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Air Filter HoleFrom $1Table of contents
It is possible to drill holes in the standard SI24 carb air filter. This allows the carb to suck air more freely and, combined with correct jetting (normally a slight increase in main jet size and check it is correct by reading the spark plug), gives a cheap and easy performance boost. It is also possible to buy a filter which does not require the drilling (see below). The reason that the holes aren't there on our scooters in the first place seems to be that the US and Australian markets were provided with detuned carbs (someone from Europe please confirm). This is just a guess and the fact that old Pinasco installation instructions talk about drilling a hole, suggests that "holeless" carbs were also sold in Italy in the past. This could also be a Cosa airfilter.
Dyno results show that the having a this hole provides a pretty big increase in power and at the cost of a main jet, is probably the best value for money performance enhancement you can get. See Air Filter Hole and Sito Plus lab results for more detail. Here are is a picture showing the difference in air filters and the approximate size of the holes. You can use a drill to match these holes in your original filter, taking great care not to leave swarf in the filter.
WarningThe fact that there are two different types of air filters means that you should also take care when replacing parts. If you have an original "no holes" air filter the get a "holes" replacement, if you do not change you jetting you run the risk of heat seizing your scooter.
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Hopefully with the oxygen sensor I'd be able to measure just how much each jet impacts over the whole range.
I tried this the other day by making a heart shaped hole (Would this thing have a negative effect?) instead. 118 is not readily available so I tried 120 and made some minor adjustment on the A/F mixture screw, so far the spark plug looks OK. edited 00:18, 27 May 2009
I'll add a close-up photo tonight.
I have drilled the extra holes and even stripped a layer off the wire mesh of the air filter.It does make a lot of difference to the power.However if the jetting is too big then you will experience delayed revving on takeoff.Have tried many exhaust combinations to this setup and found that using a Proma sports pipe is the best.Too bad here its considered an illegal mod.