UNI PK-3 Foam Filters. These very handy and high-quality filters are a good alternative for the usual power filters. They filter a bit more, especially with some extra filter oil.

Per set of 4 pieces. 

Mounting diameter: 38 mm

UNI high flow foam air filter pod set for Honda CB500, CB550, CB750 and others. Includes 4 air filters. Comes with stainless steel mounting clamps. These filters a have charcoal coloured filter element and deluxe black urethane end cap.

Keep in mind, for the bike to run smoothly, you may need to install larger jets and make other adjustments match the higher air flow from these filters. 
These filters need to be oiled.

IMPORTANT: Be sure to measure your carburettor prior to purchase. Also, measure how much room you have between the carburettor and the frame or any side covers. Use the dimensions below to see if these will work on your bike. Just because the filters fit the carbs doesn't mean they'll fit in the bike without some obstruction.

Made in the USA.

Dimensions:

39 mm ID where it mounts to the carburettors
50 mm air filter length
68 mm total length including rubber
72.5 mm outer diameter of the filter

UNI Part Number: PK-3

Fits Honda:

CB500 Four 1971-1973
CB550F Super Sport - 1975-1977
CB550K - 1974-1978
CB750A Automatic - 1976
CB750F Super Sport - 1975-1976
CB750K - 1969-1976

Fits Kawasaki:

KZ400A Deluxe - 1977-1978
KZ550A - 1980-1983
KZ550C LTD - 1980-1983
KZ550D/H GPZ - 1981-1983
KZ650B/F - 1977-1980

Fits Suzuki:

GS550 - 1977-1979
GS550E - 1977-1979
GS550L - 1979

Fits Yamaha:

DT125
XT125

...and probably others!