Ran into another odd-ball issue.
It fired up and ran great about 4 times in a row... then on the 5th time (this is after significant cool-down between start-ups). It would backfire. I had the tank on "RES".
I went chasing electrical thinking it was the points, maybe a bad switch, but no... everything was working perfect. Big, fat purple spark on both plugs in both boots... so not the coils.
I decided to pull the fuel hoses and check the petcock for flow.
On RES, I had barely a trickle out of the outlet going to the LH carb... the RH carb was fine.
On ON, I had full flow.
I looked in the tank and the fuel level was about an inch below the "ON" pickup. WTF?
I took the bowl off the petcock and a bunch of sediment was in there. Which is odd, because I soaked the tank in vinegar for 3 days in all different positions and rinsed it out VERY well with distilled water and baking soda, then again with some 2-stroke gas.
There may be some cancer inside the tank I can't see that's causing this.
ANYHOO... cleaned the petcock, added fresh gas, then it fired right up and I took it for a ride... all was normal.
Looks like some Kreem is in my future, but for the time being a couple of 90-Micron screen filters and an extra tank flush will have to do.
This was the result of 2 cups of fuel run through the petcock. I poured most of the fuel off... but there's a good amount of jet-clogging junk here.