I'm no expert but it sounds like the the a/f is way off and flooding the engine. I'm not sure about the 750 carbs but the jets shouldn't be too much bigger than those. I wouldn't think that going from 155 to 160 jets will help.
One thing I'd also check is oil contamination. A quick sniff will normally give you a good indication, if it reeks of fuel then you may have washed out the cylinders. Check compression to confirm this.