Re: 04 yamaha four stroke 150 hp heads
Your motor probably is not turning over fast enough due to a bad battery, battery cables or starter. Load check your battery and cables before you go taking anymore parts off the engine. A Yamaha will not suddenly get out of time, I have no idea why you thought it might be. Besides that has nothing to do with the engine turning over.
The gears that are "seized" are probably perfectly alright. If you are trying to turn them by hand, forget about it. They are connected to camshafts that are trying to push against multiple valve springs, you can't do that turning them by hand.
My advice???? Get a manual. Put the belt back on, following the procedure in the manual STEP by STEP to make sure the engine is timed right. Check your battery and cables, I'll bet your problem is there.