I put a K&N air filter on my model 267 Zenith (A-22- 200 cid)...Don't recall the model number but was the largest that I could fit in my space constraints...The K&N air filters are also servicable.
(Something I copied off the internet that may be felpfull)
ZENITH CARBURETOR ADJUSTMENT
Turn idle adjusting needle in lightly against its seat, then back out needle l 1/2 turns
as a preliminary adjustment.
With throttle held in closed position, turn throttle stop screw in until stop screw just
contacts throttle stop and then turn stop screw in 1 1/2 additional turns as a
preliminary idle speed setting
Set main jet adjusting needle at 4 1/2 full turns open.
Start engine and let it run a few minutes until warm enough for final adjustments. Set
the throttle stop screw for the desired idling speed.
Set the idle adjusting needle for smooth idling. Turning the needle in (clockwise)
reduces air and makes the mixture richer, while turning it out (counter clockwise) admits
more air into the mixture making it leaner (avoid using a mixture that is too lean).
To set the main jet adjustment, hold the throttle open so that the engine is running at
about 1/2 its normal operating speed. Turn the adjusting needle toward its seat until it
causes the engine to give evidence of a lean mixture by running irregularly. Then turn
the needle away from its seat, carefully counting the turns made until the engine shows a
trace of rich mixture. Set the adjustment. Then at just about halfway between the lean
point and the rich point, hold the adjustment at this point and tighten the packing nut
so there is no leakage of fuel around the adjusting needle.