Regarding your question about "rejetting"... to rejet is simply to change the precision porthole that allows so much gas to enter the carb flowstream in relation to a given vacuum in the carb throat. Vacuum comes when the piston pulls in air. Gas joins in through one of several passages including the choke, pilot (or idle) and primary (main).
Rejetting is done for one of 2-3 reasons. Bikes are built to run at approx sea level, outside air pressure about 14.5psi. Go up to Denver Colorado (elevation is about 5000ft above sea level - that is, you are waaaay up in the mountains) and when your piston pulls a certain amount of vacuum in the carb, less air passes through the carb because the air pressure outside is lower (about 13.5psi in Denver?...just a guess). Since the jet remained the same, now more gas jumps into the carb/flowstream in comparison to the amount of air. Thus, the flow into the engine is said to be "richer". Thus a change to a smaller dia jet (smaller number) is needed if Denver is where you normally ride.
Bikes/cars with fuel injection don't worry about this stuff... the correction to proper fuel/air ratio is made automatically. Old style carbs were made to meter gas/air in fixed ratios... and in the '70s cars/bikes were asked to run as lean as possible to make the EPA happy. Thus, when a bike is not in perfect original condition where the carburation is concerned, it probably starts to run too lean for proper operation... and you feel it as hesitation, poor performance, hard starting, etc.
When you put that K&N air filter/filter pods on your bike you tend to push the air/fuel ratio a bit leaner. I have seen 2-3 bikes recently that would not run at all with the low restriction filters recently installed. Also, when you put the low restriction exhaust on it tends to reduce back pressure and allow air to flow more easily through the carb venturi... the higher flow speed at a given throttle opening means more air, less gas. Constant velocity carbs do a better job handling this, thus the later Hondas (Goldwing, etc) went to CV carbs instead of the mechanical linkage slides.
Best way to know where your carbs are in this air/fuel ratio thing is to pull 'em and look for yourself. Good plugs have a tan-to-brownish color, bad plugs are either too clean or too black with soot. The bad news here is you could run properly at idle but too lean/rich at open (over 1/4 throttle) throttle OR VICE VERSA. I have a bike that regularly fouls plugs if left at idle too long, but take 'er out on the road and she runs great.
When checking idle vs open throttle ratio, one can let the bike idle for a few minutes or do what is called a "throttle chop" check. Warm the bike up with a brief ride, let her sit for 3-4 min at idle, then turn off and check plugs. Lots of color pics on what plugs look like on the 'net, compare to know what you have. For the throttle chop test, I like to warm up the bike, stop and install brand new plugs, then immediately hop on and spin it up to say 4k rpm in about 2nd gear. Open the throttle at least 3/4, hold it till you near red line (if possible...watch those speed limits), then simultaneously hit the kill switch and pull in the clutch...in other words kill the engine quickly from mid-to-high-rpm running. Pull over, check the plugs... how do they look now? This test is a pain, but if the results are significantly different from the idle test you have a prob that needs sorting.
As for deciding which jet to use... jet numbers signify fractions of a millimeter. a #32 jet is 0.032mm in diameter. Most Hondas from the '70s will benefit from a slightly larger idle jet... so I might bump to a #35. Try to get the "Tim Allen Mindset" out of your head... bigger is not necessarily better. We are seeking optimum, not maximum... quality of air/fuel ratio not quantity.
Now...with all that said... here is the bigger picture... lots and lots of variables can affect how the bike runs. There is always the dirty air filter or the leaking inlet tube that can affect bike performance as well. Before spending time tweaking the carb, make CERTAIN that ignition system is in good shape, valves are adjusted, inlet tube O-rings are sealing, etc.
In looking over this explanation of jetting I have omitted talk of the needle/needlejet. You can see how gas is metered out into the flow stream when the slide goes up...needle slims down, opening becomes bigger, more gas comes out. You can change the lower-to-mid throttle settings by moving the needle up and down inside the slide. when throttle is closed or wide open the needle hardly matters, but as you begin to screw it open it has an effect... and this in exactly where we use the throttle the most. Adjustment here is - lift needle higher and more gas joins the air earlier. Drop the needle lower and stifle gas flow during initial throttle opening. MOST bikes just need the needle in the middle clip setting... and now that I have said that maybe yours is the one that needs a change...but I doubt it.
Tired of typing now... good luck on all that.
TS99
