Ignition related backfire questions for the wizards.

Drained and cleaned the tank.

Since it was leaking after reinstallation, had to replace the petcock to fuel filter line with 4" of clear vinyl of a smaller ID. This is temporary to get gas to the carbs until I got proper line the old piece was the wrong stuff anyways. Had to use prime to run it. I figured this was because of the smaller line bottle necking fuel.


Went to start up and it ran good considering it had been sitting for so long.
Rev'd nice and smooth, idled a "tad" rough.

Tried to start the next day and it wouldnt fire. I did turn off prime after running it the night prior. I figure I have stuck float valves and then some going on in carbland, so I'm looking at ripping into the carbs. After all, they've only been sitting for three years with fuel in them.
Stuck bowl screws. I already have stainless replacements from Len.


Anyways, I noticed some strange things I was wondering if someone can explain.

What happens electrically when the ignition key is turned off?

At one point I removed the YICS bolts and the outter vacuum caps figuring it would help air out the cylinders overnight. I came out the next day and replaced vac caps and forgot one YICS bolt. I turned on the ignition, cranked it over a few times using different choke positions, then gave up.
When I turned off the ignition there was a pressurized squirt out of the YICS passage?

Today I finally got a 13/16 socket, pulled the plugs, inspected (Black btw), cleaned, gapped (two out), anti-seize, torque wrench.
Went to turn it over and nothing. Decided to pull the vac line from tank and try. Nothing. Then I got the idea to plug the Vac line to Cylinder three and try to start it. Nothing!
When I turned the key off this time "BANG".
Sounded like a .22 and my ear was ringing. Person walking by past the garage jumped three feet in the air.


And finally my questions:
Whats happening when I turn off the key?

Could something else be wrong on top of the needed carb disection?

As always, thanks.

 

The bang was your coil discharging its stored voltage to a plug and left over fuel ignited through an open exhaust valve, fun eh. Have you checked compression? Start there. Then your due to pop off the valve cover and whip out a feeler gauge and check valve clearances. Once done you will have a better idea of what is up.

I'm no wizard but this happened to my 550 before I got it tuned right.