Air filters. What's best for performance.

Prisondawg

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I've been seeing people that run the original air box with a new liner. Various pod filters. But me. I don't have any filters at all. So my question is, if I am starting from scratch, what is the best option for performance.

Side questions.

What do I do with the breathers above the carbs . From what I can tell they should have hoses that shoot excess oil into the air box. But mine currently have nothing attached to them and I don't see any oil coming out of them.

Also.

What adverse effects to running no filters is there. I also have the cheap pod filters from Mike's xs however I can see that there is a lot of people here that don't support those one bit. Are those filters better than no filters at all?

Assuming I have no air filtration system what so ever. What do I need to get up to peak performance.

Thanks in advance.
 
Running no filters will make for the worst of all possible worlds: short engine life due to intake of grit and poor performance. Now I'd bet dollars to donuts that you've already read this, but in OE XS650 carburetors, the hole in the slide opens the chamber above the diaphragm to engine vacuum, while the chamber below the diaphragm is open to atmosphere through the kidney-shaped vent at the top rear of the carburetor. The difference between low pressure above the diaphragm and high pressure below pulls the diaphragm up, and the slide rises with it.

Slide action depends on that pressure difference, and air currents moving across the vent will cause fluctuations in air pressure in the lower diaphragm chamber. This is why all manufacturers install a still air box with vacuum carburetors.
 
Carbs from a YZF600R
20180920_184428.jpg 20180920_184431.jpg
note how the "velocity stack" increases separation of the slide chamber and float bowl "atmospheric vents" from the intake pulses...
 
Sorry that was more for the grizzly bear than you. What he says; stock (CV) carbs work best with airboxes, the bigger the better. I often use uni foam filter media on the stock air filter frames in the stock air box.
 
Sorry that was more for the grizzly bear than you. What he says; stock (CV) carbs work best with airboxes, the bigger the better. I often use uni foam filter media on the stock air filter frames in the stock air box.

So should I ebay a stock air box clean it out and like it with uni foam ?
 
put up a wanted in the classies here there's lots of them around.
What year is your frame?
 
put up a wanted in the classies here there's lots of them around.
What year is your frame?
The frames an 83 the engine is an 82 and the carbs are bs38s, not sure what year, but they do have have two air intake circuits on the Venturi. I think they're from a 78 special.
 
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That's going to be tricky, 80-83 airboxes fit BS34's those 4 years all use the same boxes. (plastic).
 
I went with Uni foam filters along with jetting instructions found on this site, and I find my XS is a lot livelier. Nice intake howl too.
 
Went to a 137.5 plus a little larger pilot which I can't recall at the moment (40...42?) . I believe grizld1 actually sussed out the right ones, I just copied the recommendations and they worked very well.

edit: That was for BS34 carbs, not 38
 
If you absolutely have to use pods, Uni is a good choice. Re. jetting, click on the Tech button, find the Carb Guide, and read the section on "Tuning For Modifications." 5twins did a fine job there and nothing we've written since in post after post has added anything to it. Work the steps.
 
Before you can tackle re-jetting your carbs you will need to positively identify the year of the carb set you have. Several different BS38 carb sets were used on the 650 over the years and they are jetted differently. You base your re-jetting on what was stock in the carb set year, not the year of the bike. If you show us some pics of your carbs we can help you I.D. their year. We would need to see the sides, bottoms, and backs (air filter end).
 
I am using 2 inch ID uni foam filters that are 4 inches in length. I am also going to use the outer red pre filter sock. And some filter grease. This setup provides good backpressure and doesn't really need any rejetting imho. Of course a rejetting is good, uni seem better than pods. More expensive. Stock airbox es are the most expensive of all!
 
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Maintaining the stock air box is only expensive if you buy new OE filters. I stopped repeating myself on the subject long ago, but gggGary just recently posted for the umpteenth time that filters can be refurbed cheaply by stripping them and applying a layer of bulk filter foam.
 
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