Here's the deal. 2 brand new 540 engines, 700 hp, they run great except for one problem. In neutral they rev up fine, no issues. While in gear while throttling up from idle they start spitting. If you just push the rpms past it to about 3K it stops but if you try to ease it on up it gets worse. Builder says that the cam etc needs such a high advance that the MSD dist might need lock out kits installed and that will fix the problem? I'm running MSD 8560 dist(think thats the right number?) and MSD 7 marine boxes w/ AED 1050 dominator carbs. Thing runs like an ape after I get past that spitting point but man it's annoying most of all and imbarrasing a little. Any help would be great! Just want to know if builder is steering me right?

what Springs in the Dist? 2 light (silver) will give it the quickest advance ....

What do you mean by "spitting"????

It sounds more like a carb problem to me. Do the carbs have 50cc accelerator pumps?????

I agree with Griff. I would think it's in the carburetors. Doesn't seem to be making the transition from idle into the mains correctly. Did you set the idle speed by adjusting the primaries? If so that may be your problem. Idle is actually supposed to be set by adjusting the secondary butterflies. If you set by the primaries then the butterflies may be open to far to start off with from idle and then it dumps to much fuel in to early as you begin opening them more accelerating due to the position of the hole the fuel is coming in at.
Check out this link about adjusting. Sketch C shows what I'm talking about.http://www.candsspecialties.com/tuning1.html

Also you may need to tell AED your problem and see if they can help. Those are pretty high tech carbs.

It's not back fireing but spitting or maybe caughing

sounds like a midrange girgle to me,,are you running a power valve? could also be a lein pop,,try going up on the jet sizes on the primary side of the carbs...if you stick it in the secondarys and it goes away then your probably lein..

Did you dyno the engines?? afr's will tell you for sure in that rpm range..

Just wanted to thank all for their suggestions. I installed the MSD lightest springs which are the blue springs and the problem is solved! Thanks again for the help!