I'm trying to help my buddy. He has a 1989 Ultra with a mukuni carb and ignition dual fire coilwe have changed coils 3 times along with plug wires,went over all wiring making certain as no wire has a bare spot but the front plug continues to foul only onwhen starting the bike after the bike is running it runs great.Whenwe pull the plugs they look brand new but they arefouled. It will pop a new plug just after replacement. When it foulsthe plug it is likea backfire in the cylinder not the carb. Any help would be greatly appreciated Thanksbuck

 

Change the carb float needle and have a good look at the needle seat. Sounds like intermittent flooding. Very common on carbed bikes regardless of carb brand. H-D also shipped a lot of bikes with the wrong needle in their CV carbs that do the same thing.

 

We rebuilt the carb with great results thankyou.