Nog even over BST carb:
ADV rider.com Mikuni BST40 "Venting"...
There have been several threads in the past with comments on dirt getting into the Mikuni BST 40 carb via the two main vents.
One vent goes to the carb float bowl; the other vents the variable air space under the rubber diaphragm. Both allow equalization to atmospheric pressure in those areas.
The bowl vent uses a long hose that is generally routed up and along the top of the airbox, towards the battery; the intent here I believe is to have it protected from excessive exposure to dust and water.
The diaphragm vent is routed into a small plastic elbow/filter. The elbow contains a disc type filter element approximately one square inch in diameter. This filters the air going into the cavity under the diaphragm.
Older bikes, or non-US bikes may have additional emissions bells and whistles attached to this vent.
There have been numerous concerns posted regarding the disc filter on the diaphragm vent. The most common is that it allows dirt to enter the carb and accumulate in the body below the diaphragm, around and in the slide area. Some have blamed this filter for “slide sticking” issues.
Remember that air is both pulled into and pushed out of this vent.
Another concern, and one that seemed more a possibility to me, would be that the filter becomes so clogged that air cannot flow readily thru it, in effect damping, or at worst completely preventing atmospheric pressure from reaching the air chamber. Throttle response would be adversely affected.
One day I took off the carb disc filter and took a good long look at it and thought about what it had to do. And then I thought how much better it would be to have a larger area for the diaphragm to “breath” thru… not to mention something I could tell at a glance what condition it was in.
While I was there, I though about how I had re-routed the carb bowl vent so that when I removed my precious, suede-covered Renazco seat to wash the bike, I wouldn’t inadvertently spray water into the hose… and maybe into my carbs bowl.
Initially, I just wanted a small filter for the diaphragm vent… then I thought, “What the Hell… lets just vent the shit out of the whole carb”.
So I purchased two UNI crankcase filters, one in a “push-in” 5/16” diameter (UP-121) for the carb bowl, the other a push-in ½” diameter (UP-123) for the diaphragm vent.
This is the 5/16” diameter filter for the carb bowl.
It took awhile to decide where to put these filters. The 90° molded rubber elbow of the diaphragm vent pretty much decided where that filter would go… although I did try a few alternate locations, none seemed to be as good as the “stock” location.
Carb top view, 90° rubber elbow in lower right corner
The hose wraping around the diaphragm filter leads to the bowl filter behind the brake fluid reservior.
The carb bowl vent was a little different. I had it on the left, the right… above, below and behind the carb. Finally, the empty space between the rear brake reservoir and upper shock mount called to me… and that’s where it ended up. I’m still not 100% sure it’s going to stay there.
So there you have it. An alternative to the stock set-up that ensures no unfiltered air can enter the carburetor.
The rational for doing this is questionable... I've had my bike for almost 3 years and didn't think it was an issue. Maybe I'm bored and just looking for things to do... or maybe there is some practical logic to it.
