rebuilt motor running like its way out of time

I just recently completely rebuilding my omc 3.0 4 cyl, 181 cu bored 30 over to clean the cylinders went with flat top pistons rebuilt my heads had new valves and springs put it back together set initial timing as per manual put muffs on cranked over ran smooth for about 2-3 minutes and then acted like a fat lady on a tread mill jumping around running real rough as like timing was almost 180 out shut it down double and triple cheched everything getting spark, fuel, andd got great compression carb was not tampered with from last season so should be with in reason everything was spot on so i cranked over again same thing so i pull motor out of boat completely pulled timing cover to see if poss sheared the key negative key is still in tact and in places gears are fine so i check everything again got spark getting fuel got compression set timing on initial timing again as per book and crank it over and low and behold problem still the same

if anyone out there in cyber mechanic world that has heard of this or any trick or even if some old gramps tale of how to get it back to life please send answers this way much apreciated:confused

Re: rebuilt motor running like its way out of time

If it ran good for the first 2-3 minutes, then I would check to see if something is stuck or broken!!!
I am sure your using fresh gas, but getting fuel is one thing, getting the correct fuel/air mixture is another, I would check to make sure no debris has your fuel mix off, and make sure you do not have water in with your gas.
If I was in your shoes, first thing I would do is clean and rebuild my carb. and pull the fuel from another source.
Good luck