Even without the use of butane (see previous post), today brought an interesting development. As I was re-cleaning and reassembling the carb per the recently quoted instructions from a Montesa dealership mechanic, I noticed a discrepancy between the parts before me and the exploded view in the diagram. An o-ring and washer were missing at the inboard end of the fuel screw! When I'd disassembled the carb the first time, I assumed it was a complete unit, and simply took care to reassemble it as it had been. I didn't go through, part by part, and take a thorough inventory in comparison to the diagram. I couldn't find a properly sized o-ring in the assortment I had handy, nor a washer that would fit, so I went ahead and ordered a rebuild kit with all the exactly correct o-rings, washers and gaskets in it. Maybe all my troubles have been the result of this missing o-ring, as I'm assuming a leak at the fuel screw would result in the type of lean condition symptoms I've been having. And it could explain why turning the fuel screw throughout its range seemed to make virtually no difference in the motor's behavior. Hopefully, the kit will be here by the weekend, and we'll know if it does the trick.

I'm going to wait on the butane test for now...

Because of safety issues I feel the need to talk hypothetically: Suppose a person with a trials bike wished to elimate ( very quickly) the possibility of an external crankcase-carb-reed block air leak? Would he get quick results from opening a can of butane in the vicinity of the idling trials bike engine with regards to stabilising the tick-over speed, this could help rule out/ locate a range of air leak problems?

You don't smoke, do you?

If it all goes wrong, will you let me know and I'll delete this post

Spray some carb cleaner around the intake to locate a vacuum leak, The idle will pick up when you find a leak. Much safer than butane!

Glad you found a problem as you went back through the bike! Do not forget reply #13. The oring is needed but check that choke assembly.

Sounsd as though you are on to something there with the missing seal and wash. The carb should be clearly marked either PHBL or PHBH.

The PHBL has none that other crap, jusrt the fuel trim screw!

Bike now idles quite nicely after installing the new fuel screw o-ring. I can't be 100% certain this was the solution, since I gave the carb another thorough cleaning whilst waiting for the carb gasket kit to arrive. So I might have dislodged some gunk this go-round that survived the previous cleaning. Anyway, the change is dramatic, and I'm greatly relieved to not have to chase down and repair a main seal leak. Thanks to everyone for all the help!

Now can I remove my original post?