With that cam, a surging idle is gonna be pretty much the norm! That is a pretty hot little number you put in there (Certainly not an RV cam and well into bracket race spec).  Sounds like a Comp Cams 292H, by the specs.

Personally, I think you can tame some of the surging down with a bigger carb, I don't think the 750 is near large enough anyway given your application and hardware. Single plane intake works best on that duratrion of cam, I would think. But to get it to idle down in the 900 range (It'll still surge some, mine does with a little wilder cam than that and 2 -750's on it) You're gonna need a bigger carb at least and I would suggest a good single plane intake such as the Torker.

I doubt timing has much to do with it, but might.

If you want a smooth, low idle, something in the 268 degree range or so is about as far as you can go on that. Maybe 280, but I doubt it, most 280's I've seen run still have a good lope to them. (where the surging idle comes from).

Just my .02

Ray

I'd try it with the timing set about 14 initial and go from there.  Does your carb have 4 idle adjusment screws or two?  If it's only got two I'd check to see how far open the secondary throttle plate is at idle, there is an adjustment for it but you have to take the carb off to get to it, I know on my old carb with the two idle scres if I open that up too much it surges, you can try playing around with that. You can also open it up a bit to see if you can close the primary blades a bit more, that you can do by using a piece of paper or a very thin feeler gauge on the secondary side where the stop is, if that helps then remove the carb and adjust it.  Before you do any of that though, did you do the obvious, like check for vacuum leaks, correct float levels? 

Double check for vacuum leaks.  With that cam it will have a fairly rough idle but I don't think you sould be surging.  

For timing, I recommend setting with the rpm at 3000 and using a dial back timing light to get your total advance set.  14 initial should get you 34 total which is a safe place to start.  I'm running mine (similar set up) at 36 total currently.

Does it surge when the boat is in the water or on the trailer? you want to set your idle in the water.