Drz jetting....but not in a Suzuki

Right,i have a question regarding some jetting changes for a drz e spec engine,but fitted to a CCM404 supermoto...the bike has the E spec engine fitted,but i am just about to swap out the airbox for a k+n filter RU=1780 like this


As you can see it mounts directly to the fcr carb,and would seem to be pretty free flowing compared to a filter encased in an enclosed airbox set up.... now those that have fitted this set up here in the uk have verified the jetting needed by dyno and here is my dilema....from my own experience way back with a drz e spec sm,with full fmf exhaust and slight airbox mods,the main jet needed was a 160 or 165,now the apparent best setting for the k+n filter on its own is a 152.... why would a seemingly unrestricted inlet set up req a smaller main jet? can anyone shed some light on this? rest of the jetting is thus.. 70 pilot air jet,38 pilot jet.EMN needle on 4th clip... by the way the exhausts generally like mine are pretty unrestricted also....heres my example...any advice greatly appreciated before i do the change.


First off, it will probably make less power without the air box. But that is not the answer you want.

No airbox restriction causes increased vacuum at the the slide. Higher vacuum pulls more on the main, so you can run a smaller main and flow the same quantity of fuel.

But an open filter does not equate to more power. Think about what happens once the air gets past the filter. Typically, any airbox opening much larger than the base of the airfilter is pointless. In fact, being exposed the airflow is subject to ebbs and flows due to cross winds and the like and therefore the jetting can vary. Looks cool but is not realistic.


Thanks very much for the reply,very thought provoking,and indeed explained why the smaller main jet would be needed,even though drawing the same ammount of fuel,im unsure what to do now,as the open filter to me seemed to be the way to go for my sm build,im not aware of any DRZ bike running one though to see if its the way to go or not,i guess a dyno run as is,and then one with the iarbox removed would be the way to go,but all adds to time and expence..... other option would be to open up the airbox lid as per this pic


Increased airflow,but still retaining the airbox.

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