Air Box Modifications ???

I read the article here in the KZR site under the KZ Information tab / Articles / Technical Tips / "Making Power With Air". The article recommends building a fitting that allows air to enter the filter from the bottom in addition to the stock set-up, which is only from the top. The inside diameter of my stock top collar is only 1 9/16". I would think that the 4 carbs can probably pull more CFMs than the top collar of the stock air box.

But before cutting into the air box, I decided to try a short ride with no filter and no top collar on the air box. This opens the top of the air box to about 4". The bike ran so lean that it would bog badly after about 1/4 throttle. It ran terrible. No mid or top end power.

I screwed the top collar back on, but without a filter. The bike ran very well. So it appears that the carb tuning is depending on a certain amount of restriction from the air box. I expect that if the air box is modified, then it would be necessary to go considerably richer on the jets and jet needle settings.

Does anybody have any experience with modifying the stock air box? What can be expected regarding performance, and what other tuning is necessary? Thank you so much.

Put a K and N filter in the stock air box,put the screws back in the box and forget about it.No rejetting needed ,no futzing with hole drilling,clean the filter once a year and use it until u or the bike dies of natural causes.bj

It really depends on what you are after. If the engine and exhaust system are stock, then hocbj23's suggestion is probably best. If you are also going to add an aftermarket exhaust and cams and a port job, then a more open box can reap measurable benefits. Several guys over at zrxoa are running modded boxes with some pretty stout engines. After a bunch of dyno runs, several of them concluded that a properly modded box is at least as good as pods, but it is not as simple as grabbing a hole saw and attacking the box.

Try leaving the airbox top off & see how it works.

WABBMW wrote:

I read the article here in the KZR site under the KZ Information tab / Articles / Technical Tips / "Making Power With Air". The article recommends building a fitting that allows air to enter the filter from the bottom in addition to the stock set-up, which is only from the top. The inside diameter of my stock top collar is only 1 9/16". I would think that the 4 carbs can probably pull more CFMs than the top collar of the stock air box.

But before cutting into the air box, I decided to try a short ride with no filter and no top collar on the air box. This opens the top of the air box to about 4". The bike ran so lean that it would bog badly after about 1/4 throttle. It ran terrible. No mid or top end power.

I screwed the top collar back on, but without a filter. The bike ran very well. So it appears that the carb tuning is depending on a certain amount of restriction from the air box. .

That also proves the "restriction" of the air filter has negligible effect. I think if you screw with the airbox and you will have a new hobby that will last a lifetime. The makers are so good at squeezing power out of these engines, there is not much fruit left to pick. You might get a little more power one place at the expense of somewhere else.

I did the airbox mod on my kz650 after I changed out my carbs to VM26's. I kept the stock pilot jets because it didn't make any difference in either starting or idling.
On the high end depending on exactly how much you mod your airbox needle clip to #4 or #5 and main jet 120 or bigger.
With only the top of the airbox opened up to the max, main jet 115 needle clip#3.Both top and bottom of airbox fully open you will need at least a 120 main jet and the needle clip to #4 with the stock air filter. No need to spend $$$ for a K&N when a stock filter works just fine.Good luck!