Engine backfiring upon start up

i have owned a 1980 kawasaki kz650 for about 5 years now and has always run great i had rust in the gas tank and had to get a new tank and clean all the carbs really good... now that i have done that the exhaust backfires upon startup and gets worse as i crank the throttle... also small flames popping out the back any ideas on what could be wrong with the bike i would hate to get rid of it since it is such a great bike

Clean the pilots again.
Steve

Insufficient intake valve clearance can cause backfiring through the carb. As can any other reason behind why the supposedly closed valve isn't fully seating, such as burnt valve or seat, bent valve, needing a valve job, etc.

Popping through the exhaust on deceleration can sometimes be remedied by slightly enriching the pilot mixture(s).

If not already done, would also assure proper functioning of the ignition timing advancer (most easily done with a timing light). It's sometimes necessary to clean and lube the advancer.

Hope the advancer doesn't look like this:


Good Fortune!

Spark plug wires swapped?

Lot of these problems popping up lately (including me)..... all of the above and be sure you have a good inline filter..... carbs can look clean enough to eat off of and pilots can still use another cleaning with high pressure air, cleaner, wire and sometimes a soak depending on how bad they were at the start....... could be leaking air boots too (new boots for me) .....thinking it was in my case hitting the pilots one more time and buying a better gas filter.