Left carburetor backfire?

[BlackGs500]

I just recently got done putting my bike back together. I super-cleaned/rebuilt my carbs. I chased down some intake leaks and found out the filter I was using was also the incorrect on for the bike. The bike has a K&n lunchbox filter and aftermarket exhaust(not sure on the brand). I have 40/150 jets in the carbs. They are bench synced and adjusted to 3 turns out. All o-rings that I know of are in place and great condition. The valve slide rubber is also in good shape, no holes or cracks/tears. Has new needles.

Anyway, I can get the bike to startup and idle just fine. However, it is now backfiring through the left carb. So much that you can see a flame come out the side of it.

[gregjet]

Possible stuck inlet valve or a bit of crap stuck under the valve seat.

PS that's scarey...

[BlackGs500]

Ya I know, it looks like the left carb is just super lean. They were so out of sync, the left sparkplug was white, and the right was wet of gas. I also found out the incorrect sparkplugs were put on the bike, with incorrect gaps for them too(never taking it to a shop again). I have new ones, then I will try to sync it again and see if the problem is still there. The only other thing I can think of is clearances but I will tackle that when it comes.

[mr72]

If you're absolutely sure of the jets, then in order:

1. torn/damaged diaphragm on the lean carb
2. bad vacuum leak on the left side (intake boot o ring, maybe missing o-ring under the vacuum sync port, missing cap?)

Too lean may be caused by air coming in from a place other than the main carb throat (venturi). Vacuum leak is the best way for that to happen.