#2 plug carbon fouled

[xszgrider]

My #2 plug keeps getting carboned fouled and the bike misses above 7500 rpm.

I have performed the following:[list=1][*]Checked the valve clearance. #2 intake was at the tight end of the tolerance, so I put in a new shim.[*]I measured the compression after the valve adjust and #2 was at 135 psi.[*]I measured the resistance of the coils with the end caps off, and they were in spec. [*]Put in new NGK BP6ES plugs.[/list=1]

Is this a carb or coil problem? I still have the stock coils.
Thanks

[randy]

You've pretty much eliminated ignition ( did you check the caps?)and compression as culprits, the only thing left is a carb problem. check the float level and clean/replace the float valve in #2?

[prometheus578]

Could be several things:
Carb #2 could be acting up. Time to pull the carbs. Float height could be off... Float needle might not be sealing properly and causing a higher fuel level, main jet might have fallen off and is lying in the bottom of the bowl, etc.
Could be an odd intermitant breakdown of the plug wire going to cylinder #2. Switch plug wires from #2 and #3 and see if the problem switches, too.

[81xsproject]

why would you change the shim if it was in tolerance? I tried to get all mine to the smaller side of the tolerance range... figured I was giving myself more wear miles before I had to do that horrible job again.

[xszgrider]

valve shim tolerances

It has been my experience with my Honda CB1000C and a friend who has CB900C, that the valve clearance gets smaller with the shim over bucket types. I do not know why.

It can get so tight that you lose compression in that cylinder. I saw this on my friends CB900. We corrected the clearance and his bike has been fine for the last 2 years. I have heard that you can even damage the valve seats if you lose compression.

I only assume the same thing happens with the Yamaha's. I am not aware of any problems from being a little loose. Maybe there are some experts on this site who can help.

[prometheus578]

"No experts, just me."

Over time, as the valve wears, it seats itself up further in the head, reducing the valve clearance.
Valves that are too tight, may actually hold the valve open, thus losing compression, to say nothing about the hot gases burning the valves as they slip past.