What exhaust and air filter do you have?

 

BMC Race Filter (De-snorkled), and Scorpion slip-on. No spacers on tank at this point in time.

 

I had a K & N that I Dremel'd out to the filter element, and with a 150 main it was lean. Did you shim the slide needles, or replace them with richer needles? Less likely to be causing a problem at WOT, but you should do that anyway.
It might be lean, I'd say. Sort of strange for it to only happen in 6th gear though. Try taping off part of the opening in the filter to restrict air flow a little bit, and see if it helps. If it does, that would tell you you are running lean.

 

The needles do have a single shim on them...cue my ignorance...but I don't fully understand how the needles being shimmed (or being aftermarket) affect the fuel mixture. I am safe to triple shim the stock needles, or should I just buy something akin to the DJ needles that are adjustable? Just trying to learn as I go without breaking anything...doing well so far.

 

The slide moves in and out(with the needles in it). Out =more fuel, in=less. So, when you shim them, you move them slightly more out, and richer. I have 3 shims in my needles. Seems that the norm is one or two though, so take that for what it's worth (I had a dyno tuner help me figure out my jetting).
Use some duct tape, or gorilla tape to block off some air in the filter opening. I'd just block it a little bit, and see if your problem goes away, as the easiest way to tell if you are lean. Less air, same amount of fuel means the ratio is richer.

 

Sorry for the long delay before an update...I tried taping the air filter. I taped over approximately 1/4 of the inlet hole (probably way more than needed, I know, but I wanted an immediate noticeable effect). I did accomplish getting the effect, but not the one I had hoped for. The taping seemed to accentuate and amplify the lurching at WOT. Could this be an indication that the reverse of my initial guess was wrong, and that it's actually running a bit RICH at WOT. I'm not familiar with the symptoms of either condition. BTW, I earlier posted that the condition was only in 6th gear...not true, just much more noticeable in that gear...but happens in others as well. Trial and error may be my next option with regard to jet sizes.

 

I'd experiment with the slide needle also. You can either buy an aftermarket needle, or go to radio shack and get the "small washer pack" they sell with assorted small washers (I guess you already knew that) of different sizes. It has a bunch that fit between the c clip and the rubber /plastic stopper on the needle.

 

Questions: Is shimming the needles a sort of "bridging the gap" between jet sizes, as a way to fine tune the setup? Also, is there any true benefit to an aftermarket needle beyond ease of use (i.e. clip positions)?

On a side note, LKB3rd (and others), thanks for your non-condescending replies to my posts. It is the folks like you who make these forums a place to which I find myself returning religiously. It does not go unnoticed.

 

The pilot jet, and fuel screw effect idle and very low revs. The needle controls fuel at mid rpms, and the main jet effects wot. There is overlap between the three, and they will all have a smaller effect at the other ranges. It's sort of a balancing act to get all of them just right.
No problem on the help. I get a lot of help here when I need it, so I look at it like "paying it forward."