A few days ago I was out riding with the kids.  We had been hot rodding a bit with the bikes, 75-80 MPH,  and I pulled over the S40 stalled when I went to idle.  This has never happened before.  It did it twice.

Is the idle mix rich or lean?  It has been jetted and a Harley muff installed.

That is what I found strange about it.  It has been over a year with the same jetting and idle speed.  I always run premium gas with no ethanol.

I filled up again  today with 100LL.  Maybe a bit of gunk on the idle jets?

Flint,

If the Transient Enrichment Valve is dirty you can get the engine to stall when gearing down or coming into a stop.

Before you start cleaning the TEV, stuff some rags around the starter because if you drop any of the screws you will never find them again. While you are at it it would be a good time to replace the 3 M4 x 10MM screws with stainless steel Allen head screws, or at least plated Phillips head screws from Ace Hardware.

Hold your thumb on the cover while removing the screws as it is spring loaded.

It is easy to clean the TEV, after disassembly use soap and warm water to wash the rubber diaphragm.

While you are inside the TEV you will find a small orifice that you should blow some air through to make sure it isn't clogged. The little air tanks you can buy for cleaning computers will work if you don't have a compressor.

As Dave suggested speeding up the idle a little will help.

Kenny G

100LL isn't doing anything for you but making your wallet lighter. It doesn't improve performance on this bike nor does it clean the carb as some have said. This bike has 8.5:1 compression and only requires 87 octane.  

kris  100 low lead is more like 91 octane if measured using the cargas method. it is good as it has a ton of lead to cushion the valve seats. also it has zero alcohol and keeps the tank .carb and rubber stuff like new.

There was a period of time when I used Avgas in my chainsaws and yard equipment.  I did this to avoid the ethanol.  Avgas doesn't burn at the same fuel/air mix as pump gas, and it runs rich if you don't rejet to account for it.  The WOT speed of my chainsaws would drop 1,000 rpm when switching to Avagas.....until the mixture screw was adjusted to get the correct fuel/air ratio.  The E10 requires more fuel to mix with the air...and if you are jetted to run correctly with E10........the Avgas will result in a mix that is too rich.