1980 mercruiser 305 228hp inboard problem

I have a twin 305 228hp inboard quadrajet that ran beautifully until after almost running out of fuel...refueled...immediately my port engine started "hunting" and had erratic rpms...stalling...I restarted...it starts and runs for a few seconds hunting the whole time...sometimes backfiring through the carburetor and then dies...will not idle. I've done the following:
switched carburetor from good engine to bad...no fix
switched carburetor from bad to good... good engine worked fine with bad engine carb
replaced fuel pump
changed fuel filter
switched fuel line from good engine over to bad engine...no change in bad engine
I am looking for suggestions for what to check next.
Thanks

After endless hours of trouble shooting toward fuel issues I turned my energy toward the distributor, I checked the gap and decided to change the condenser even though it was new,guess what the condenser was faulty and now the engine purrrrrs like a kitten, this stuff can drive you crazy. If you have backfiring through the carb and it is huntin.g or hardly running change the condenser first.