It aint rocket science
Join Date: Nov 2010
Location: Glastonbury, Connecticut, USA
Model: The best
So the problem appeared after a winter layover. You are a long time owner and presume riding less as we age.
I suspect the older unused fuel left in the bike finally caught up to you and fouled the idle jets.
Like Ivory soap 99.7% of the time running problems on a 1500 are carburetion/emission control related. The first thing an owner does is crank up the idle and maybe jack with the sync screws on an engine that won't run. An engine surge is then produced if the engine is not constantly being fed the metered fuel from the carb(s) due to a blockage. The limited fuel flow in the carb circuit collects in a pocket then gets sucked into the engine causing the surge and repeats over and over.
Remove the air filter and with a little carb clean spray dribble it down into each carb throat. The idle RPM will go way up on a faulty carb/cylinder bank.
There is NO substitute for experience
I'll be around when I come around and not before
Internet myth buster
WS Grand Chapter CT
If local and need help with a 1500, Valkyrie or ST repair please PM