Grinder: Four-strokes have ports that
can be reshaped for better flow.
PORTING TOOLS Dear MXA, As a two-stroke racer from the old days, I understand cylinder porting and even tried to port my RM125 myself once. But, how can you port a four-stroke? It doesn’t have ports. Unlike on a two-stroke, four-stroke porting is done in the cylinder head. You are right that there are no ports in the cylinder wall of a four-stroke, but there are still ports that lead from the fuel injector to the intake valves and from the exhaust valves to the exhaust pipe. By massaging, reshaping, smoothing and blue- printing these tube-like ports, a mechanic can help a four-stroke engine breathe better. To properly port the head on a four-stroke requires an understanding of how the air is drawn in and out of the intake and exhaust ports. Porting and polishing the cavities of a four-stroke cylinder head can increase the flow rate, but it isn’t as obvious a modification as raising or lowering the ports on a two-stroke.
Stretch: The geometry changes
when the rear axle is moved.
THE LONG AND SHORT OF IT Dear MXA, I bought a new chain and sprocket for my bike, and when I put them on, I had to move the rear axle adjuster back about an inch. What will that do to the handling of my bike? There will be three effects, but you would have to be fairly sensitive to notice them. (1) When the rear axle is moved rearward, it increases the leverage ratio of the swingarm—thus, it could make your rear shock feel softer. ( 2) By making the swingarm longer, you will effectively move the engine forward in relationship to the wheels. This will put more weight on the front tire—thus, it could make your bike turn better (or worse). ( 3) Moving the rear axle rearward increases the bike’s wheelbase—thus, the handling in corners could slow down. You will have to judge whether the effects of the longer chain make your bike feel better or worse, but you can always take a link out of your new chain.
Extra part: You can remove the
KX450F backfire screen, but you
have to replace it.
SCREEN-DOOR ADAPTER Dear MXA, I wanted to remove the backfire screen on my KX450F, but once I removed the back- fire screen, there was a big gap. I need a spacer. Who makes KX450F airbox spacers. Both Cometic and Pro Circuit offer KX450F backfire screen spacers ( www.procircuit.com) that cost about $40.