use of old school mikuni flatside cv carbs in ktm lc4 like the one xrsonly sells for xr600/650/400 at $289

i ask this question because of all the complaining about the bst carb used in most lc4 based ktm's

i am not asking about kiehnen fcr or mikuni "pumpers"

just the basic mikuni cv flatside carb vs a mikuni cv bst carb


mainly the three on the right
 they are cv carb and not pumpers so they are as good with altitude as a bst without the issues you get from running over rough terrian

i am not refering to the kiehnin fcr pumper carb which many people want for it's pull from the lower rpm's

yes they look/are what is normally used on twostrokes

does anyone know of someone that has run one on some form of lc4 based moto

i did use one on my xr600 and it worked better than the stock carb up until i put in the full on fcr pumper


basically do any of you guys with carb knowledge know why the lc4 is mostly stuck with the mikuni cv bst carb even though it has issue's in rough terrian

what are it's advantages over the cv flatslide ??? does it handle altitude better ?? or was it easier to attach to the charcoal can or other emissions stuff ??

and if it has been done what was your jetting and year of lc4

All carbs that use a slide controlled by vacuum have the same issue of severe bumps causiing the slide to move because of gravity and not engine vacuum. It makes no difference on these carbs if the throttle cable moves a flat slide, round slide or butterfly.


 Any moder flat slide is going to fall flat on its' face off of idle unless the throttle is moved very, very slowly. Just too much vacuum loss on a big bore carb, hence all have AP's. A old style round slide has a slight edge because of the added control of the fuel mix the cutaway provides but it is still going to be a pain.

cv was used for emissions and usually get better mpg 

hey william

any thoughts on altitude on a pumper tm40 vs a bst

i have talked to a couple folks in the biz and they thought you could go pretty damm high and low with a properly jetted tm40

that kinda kills the main reason to keep the bst for me

Constant velocity carb will always run better at altitude. Inherent in the concept of a vacuum slide.


Typically, a 3,000' change it pretty tolerable by any carb. So if your riding range is 0 to 6,000', jet for 3,000. Keep in mind, it is not really altitude, it is air density which is made of Temp, humidity and actual air pressure. As you go up, the air gets colder and drier and this somewhat offsets the lower air pressure.


A trick some people do is put holes in the airbox side. They cover them up at sea level and then as altitude increases, they uncover them.

ah so there is a "trick" that'll get you a bit higher with a pumper

my guy was challenging the standard thinking on cv's and the vacuum slide a bit

i am considering a tm40 for a 640 adventure at some point  and would be stoked to cover mos of california with it at one jetting and a couple holes

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