Modifying Stock Airbox

ThatXS650Guy

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Hey Gang, I have the stock air box out of my '79 Special while I am working on the new seat/tank combo. This thing weighs a ton and seems unnecessarily restrictive. Has anyone modified these to flow/sound a little better or reduce the weight? I am not ready to move to pods yet and I will probably keep the side covers so looks don't matter... I was thinking maybe some pop-in vents on the side. Bad idea?
 
.. the factory... carb designers build the box to work with the carbs as designed... they say if you change to pods the carbs need jets.. etc etc .. I'd assume the same would apply of the box is modified. ... ?
 
xsjohn did some tinkering with air flow volume on the later plastic boxes. You might get an idea from his posts on the 650rider site.
 
++WER #3 post regarding XSJohn's air box mod - I did it on my '82 plus trimmed away some of the rear flange on the plastic side covers to give incoming air a straighter shot into the air boxes. I already had installed his needle kit in my BS34's, so don't know how his airbox mod worked with bone stock carbs, but I was pleased with how they worked on mine - seemed to wake up the carbs a bit.
 
Thanks all. I saw John's air box mod but that was on a plastic air box. Mine is made from steel and therefore different. It has an oval connector but it seems restrictive in that the air enters from the bottom from two, small 40mm x 20mm rectangular openings, goes all the way to the top of the air box and down to the bottom thru two, 40mm diameter holes and into the filter. Seems restrictive to me but maybe the Yamaha engineers had different goals for the Special... IDK.
 
Hey Gang, I have the stock air box out of my '79 Special while I am working on the new seat/tank combo. This thing weighs a ton and seems unnecessarily restrictive. Has anyone modified these to flow/sound a little better or reduce the weight? I am not ready to move to pods yet and I will probably keep the side covers so looks don't matter... I was thinking maybe some pop-in vents on the side. Bad idea?
Hi 'Guy,
I fitted my Heritage Special with Unipods a few years back because I grew weary of removing & replacing the stock airboxes to do carb work.
Then I needed the Unipods for another project so I put the stockers back on again.
Admittedly I ride like a timid old man (because I AM a timid old man) but I didn't notice any difference in engine performance between them.
Note:- that's Unipods, not those pleated tapered things even if they do say XS on their ends.
 
Thanks Fredintoon, I am not sold on pods (don't run them on any of my bikes) but the Special air box is so damn heavy and restrictive I was thinking about some ways to let it breathe a little better while maintaining the advantages of having an air box.
Here is what I am talking about:
Special air box.JPG
 
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Those look like the same air boxes on my '78 Special. Over the years I tried drilling holes in the covers and in the backs of the boxes along with lots of experimentation with jetting. In the end I fiberglassed the holes shut and found that the performance and gas mileage remained about the same.

I have a theory on this, the labyrinth design of the stock box increases the tuned length of the intake tract and helps the midrange power band where the engine operates most of the time.
 
That is what I was looking for dps650rider. Thanks for the input.
Jim
 
Current air boxes are a lot bigger in relation to motor displacement than the old stuff; perimeter frames allow a lot more room. Back in the day engineers had to work with the space they had.
 
Current air boxes are a lot bigger in relation to motor displacement than the old stuff; perimeter frames allow a lot more room. Back in the day engineers had to work with the space they had.
Sure but the XS650 Special air box is particularly complex and heavy even when compared to my other bikes from that era. For instance my even older KZ400 (not a high performance MC by any means) has a large, lightweight, plastic air box with a single filter as does my slightly newer GPz.
I was thinking the Special box was tuned for quiet rather than performance.
I'll probably just leave it alone...
 
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