if this is a problem that just started out of nowhere, and it was previously running fine, then I'd look at valve springs.

however, I had a mishap once... mis-read the timing marks on the gears when I installed them. cam ended up about 30 degrees advanced. engine fired right up and ran great. but it would NOT rev over 3,000 rpm.

so don't be afraid to check cam timing either if you find that it's not in your ignition anywhere, and the valve springs are ok. although it's unlikely, I suppose it could be possible somehow if the chain is old and very slack, to have jumped a tooth or 2...but if it was that loose, I would also think that your timing would be eratic as the slop in the chain allowed the cam to lag behind, then get yanked forward, then lag behind...giving you a tick-tock back and forth on your spark timing.