1980 Yamaha XS400SE sooted plug


Q: Well I’m truly stumped with this bike, I have no idea what is going on with it or what is causing the sooted plug on the left-hand pot and which I think goes hand-in-hand with the inability to rev cleanly beyond 4K revs: every ignition component has been replaced and where possible swapped with the right hand pot to no effect. I still end up with a sooted plug! I have: renewed plugs (several times), points, condensers, coils, HT leads and HT caps. On the carbs I have: renewed float valves, and all jets in the left-hand carb. The diaphragms are perfect, both slides operate cleanly and in harmony and the carbs have been balanced. Air filters have been renewed (even tried running without them to no effect) the choke mechanism has been checked and functioning as it should. The needle clip is at slot four, from the top. Additionally the points gaps are spot-on at 14 thou, timing (bulb lights exactly on LF and RF point), valve gaps are okay and compression on both sides is indicating 150psi. The bike starts on the button, ticks over nicely and runs well below 4000rpm!


 A: It sounds like a carburettor problem to me, in that the sooty plug is showing excessive richness. I take it that this only manifests itself when larger throttle openings are used and this would point to the main jet or needle jet assembly being incorrect. The main jet is supposed to be a 135, and am I right that you have just fitted a brand new one that cannot have been damaged by being prodded with a piece of wire? There should be a metal washer between it and the carburettor body, if it makes any difference. Otherwise maybe the needle or needle jet are incorrect. You might try swapping over the various bits between the carbs to see if it makes a difference. You fitted a new O-ring with the replacement float valve assembly of course? You might be able to rig up a piece of clear pipe connected to the drain screw hole to check that the fuel levels are the same. Lastly, are you sure the choke mechanism really is working correctly so the plunger is shutting off the passageway fully?