By: Anonymous: 400man ()  Thursday, 13 October 2016 @ 09:14 AM ICT (Read 268 times)  

I have a '80s Yamaha XS400 and I have no idea what is going on with it or what is causing the sooted spark plug on the left cylinder and which I think goes hand-in-hand with the inability to rev cleanly beyond 4,000rpm. Every ignition component has been replaced and where possible swapped with the right side cylinder to no effect. I still end up with a sooted spark plug!

I have renewed spark plugs, points, condensers, coils, HT leads and HT caps. On the carburetors I have renewed float valves, and all jets in the left-side carburetor. The diaphragms ae perfect, both slides operate cleanly and in harmony and the carburetors have been balanced. Air filters have been renewed 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, iming valve gaps are okay and compression on both cylinders indicating 150psi. The bike starts on the button, ticks over nicely and runs well below 4000rpm

By: news (offline)  Thursday, 13 October 2016 @ 11:19 AM ICT  

It sounds like a carburetor problem to me, in that the sooty spark 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.



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