Backfire from Hell!

So I have to ask the experts. I have been rebuilding my 2003 yamaha roadstar for the last nine months, and today was the day that I finally started her. Well she started up and mostly ran fairly well. BUT she back fires like crazy only when the crankcase vent is open if it is partially occluded with my finger there is no back fire at all. I have no idea what is going on. There is a huge laundry list of mods that were done but the main thing is it sat for 6 years, and almost everything was redone including a complete carb rebuild, granted it was the first time I have even seen a carb. I need some expert advice

Thank you!

Can't speak to the odd backfiring, but pics would help get an idea of what you have going on.

Did you have someone helping or at least guiding you on the carb rebuild? They are simple but there are some little passages that need some attention.

Video

I attached a video of it. I had the manual and the instructions for the jet kit and spent for ever cleaning every single passage, all the jets are new o rings gaskets, new accelerator pump new idle circuit, new enricher  and cover. I just messed with the pilot some more and its a little better but not a lot. There seems to be a ton of air coming out of the crankcase vent

Why did you change the jets? It's not like they wear out.

Have you changed the exhaust? Air filter? 

Have you messed with the ignition system?

You may have to un-mod the engine and take it back to oem spec just to verify that it runs properly as designed and then do your mods one at a time to verify each one operating correctly.

It's going to be very hard to diagnose something that's had a whole bunch of changes made with no operations check in between.

Last, it's a V-twin. It's going to move a lot of air in and out of the crankcase.

Backfire into the intake or popping in the exhaust?

It just backfires out of the exhaust, in fact only out of one of the cylinders. As for mods, well I messed with everything,  new air intake deleted the air injection system, all new carb parts because they were so corroded they disintegrated, new ignition module new ignitor, and I can't back because the old stuff was trashed but it did run when parked since I parked it lol

Just a guess but you've probably got an exhaust leak letting fresh air into the exhaust and causing a pop. That's what does it on my bikes.

Good luck.

In reply to ShawnG :

Agreed. I'd definitely look into that, especially given that the problem appears to be confined to one cylinder.

Isn't a backfire unburned fuel igniting on the hot pipe?  Eliminate the cause - cease unburned fuel with a hotter plug, better valve seating, etc.?

Just thinking out loud ....

Yep, I would check your exhaust gaskets. Just went through this on my Guzzi. 

Thank you for the advice, I went through the gaskets on the exhaust and there doesn't seem to be an exhaust leak. I am a bit concerned, I wrapped the headers and the exhaust cans seem to get pretty warm. I went back and looked through the carb and the pilot screw was really really out like 15 turns instead of 2.5 to 3. I know for a fact that I set it when I put it together, so no idea but I do have little gremlins in my shop that are 6 and 8. However it seems to run well. A little choke to start then within a few minutes no choke and smooth throttle, on off, partial chop it no back fire. I rode it at speed for 20 minites and it did well from low rpm all the way up. So im going to call it good and see what kind of gas mileage i get and go from there. Thanks all!