you can bump the timing up a bit likely. a lot of people run 20 deg base timing. I cant seem to get mine running right at any timing below 15. be mindful of spark knock, but many people including me haven't had any issues on normal gas up that high.
also the webers are sensitive to fuel pressure, where the hitachi isn't. you will want to check your pressure if your gonna run the weber. should be between 3 psi and 3.5 psi. I just installed a $17 inline reg. its worth gold to just pop the hood and be able to see that your getting fuel pressure. Also get a 3 way filter and install the return line again. helps take some of the pressure off the carb
another common problem is water leaking into the manifold from that coolant passage. the carb gaskets and adaptors aren't generally set up to accommodate them. some JB weld, epoxy, whatever you want that will fill and block it before putting the carb on. I welded mine shut.
might double check your jets, 3 turns out is quite a bit, should be closer to 2 if the jets are standard. maybe a vacuum leak as well requiring a rich carb to adjust it out? also check your float adjustment and make sure your choke is not interfering in some way.