You need a variable speed Dremel tool with a course sandpaper covered cylinder
that is about ½" in diameter.
I use the one that came as part of the Dremel kit. Use the low speed setting.
This tool really "Hog's" off the rubber! Take it SLOW AND EASY !!! At least
the first time.
Look inside the manifold and you will see some very "Abrupt" corners
where the mixture is routed left and right to the cylinders.
Stick your finger in there and feel the thickness of the rubber at the corners.
My guess is that it's about ¾" thick. Most of this can be removed
with the Dremel.
Chamfer this sharp edge so as to have approximately a 1" radius.
The radius isn't that important! Just form a nice, smooth transition around
In your mind, visualize the fuel mixture flowing into the stock
Those abrupt , sharp corners disrupt the mixture as it makes a right or left
hand turn into the intakes.
You can just picture the "Dead" areas where the mixture swirls around these
sharp corners and creates turbulence.
This turbulence is the cause of problems and must be reduced.
Now visualize the fuel flowing around the smooth corners. Much more efficient!
The two pictures give you an example of what the manifold looks
and what it should look like after porting.
Click on any picture to enlarge.
Your going from this
As you are grinding the rubber away , you will eventually come to
the aluminum casting that feeds each cylinder. Don't panic ! This is where
I usually stop, but I am sure you could go a bit farther.
Keep on checking the thickness of the rubber at those corners.
Do a bit on each side, alternating between sides.
Remember : If the rubber is cut too thin, your carburetor will fall off!
Not really, but don't go too far.
The idea here is to "Smooth" the flow of mixture as it splits left
and right to each intake.
As you are grinding, constantly check that both sides are "EQUAL" in size.
This is EXTREMELY important for a balanced fuel flow to each piston.
The shape and depth of cut MUST BE THE SAME on both sides !!
If not, your bike will veer left or right !!
This operation typically takes me less than 1/2 hour - being very
careful to "Balance" the cuts.
Just hold a firm hand on the tool , and check often with your finger.
I hold the manifold in my left hand. You may find it easier to mount it in
Bright lighting is necessary to see where you are grinding.
I would not worry about smoothing the surfaces too much.
It just won't buy any performance improvement. Increasing the "Bore"
diameter will not
gain you any performance improvement. Just attack those sharp corners.
It's virtually impossible to mess up a manifold, as long as both
sides are cut the same !!!
Actually, nothing can be much worse than stock!
Hey guys, put another notch in your butt stock !! Just do it !!
I promise you will feel the difference.