KTM LC4 Mikuni BST40 "Venting"... Or, I got bored again.

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.

You’re call.

Nizice

You just do those? I don't remember seeing them.

I've got a 620 needin some TLC if your bored.

Naa, I did it about 2 or 3 weeks ago. As my paint is still out, I decided to hang the Motion Pro AUX fuel bottle and fire the bike up... get the gas flowing again, check for leaks and noises, check the coolant after the rocker cover reseal, bla bla bla.
Then, seeing as how every thing was working fine, I figured I do a little write-up on the filters, along with some thoughts and theories about filtering the vents in the first place.

Do I realllllllly want your SXC to "take up residence" in my garage?

C

thats cool creeper, and we must be on some kinda alien controlled connection cause i addressed that this sat. i went a slightly different route with the small plastic VW fuel filters.

Great minds think alike?

I plugged both of them completely. Tested it using a shop vacuum and it didn't seem to affect it, so I did it on the bike and have noticed no ill affects

once again thank you for the info
where can l purchase them ?

also who has a good online source for filters and oil ?

My 2000 came with your filter already installed on the elbow up top . I was told ktm in ohio was aware of problems with the mikuni bst hiccupping and farting , mine worked well , replaced with a kehin fcr that works great.

So these are an easy find at a good auto parts store Creep? I'm currently waiting on my Hondabond HT to come in so's I can do the Valve cover fixy thingy.

Randy.

They are available from bike shops that buy from distributors Parts Unlimited and/or Tucker Rocky. Retail pricing is around $13 per filter.

They may be available from auto parts... I really don't know.

C

I don't own a ktm, so I'm not sure how that carb works. That said, wouldn't that setup allow water into the carb easier then a length of hose routed higher up?? It just seems exposed.

Short of being submerged, it's a fairly protected area 12. The bowl vent filter location is actually only a few vertical inches lower than it's original position on the bike, and the diaphragm vent filter is in it's "original" location.

The BST carb is not unique to the LC4, there are a few Japanese DS bikes that use one as well.

C

There's your problem Creeper, there's shit coming out of your carb. You need to keep it from getting in there in the first place

Damn you are bored man, aren't you? Another 'index worthy post.' Thanks for clearing this all up. While I don't have a BST, I was reading the debates and thinking there must have been some kind of plumbed venting provision. Luckily for me, all of my bikes have the PHM40.

Any opinion on the Dellorto vs BST? It sounds like you've got the BST fairly well sorted. I'm wondering which carb you would choose for the LC4 if given a 'cost is no object' choice. Dellorto? BST? Quicksilver II

My 99 LC4 has 32,500 miles on the original BST carb. I finally had to remove the carb for the first time ever last month to clean a piece of garbage from the air pilot jet.

My slide showed no wear on any of the plastic and the dust under the diaphragm was minimal. I have had my LC4e as high as 4' in the air coming down on flat Mother Earth and I had no bogging. I believe the carb can faulter if taken over deep woops at high speed, I just haven't the talent or balls to do it.

Having a non CV carb is better in the mid and low because they give you instant air flow. The BST is good at what it was made to do (precise fuel metering and long life). It is not as performance minded as the FCR carb, but makes for a very reliable mixer under all but the most severe conditions.

Hey, you said...
good job creeper, very diplomatically put!

ps - obviously (if you had been back you know where) you were right... again.

now when you have some more free time lemme know if Mikuni put any Bottom Traps behind the jets. just curious; if'n you know, cool!

Bottom Traps?... Que?

hey meat i dunno if they put in traps, but it sure would be easy to make some up and epoxy them in place.

Thats why I have mine up under the tank along the frame. Works fine there too.

I think Meat wants you to put your bottom in here: