I'm the owner of a newly aquired 1995 KLF300c 4x4. The engine has been rebuilt. Standard side rings were used. No modifications have beed made to the engine. The problem is that at mid to high rpm, it is too rich. Black smoke out the muffler and fouling spark plugs. It starts up immediately, idles perfectly with no backfire, and revs quickly with no bogging. I purchased a set of stock sized (not optional) Keihin jets (starting, primary, secondary, and air idle). After the replacement the atv is harder to start, pops through the carburetor at idle and low-mid rpm, bogs at 1/4 throttle and up. When revved, it slowly returns to idle. Anyone know why stock jets on a unmodified engine would bog like this? The fuel is confirmed fresh and unrestricted, the carburetor is clean. The air filter is new. The ignition coil, trigger, and cdi are new Kawasaki. The plug is a fresh NGK.

 

Most commonly in my experience, if it's popping through the carb, and bogging down under throttle, this is almost certainly a "lean" symptom. Meaning to much air is getting into the engine. So much so that it's blowing gas all the way out of the back of the carb. The bogging down also points to it, not enough fuel, to much air, engine wants to die. Even tho it seems like your flooding it, and can probably smell gas like crazy, you have a vacuum leak somewhere. Carb manifolds, orings, airbox couplings, somewhere. Also, if you havent set your float height and bench set the carb, taken it to rhe church of clean as they say, then your just gonna keep banging your head on the wall adjusting in the blind. Removing and installing the carb over and over until you get lucky or ruin something. Just do the chores and get them behind you. It sucks, it's easier to just say "eh close enough, I sprayed em pretty good with air" but if you just do it right the first time it makes everything after come easier.

Anywho. Spark plugs will tell you everything. Are they white and crispy? (Lean) or black and sooty (rich).

 

The plugs are always black. I failed to mention that the "popping" and bogging issue is only when the engine is cold. Once at operating temp (5min.) It runs great with only a rich exhaust at full throttle.
the float level has been set via service manual specs and the intake boot and seal is new (no known air leaks. What would make it run poorly until hot? I initially thought it could be a air leak at the intake boot that sealed with expansion. RTV silicone was applied to rule that out, but did not change the issue.