By: Charles Salyers
today's pursuit of gaining a more powerful engine, most
companies have charged into building the monster motor.
But most people cannot afford a high cost engine buildup
so I looked into a low cost performance kit. I
choose to use a TRX 400EX since it has become a very
popular quad on the race track, trails and not to
mention the dunes and found a few that stand out.
My choice was Powroll
since they have been doing 4-strokes just
about longer than anyone. I contacted Powroll and
spoke with Pam. She informed me of several options
to go with to make it a low cost buildup. I went with
the 415cc Big bore kit. This kit cost $401.90 and
comes with a 87mm JE piston with rings, clips and pin ,
CDI, vented air box lid and a new head gasket. It
gives a 20% power increase.
start I tore down and inspected the engine (using the
factory repair manual ) I took the cylinder,
piston and rings to a local machine shop with the
clearance specs. provided by Powroll to have it bored
and honed. When I received the cylinder back from
machining it was ready to be reassembled. I
started by replacing the base gasket with a new Honda
gasket. Next, I installed the Powroll piston into the
freshly bored cylinder and set it into place.
installed the piston with the arrow facing the exhaust.
Then I reinstalled the new head gasket and head
torqueing to Honda specs. Next was reinstalling
the cam and valve/rocker cover. Afterwards I
checked and adjusted the valves to factory specs.
Total time for kit was about 5 hours for teardown and
reinstallation. After everything was put back
together, the need for a larger jet in the carb was
needed to allow a rich fuel mixture for proper break-in,
also the clip on the needle was moved down one notch.
Once I was satisfied that everything was ready, the
break-in was done by the guide lines given by Powroll.
I then rechecked and adjusted the valves again. After
that I rechecked my jetting to make sure it was jetted
A recap of parts are as follows:
1. Pro-circuit pipe-
2. K&N filter w/outerwear- $55.00
3. K&N powerjet kit- $45.00
4. Powroll 415cc bigbore piston kit- $234.95
5. Powroll vented airbox lid- $49.95
6. Powroll high rev CDI- $117.00
7. Honda factory cylinder base gasket- $6.50
8. Honda factory oil filter- $3.25
Seeing I already had a pipe on the 400EX , I added
the price seeing it was tested with it on. So now
on with the test ride! We got a new 2001 400EX all
stock to test out this new 415cc hop up, man were we
In a 300ft. drag the 415EX pulled away every time, this
puppy was hitting hard from bottom to topend. It
had the feel of a stock TRX250R not your normal 4-stroke
grunt. We even switched the riders back and forth to
make sure it wasn't just the rider. So next we hit the
trails to see what it would do there. The 415EX
hits the top much quicker than a stock 400EX so we were
flying up hills and around the trails like it was meant
to do. For the money invested into the 415cc this
was a serious deal for a fast rev hard hitting power
For more info about what Powroll can do for you
give them a call and ask for Pam or Pete.
They are very friendly people and can answer any
questions you may have about building your own 415cc or
larger 400EX or any 4-stroke you may have. Just
tell them that you seen this article on ATV Source Magazine,
and they'd be glad to work with you on your quad projects.
P.S. Next we try a 38x40 HV Lectron Carburetor on it
4115 SW 19th Street
P.O. Box 920
Redmond, OR 97756