Motorcycle Repair: cylinder no. 2 not getting hot 71 cb750, air screws, brown hue

Question
hi guys, having a problem with my '71 cb750. i have 4into2 exhaust, and no. 2 isn't getting as hot as the other 3. after a moderate ride(50mi. or more), all 4 headers will be the same temp, but never while idling. i've reved the motor while sitting in neutral, and this still doesn't get no. 2 hot. i've had the carbs rebuilt, and new main jets put in, but have old idle jets in. i've pulled them and made sure there is nothing blocking them. i've ran wire through the jets and cleaned and hit them with a wire brush to clean anything off the outside. did the same to the air screws. the part that has me confused is whether it's idling, or after a run, when i pull the plugs, all 4 look exactly the same. a light brown hue. this is driving me crazy, as i'm lucky enough to live in the only place in the nation where you have to get a bike to pass emissions( PHX!!!) every time i have taken it to emissions, they say the readout looks like one of the cylinders is misfiring. but the plugs look great. i've tried all new plugs, 3 different sets of wires, a new set of coils. oddly enough, after i put the new coils on, all 4 headers warmed at the same speed. ONCE. then back to the same old, same old.
i'm loosing my hair(that's where my mind was)! any help?  

Answer
Randy, start at the beginning. Check the valve adjustments first, then do a compression test to see that all cylinders are equal/close to 175 psi. If all are close to the same, then you can move onto electrical issues, like checking the spark plug cap resistances. They should all be about 5k ohms. If they are all good, then the problem narrows down to the carburetor, assuming that the ignition timing is correct and the coils are properly firing.

There are air bleed ports in the carburetor throat to supply both idle and main jet circuits with air to blend with the jets. The stock idle jets are #40 and float levels need to be set at 26.0mm. IF you are using KEYSTER carb kit jets, try something else. They often have quality/calibration issues.

If the intake manifolds are original (38 years old), plan on changing them or at least checking for air leaks at both ends of the intake runners. Make sure that all the slide needles are in the same clip settings and that the air mixture screws are about 1 turn out. Turning each idle mixture screw in/out should affect the idle speed and smoothness. Check carb synchronization carefully. You will see cylinder imbalances on the carburetor synch tools.

Bill Silver