1997 mercruiser 4.3LH backfire/other issues

[bman440440]

Hello all... I have been lurking on this board for a wile now and now its time for some help...
I have a 1997 1950 ME(sr1900) w/4.3LH and I bought the boat about a year ago and I love it but now I have some problems with it... when I got it I did plugs, wires, dist cap and rotor as well as water pump and engine / drive oil changes just to have a good starting point... and the 1st time out it was a little hard to start IMO and it wanted to diesel(it has all fresh gas in it) when shutting it off. well I chalked the hard starting up to being a carb engine and my other boat has a 350 MAG MPI in it and starts with a flick of the key... and for the dieseling after a little poking around found that these motors tend to do that when you don't let them idle for a wile to cool down after running.
so the next time out after running it I would let it idle for a minute or two and that did help a lot but did not completely stop it... so I check the timing and it looks like some one at one point tried to set it with out locking it into base mode, so I set the timing properly to 10 deg BTDC and take it out again and it was running good for a little wile then it started to back fire threw the carb when trying to make power... so now before I do anything more I would like some input from the people on here if I had done something wrong or what I should do next... BTW it does start way easier and rungs good up until about 2000 RPM +/- then backfires.... any help from will be great!!