The cause is the filter has a stepped rubber ring and this seals off the three holes circled on the carbs which I forget the explanation but have something to do with return flow and sensing.
These need modification to the stop lip to allow proper air flow to the atmospheric pressure chamber.
|From XScafe, not same as mine above but shows the same issues.|
Big inner lip...everything restricted...diaphragm vacuum port at the top: pilot air jet-lower left: main and mid range air jet-lower right: bowl vents in the middle
Best ones to use are the K&N at $80ea or the Mike's XS copies at under $20ea plus min postage $30..
I thought I'd give mod-ing these a try in the interim so I cut three chunks out of the step but it cuts right through and in those places and ruins the integrity of the filter.