The bottom end is now removed (two screws). The two screws sandwich a couple of items: (1) gasket and diaphragm, (2) cast aluminum plate with adjustment screw, (3) gasket and diaphragm.
When the screws were removed and the housing with the vacuum feed is taken off, I discovered the source of my problem. Two pieces of debris were in lodged in the components. One was afixed to the diaphragm, the other caught in the spring. This would explain the erratic behavior, why the machine would run fine at times, other times would choke out. The location of the debris was impeding the flow, and the passageways were partially clogged. Apparently, when I switched the trimmer right hand to left, that motion was enough to move the debris into a place where flow was cut off, or at least choked down. It also explains why my action of vigorously shaking the machine would solve the problem sometimes, but not others. The shaking moved the debris.
The debris was a red, rubbery like material. It could have been a shaving from a drill bit in a piece of plastic. It was not obvious the source. It was clearly not a fuel tank issue, because it was lodged in the fitting coming from the vacuum line, not either of the two smaller fuel lines. Yes, a mystery of what it is, and how it got there.
The appropriate gaskets and diaphragms were changed out for new ones. I probably could have eliminated the replacements if I would have just taken out the debris, and closed it up again. But, "while you are in there" prevailed, and I had replacements at hand, so they were installed.
The arrangement of gaskets and diaphragms are obvious. In other words, which one goes where, and how it is to be oriented is without question. There are holes and pegs that are used to index the casting into the right position. Small holes in the gaskets and diaphragms insure that the G & D are properly placed by using the pegs.
All was closed up, the carb remounted on the engine. Be sure to get the fuel and vacuum hoses properly connected. Mark the two small ones before removal if you think you might get them mixed.
It started second pull, and has been running top notch since the repair was done a week ago. It starts first pull, it winds up nicely, runs well at WOT, and has good power. All is well with my FS90R trimmer.
Oh yes, I like the gas cap too! (sorry, just had to add that in the mix)
I hope this was useful to somebody. And, maybe a skilled Stihl mechanic can tell me what I did wrong as well. Thanks.
Last edited by Roger; 09-26-2010 at 10:12 PM.