I've started a new thread as this was getting a little confused with lowering.....
Thanks to Bobn on the XRV forum I've discovered another probable cause. To quote him:
"My money is on the slide lifter mechanism. I've seen two carbs now where this has been seized.
If you look at the pulley that the throttle cables wrap round you'll see that there is a cam profile running around the periphery. This acts on a little lever with a nylon bearing that runs on the cam.
When the throttle is opened the action of the cam lifts the lever and this in turn rotates a shaft set into the carb body. The shaft acts on a fork inside the carb body which lifts the whole slide assembly of it's boss. If the lifter shaft is seized, it will be seized in the "up" position which means that the carb slide will not be seated on its boss when the throttle butterfly starts to open. This can cause the misfire."Mine wasn't seized but the return spring wasn't (returning that is), so the lever wasn't returning to its lower position - the flat part on the cam below. I've temporarily fixed it with an auxiliary spring - not pretty but does the job.
Actually, thinking about it and looking at the picture, it probably hasn't fixed it at all as the pivot is probably attached to that forked part just above the lever and that doesn't move so probably the spindle is seized after all.
