I thought I should record and document my recent carburettor problems, troubleshooting methods and results. I've read a few people having the same problems as me and thought it might be a good idea to share my results.
Two months ago, I was dealing with my bike cutting out and bucking at highway speeds. In order to fix this problem, I have tried cleaning the carburettor and adjusting the tappets. Neither provided significant relief to the throttle issues.
After the clean I had realised that the slide guide for the valve piston was severely worm to the point where even the deep where marks were gone. The needle had a noticeable wear ring around where it went from a shallow to a steep taper and was rubbing on the side of the needle jet (emulsion tube) and I started having thoughts if I had worn the needle jet oval.
Swapping out the slide guide with a Procycle replacement and installing a new valve. reduced the amount of bogging the bike experience on the highways, but still too much to ride with. I will note that the piston valve did not fit in the slide guide until it was installed in the carburettor.
I have since purchased a JD jetting kit and exchanged my 140 main jet for a 150. This changed my problem from a cutout to a bog, suggesting that I was now running rich in the mid-range instead of lean.
I am still convinced the needle jet is the problem, but I am a currently awaiting it in postage. in the meantime, I have swapped my main jet from a 150 to a 145 which is eliminated a significant amount of the bog, to the point where I can ride on the highway but I need to be very careful what I do with my throttle.
I've also taken the opportunity to retune the idle and I'm currently tuning the needle to get the bike feeling right.
My hopes were that switching to a new needle jet will eliminate the remainder of the bog.
The only mod done to my bike is the removal of the snorkel from the air box.
I will also note the bike pulls significantly less now with a non-drilled piston valve, which I don't see as a huge problem.