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Not much to write about here. Yamaha breathed all over the YFZ powerplant
to get every pony out of it possible. Then, unlike typical past OEM
quads, they amazed me by releasing all this valuable know-how in the
made for public version. This motor is packed with just as much hi-tech
modern-day horsepower as the bike guys get with their 2004 machines. In
case you didn't know, it includes five titanium valves, an all aluminum
body, ceramic coated cylinder, ratchet type five-speed shifter, direct
ignition coil, and magnesium valve cover all shared from Yamaha's YZ450F
dirtbike.
We did make a few minor changes to get a little more oomph out of our
YFZ however. Here's what we did.
Add a pipe or don't plan on running up front
Yamaha supplied us with their GYT-R Full Titanium pipe. Power gained
from this simple ten minute mod is simply incredible! You'll want to
re-jet the carb and toss the airbox lid. After that, be prepared for a
whole new quad - one with ample power throughout. I was amazed how the
motor responded. The powerband was greatly improved everywhere! Of
course the pipe is louder than the stock pipe but no louder than any
other aftermarket pipe, and after all this is a full blown race quad,
it's supposed to growl a little.
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The stock forged piston is based on the YZ450F, but not identical. The
compression ratio is 11.9:1, not bad but we thought we would try to get
more compression out of the motor. I raced the Mt. Morris GNC with the
stock piston. The motor performed so well that I almost didn't want
anyone touching a thing. We couldn't help but tinker though, so we
installed a 13:1 GYT-R high compression piston. I noticed a slight
improvement but surprisingly it was minimal, especially compared to the
gain we saw from the pipe. All in all, in the pro class you need every
little bit of power you can get. So far the reliability of the higher
compression motor has been great.
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Yamaha hooked us up with a GTY-R billet clutch basket, GTY-R inner hub,
GTY-R pressure plate and capped the clutch package off with a GYT-R
clutch cover. These improvements are lighter than stock, aid in heat
dissipation and improve the clutch performance. Their pressure
plate/inner hub also incorporates a nifty oiling system which help
extend clutch life. Long twenty minute pro motos even in Loretta Lynn's
kind of heat showed no problem for our YFZ's new aftermarket clutch
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Eventually we swapped the stock CDI box for a GYT-R Rev Box in order to
let our YFZ run to its maximum RPM ability. It definitely did just that.
The rev limiter location is increased to 11,200 rpm (stock is 10,750
rpm), which is very similar to the YZF's.
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You'll want to retard the exhaust cam timing if you want to move your
powerband a little more toward the top end. If you elect to switch to a
kick starter, this mod is a must do. The cam timing cannot be left stock
if you want to kick start it.
This mod simply moves the power a little more toward the mid-to-top end
and away from the bottom-to-mid end. This is basically a personal
preference, unless you elect to bolt on a kick start system, then it's
mandatory. If you like your power a little more on top, it's safe to
move the exhaust cam one tooth from top dead center.
To retard the timing on the exhaust cam first find top dead center
(refer to shop manual), relieve the pressure from the cam chain
tensioner, pick up enough slack in the cam chain in order to move the
exhaust cam one tooth on the chain in a clockwise direction. Know that
the intake cam will have to be moved in order to create enough slack to
move the exhaust cam. In this diagram, at top dead center, you'll notice
that on the left (stock) there are 14 chain link pins from the white
dots that we painted on the chain. On the right, after we moved the
exhaust cam, at top dead center, there are only 13 cam chain link pins
from white dot to dot. As you can see, we're actually retarding the
timing by one tooth. This lets the motor rev a little more on mid-to-top
end of the powerband. When adjustment is complete make sure the
horizontal timing dot on the intake gear is visible and horizontal with
the gasket surface on the cylinder head.
