Really won't start - Help please!

[spr402]

I have an 1989 GS500E.  I bought it from a buddy who did not run it for 2 years (questionable if he stored it properly as well).

It ran fine when I got it, good oil levels, only premium fuel.
I had to replace the gas tank but used the original fuel filter and petcock. 
I started having a problem with starting the bike.  It would not start and sounded as if there was no fuel reaching the engine. 
There was fuel in the float bowls but still no luck.  javascript:void(0);
I decided to take off the carb, give it a superficial clean and also replaced the fuel lines coming off the gas tank.
The bike started and would idle okay but would quickly stall out with any load (in gear) or any throttle.  javascript:void(0);

I took off the carb again and gave it a detailed clean as per the "How to clean the carb" page on this website and the Clymer manual. 
I did not replace any jets and the jets in the carb are not adjustable so I only cleaned them and put them back in.
I did replace O rings as required, keeping only the O ring on the fuel float assembly.
And I took very special care to ensure that the hoses went back on properly.
I also changed out the air filter for one from a Suzuki dealership and changed out the plugs with a 0.76 gap.

Now the bike starts but once again stalls out with any load or throttle.  It also backfires now.  javascript:void(0);  When I have ridden to ensure it is in working order, it quickly starts to bog down when the fuel selector is in ON and stalls out.  It is very hard to start, even on full choke.  If I switch to PRIME on the petcock, I can get the bike started but the backfires are loud, there is no power or pickup and stalls after a few minutes.  After this, when warm, it will not idle either.  The bike will start, supper then die, and sometimes it won't even start.

I have tried everything, checked every site I could and still nothing. javascript:void(0);

I know it isn't rust or fuel flow.  There is spark and nothing is interfering with the air intake.

Any help would be very much appreciated.

Thanks

[PachmanP]

Are your float levels ok?  Getting gas in the airbox would be one sign they're too high.
Your petcock could be bad.
Is there any change with the choke on or off?  Maybe that's sticking somehow.
Have you checked the valves??