Predator 212 hemi.

SRTsFZ6
A bit more information please..
 
Has it been run low on oil, if on a mower, the blade hit anything large?
 
Is the engine currently attached to anything?  If so, did you try disconnecting it (verify it is the engine, not the device it's powering)..
 
Been sitting for a long time?

 
Does a Predator engine have a flywheel brake like many Briggs used on push mowers?
There's an old say that goes: "Garbage in Garbage out!" not intending to insult OP but there is a serious lack of information for one to give a good answer. One could probably think of all kinds of off the wall theories but 99.999% of them would be wrong.
 
Like SRTsFZ6 asked "we need more information"

SRTsFZ6
 
Is the engine currently attached to anything?  If so, did you try disconnecting it (verify it is the engine, not the device it's powering)..
 

 
You have not answered SRTsFZ6 question yet.
 
You did mention it being on a generator so the problem might be the generator and not the motor.

If the heads still off, try, by hand, just pushing the valves open, checking for binding..
 
The valve seats look ok,  not sticking out any? 
 

Something is odd here! Why would motor turn over easy with head off but not with spark plug is out? That would say problem isn't a compression release problem, drag on generator, main bearings or other moving parts. 
 
Is this hard turning steady or is there just one point in a revolution that it gets tight for a short distance then easy the rest of the time? Or is it only turning hard when head is on?
 
I'm sure many here could figure it out if they had hands on but it can be very difficult when depending on someone else's description of what is happening.
 
One question that I don't recall being asked is when did this "problem" start? Did it turn over and fire up and run good one day then the next time you went to start it turned over hard? Or is this some unit that was bought at a yard sale with out seeing it run and the problem was found once it was home?
 
Very often in situations like this the back story is what will contain the solution but too oftn it is never shared.