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Old 03-14-2013, 12:13 PM   #74911
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eakins's Avatar
Joined: May 2002
Location: Fort Collins, Colorado
Oddometer: 19,403
BST carb - Off-idle bog issues and things to check:

Originally Posted by motolab View Post
Make sure the slide guide and emulsion tube are not worn out (a worn out emulsion tube will cause an excessively rich mixture at low rpm at all openings), set the float height, install a non-USA adjustable OEM needle, install the correct main jet for WOT operation, set the clip position for correct 1/4 opening operation, install the correct pilot jet for 1/16-1/8 opening operation, set the idle mixture and speed.
Thanks! Bill

I also found this you sent me before of things to check so others can see if they have carb issues.


"I would pay particularly close attention to:

The float needle's tip: Check with and without magnification for a witness line where it has been contacting the seat. If one is found, the float valve either leaks or will begin leaking soon, causing a high fuel level or overflowing.

The needle seat o-ring: Make sure it fits snugly and is not cracked/hardened/deformed/shrunken/otherwise damaged. If it's not in good shape, it will leak, causing overflowing.

The slide guide: Make sure that the indentations concentric with the bore in the bottom corners adjacent to where the slide bottoms are still .010" (.25mm) deep or more. If they are not, the emulsion tube is at risk for wear. See,,,, and

The slide: Make sure there is no vertical grooving on the downstream side and that the lift hole area has not been increased by drilling. Drilling causes excessive slide guide and slide wear, which in turn accelerates emulsion tube wear. See,,

The emulsion tube: If the outlet has ovaled, the mixture will become overly rich at low rpm at all openings. The wear can sometimes be hard to see, but keep in mind that .0004" (.01mm) of wear will be discernible on the dyno as an approximately 1 percentage point change in CO (i.e. it the CO was 4% before the wear, it would now be 5%). Sometimes spinning the tube between your fingers can help you see the wear, as can inserting something known to be round, however if the slide guide is worn more than .010" (.25mm), you can pretty much be assured that the emulsion tube is worn. See, and

The pilot jet: I recommend replacing it as a matter of course.

The float height: Using a float height setting tool, set it to it to 14.7mm.

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