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In the past, when OEMs didn't make ATVs especially for racing, we would
spend countless hours trying to get rid of dead weight. The YFZ comes
stock at 350 pounds - not too bad, but thanks to Yamaha's planning
ahead, we could easily shed additional weight by scrapping the electric
starter, battery and other needed drive gears and clutches. We felt that
the weight loss and reduction of rotating mass could enhance
performance, so we took up Yamaha's kick start weight-loss offer and
installed their GYT-R kick shaft kit as well as their kick crank kit.
Yamaha had enough sense to equip the YFZ with the same exhaust cam shaft
found on their WR450F bike, which offers easy starting with its auto
decompression cam pin and lever. Yamaha's hot start button is integrated
on the GYT-R clutch perch, but but does not come with their kick start
kits. However their hot start cable does - just screw it into place on
the carb. For those wanting to remove the starter motor like we did,
you'll need the GYT-R starter plug to cap the spot the motor used to be
in. Yamaha's kick starter accessory kit even includes a template for
cutting the perfect amount of material off the rear fender to allow your
foot to pass by the fenders. As you can see, they thought this out
pretty carefully at the drawing board. Make sure to unplug the radiator
fan if you remove the battery. If you don't, the fan will rob precious
voltage away from the spark plug and the quad will not run properly. You
wont need the fan for motocross, but if you plan on racing cross country
events or dune riding, you'll need the fan to properly cool the motor,
so don't remove the battery. Also, in order for the kick start kit to
work the camshaft timing must be changed to the YZ timing by retarding
the exhaust cam one tooth as described above. As a motocrosser, I was
happy to shed some weight but found one problem along the way. Although
the motor starts easy enough with the kick starter, I have to admit, I
grew a little attached to starting the quad with my thumb. I'll even
openly admit that I stalled the quad at the YFZ's debut at Mt. Morris
but since it had the electric start at the time, I fired it up instantly
and only lost about one second of time. Not to mention, we only saved
about seven pounds of weight, so I'm not so sure if this mod is such a
hot set up or not.
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As far as performance goes, the PEP ZPS shocks and long travel +2" Lone
Star A arms top our list of must haves for pro-level competition. Check
out the roost deflecting Quad Tech Stealth SPS (Shaft Protection System)
on the PEP shocks.
Click the photo for a handy feature on dialing in your shocks.
Priority number one for motocross racing is to widen out the quad.
Thanks to Scott Butler at Lone Star this was no problem, even in our
hurry up rush. We fitted our quad with a set of chrome Lone Star +2"
A-arms. In the rear, we swapped the YFZ swingarm with a Lone Star unit.
We did this mainly so we could easily slip an in-stock LSR Twin Row
bearing carrier in, all so we could slide the Lone Star's Axcalibar
variable width axle through. LSR steel-braided brake lines were added at
this time as well. We locked down the axle with LSR's Disc Brake Hub and
a LSR sprocket hub with a Honda bolt pattern so I could use all my
current gears. LSR's Micro-lite hubs with Honda lug patterns were used
so I could use all my current rims. After that we mounted up Kenda front
and rear tires with 1/4 offset Douglas Wheels. These mods widened the
quad to 49 1/2".
Next on the improvement agenda were the shocks. Wayne Mooradian at PEP
made a set of ZPS (Zero Preload System) long travel with adjustable
rebound and compression front shocks for us. Wayne never even saw the
quad, yet built us phenomenal shocks based on our specs. They were
absolutely perfect. We've yet to even adjust them. As for the rear
shock, we hurriedly shipped Wayne the stocker. The next day, He re-valved
it and replaced the spring, and returned a much improved shock. Since
the YFZ starts with a great set of fully adjustable shocks and a well
designed rear linkage, the end results were extraordinary! The rebuilt
stock rear shock performed as well as any shocks I have ever had! With
these mods, and since the YFZ's center of gravity is positioned
perfectly, the end results offer an extremely predicable and well
handling race quad! Click the photo for on the left for a handy feature
on dialing in your shocks.The last thing we did at Lone Star was bolt
up a +1" forward 4130 chromoly steel steering stem. It's a standard
height stem with a one inch forward design to give me a little more
room.
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Information courtesy of
atvscene.com. |
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