for the most part it's the airfilter, but fuel also does it.make sure you use gaskets on top of the carb,and then again on top of the linkage bracket so the lower filter housing seals to the throttel linkage plate/bracket. lastly is the velosity stacks, they are too big for the filter and eat the filter at the ends letrting the filter screen debris&crap go into the carbs&your motor. I mark each velosity stack&either bend them up some(but you have to be very carfull you dont want to crack the chrome as it is very hard for the engine to digest if it flakes off. a better way may be to take off about1/8"~3/16" with a belt sander where it has been marked st the end where it contacts the filter, it dosent take much and only there a the end, but be sure to mark each stack and do them 1 at a time then clean&replace them back on to the carb 1 at a time. the reason for 1 at a time?? well there bolt pattern is angled and they will only fit 1 way, if you do them all the same 2 of them will not fit.and when you remove them the linkage top plate can move and your nolonger synced up with the linkage. Do them 1 at a time and nothing moves, you get each one just right. 1 more thing the main jets dont come out with the stack in place. you could also mark it where the main jet stack is located in the center of the carb & knotch for easy removal of them with out having to remove the velositystacks. the old aluminum webber delorto stacks were nicer and didnt need these mods.I personaly think it is a ploy to sell more filters when they get eaten up, but just what is the job of the filter?? to keep crap out of your motor.if it's eaten up it isant doing it's job at all. in addtion to this there is some filter covers(outerwears)(empi makes them too) and they fit over the filters and keep dust sized particles from clogng or going through the filters.I have had them on my bug since I put the carbs on about 10 years ago, I still have the orignal filters on it.it is a daily driven car , hear in hot sandy Fl, with no trunk on my car(engine is just hanging out in the breezzee, baja style) Ive had 2 sets of outerwears in 10 years.and my 16"x31"x15 rear hoosier tires are right there next to the carbs with no inner fenders at all.(this 356 will be a compleatly different learning contortionest expearance for me,I hope the biger motor will fit it as the engine will be almost 3/4" wider + a lot taller intakes too.might need some scoops or buldges for the carbs&aircleaners.