Motorcycle Repair: Air filtering cb750 1972, velocity stacks, element system

Question
hi bill, i have a chopped honda 750 (1972), at present we have a custom air box, which I feel at the top end restricts the air flow, because i'm limited to 110-115 km/per/hr, and a lot lower going up hill, even when downshifting.  Last summer i tried 4 k& n pods, but that made the bike run terribly. Ran up and down the hiway with no filter (I know, not a good thing) just the velocity stacks, and it ran a lot better, than with our custom box.  I think i have 2
options, the first is to modify the custom air box, ie. open it up more and design a new element system for the inside, and 2; try the pods again, but get into re-jetting.  If I choose the later, what all is involved in re-jetting the carbs ( i feel that it will not be too easy)  any help would be greatly appreciated.

thnks  

my other email is happyharrys@nb.aibn.com

Answer
Giles, most modern motorcycles draw air from a large air box that stills the incoming air and pulls it from a common chamber. Changing to 4 small filters changes the air flow characteristics as you have found, but jetting can be done to make them run successfully. Here's a lengthy description of carburetion and jetting...

http://roadstarclinic.com/content/view/56/96/

I have to say that you want to do carburetion work LAST, after you have established that the engine has good compression on all cylinders and that the ignition spark advancer is free to move correctly and that the point gaps and timing are all set correctly. Once you know that the basics are in place, then you can carry on with carb changes.

If you have the stock K2 carbs with push/pull cable setup, then changing needle positions is a lot more complicated than the original 4 cable/slide sets from the K0s. But, you have to establish the main jet settings first, so you don't seize the motor from being too lean. Generally, if you have changes to the intake and exhaust systems, you have to increase the main jets by about 2 steps. If you have 105 or 110 mains, try 115-120s to start with. Have fresh plugs in it, so you can read the mixtures more easily.
When you get into 3rd gear at wide open throttle, listen and feel the motor to see how it is running. If it flattens out going from 3/4 to full throttle, pull the choke lever up a little and see if the engine performance improves or declines. If it gets better, then the mains need increasing again. If worse, then lean it back down one step and try again.
Access to a dyno will speed up the process greatly....

Once the main jet sizes are established, the rest of the tuning has to do with raising and lowering the needle positions. If the idle is good, leave the idle jets alone.
If you have a hesitation from idle to 1/4 throttle, try richening up the idle circuits a 1/4 turn. If you can't adjust it out, you may need to increase the idle jets one step.

Once you change out the intake and exhaust, then all bets are off for the stock jetting and it is trial and error there until you find the right combo.

Bill Silver