We call them butts down here. No particular sex assigned to those butts though from high school French I recall they would call them le butts or la butts. No matter, both nasty habits.
On a v4 with a collector box, swapping the filter to a free flow design gives the potential for the engine to pump more air. Only a potential as the collector is the bottleneck. Add headers, ditch the collector and you just installed bigger fire hoses to your pump and now you are moving more air. More air same fuel gives a leaner mixture so post headers you gotta adjust jetting to get that mixture back up to 12.x:1 for optimal power. Theoretically, leaving the jets stock can yield more power with the leaner mixture. The old saying "lean is mean" applies. At the expense of more power you also get more heat which is no bueno for those commercial built for a profit margin cylinder heads. I remain convinced the reason many of the v4 models run larger mains in the rear carbs is for cylinder head cooling. But then you have the 83 which is 140 on all 4 corners? The basics are more air into the pump requires more fuel to maintain that power vs heat 12.x air fuel ratio. More air in requires more air out aka headers. Lower altitude or cooler temps means denser air ie more fuel needed. Our CV carbs are self adjusting for temp altitude via the vacuum slides for the midrange, not so much idle and WOT. No really, it is in the book. Confused yet? Me too. I am pretty darn good at making a rack into 120 parts in a bin, scaping all the grunge out and reassembling them back into a reasonable facsimile of what they resembled when they left the factory but I am not a master tuner. But, the guy that repairs the power plant down the road probably lacks a nuclear physics doctorate and manages not to wipe a medium sized town off the map if you count Chernobyl as an outlier event. Keihin carbs are wonderous mechanical devices that manage to mix air and corn juice at an appx 12.x:1 ratio from idle to 10k rpms reliably, unless you park them wet for too long and ya can't blame Soichiro Honda for that one.
40/142 will do ya fine as frog hair. Gonna idle cool on an August EBGTX day and rev sweet to past redline if you find yourself chasing a litre class crotchie.
