Originally Posted by
Bellgate
Shane, thanks for reposting the information from the local NZ forum. After reading it over I am not sure if the following situation is a tuning issue or not.
An engine will run fine until the throttle is closed. When reopening the throttle there is a bog in the mid range. Is this the same thing as a "lean bog" condition you mention in your post?
Hi Rich,
Lean bog is normally right from the get go. The high speed needle is closed off too much. With an emptying tank, the tank pressure takes a little longer to build after an off throttle period. The high speed needle is the first restriction the fuel comes to in the fuel system after it leaves the tank - if you can't push enough fuel past the high speed needle to feed the low speed needle sufficiently - lean bog.
If the engine is responding initially, then fading mid range it probably points to something else.
If the high speed needle jet has any sort of small blockage (dirt, grit, shredded O Ring bits etc), this can cause all sorts of funky behaviour.
As a precaution, remove the high speed needle and blow fuel back through the high speed needle housing to back flush it. Re fit the needle and give it a run - might be the issue. Also see post on worn collet and slipping flywheel.