no offense, but this isnt my first kick

at a modded motor.

Saying it wont work is like saying Yamaha

could never have built a 4 stroke mx bike.

How did they do it ? Didnt they use one cylinder

off of a GP bike ? Airflow is everything.

 

Lets compare apples to apples,  2s and 4s are two completely different types of power, as is constant high rpm  road racing hp vs more varied rpm type MX hp requirements. So at this point you and I can agree to disagree, when your finished (and I hope you do follow through on your project it does sound interesting)  show me the dyno sheet for a MX based YZ 125cc making a broad midrange AND also capable of 50+ hp , and at that point I will be a believer.  

Edited by Polar_Bus
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This one is an experimental

2 stroke "diesel", built with

scrap, outdated, and unusable /

un raceable parts, just to see if

it would work.

It starts, but need to work on the

howebrew fuel mixture ,

/ fuel delivery system to

keep it running.

It doesnt need an ignition,

or a tuned pipe, and C/R is about

24 to 30:1, depending on fuel

componenent percentages

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Edited by yamaha7442
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1st failure.

The bolt pattern of the gp cyl and

the yz cyl are off a bit.

The original thought was to make some

alum studs to put in the yz cases, then

just drill new holes to accept the gp cyl.

But that would mean scrapping a set of

cases just for this test, plus the front right

stud was in the waterjacket ...no go.

Plan B was build an adapter plate,

but the surface grinder decided to

chunk the wheel right near the end

of the process, and tig welding the

damage warped the plate.

Start over with a new plate

Pipes are ready, cyl is ready,

...as soon as the plate is done (?)

will be able to fire it up.

cool part is, have heads available

that can run anywhere between

15cc to 7.5 cc head volume ( pump

gas, to VP's best ) and a carb setup

that can run meth

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Even though its a similar design, the YZ-F is a far cry from GP bike specs, and like was stated earlier, an apples to oranges comparison. A GP engine used in an off road application is past rideable, which is why you have a completely different cylinder design in the first place. Not that it cant be done, I think you'd be farther ahead in the end bumping a stock ~35hp engine designed for MX up to 40-45hp instead of tuning a GP engine down to that level. Cool project nonetheless.

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Have gotten into the 40 to 42 hp range

with Yz 125's, but that seems like as

far as it would go, and power is narrow / peaky.

Have seen others get more, though

The GP cyl on another bottom end has

a broad curve, but seems to sacrifice

bottom end due to the larger case volume,

...which does help the overrev in this

cyl set up.

1, or maybe 2 other benefits of putting

this top end on a yz, ...smaller case volume,

since the base of the cyl has been cut 5mm

to accept the 5mm plate, ...and can run

the pv off of the yz governor, instead of

pneumatic, or electronic pv control.

Dont mean to bring up 4 strokes

again, but look at basic comparisons

Some of the 4t mx bike builders

and engines are getting insane

numbers now.

Maybe look at it this way

350ci in a car, maybe 200 hp

350ci, well built, maybe 400 to 500hp

Nascar (roughly 350, 360 ci ?) 800 hp

Whats the difference, other than quality

of parts, rpm, lots of dyno work, and airflow

similar thing with 2 strokes,

garden equipment, not much hp (haha)

125cc stock dirtbike, 32 to 35, approx

125 gp, 50 + hp

So far, has'nt cost much other

than spare parts, and spare time,

...but wont learn anything unless

its tried

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