Motorcycle Repair: 1975 Honda xl 350 sputtering

Question
I saw another Greg With a question about the same bike doing the same thing. My bike seems to be doing the same thing sputtering out after a few minutes with a new plug. The plug gets really carboned up. The bike has pretty good compression and doesn't smoke and almost runs perfect after the new plug is installed. I had a entire carb kit done, all jets and needle but my machanic thinks it could be a worn needle jet holder gone worn. The bike did sit for three years and was takrn in to a honda shop in New Mexico and had a carb cleaning and 110 main jet installed as well as new points and air cleaner. bike worked well for 45 minutes. I brought the bike to Oklahoma and put in a 125 main jet and a new plug. This is when I noticed the plug looking like the bike was running real rich. could it be time for a new Mikuni carb or could it be more.
Thanks Bill and look forward to answer

Answer
Greg, I can't say that I have messed with the XLs very much. I see that the stock carb specs are #130 main, needle clip in the middle, #48 idle jet and 18mm float level. If the bike has been changed in regards to the intake (air filter) or exhaust systems, then the jetting may have to be adjusted.

Always start with the basics...  valve adjust (.002 intake and .003" exhaust), compression check (180-220psi), ignition timing adjustment, including testing spark advancer function. The carburetors are highly sensitive to changes in ignition timing and intake/exhaust tract changes. Verify that someone hasn't poked a bigger hole in the jets than what they are indicated. If everything seems to spec, check for blocked air bleed passages in the carb throat. A "rich mixture" can either be too much fuel or not enough air. Check your air filtration system and any blockage around the inlet.

Bill Silver