Bring number one piston up and advance the timing, depending on what kind od vehicle it is!

Sounds like the valve timing is off, but you need to give more info. to answer question. make, model & year of vehicle, was the engine set at top dead center before it was taken apart, ect.? A common mistake when reinstalling the timming chain ( if it has overhead cams ) is getting the valve timing 180deg. off. Recheck valve timing.

I agree that you have a timing problem. Its not that serious. 1: Find a repair manual for your autos make and make sure you have your spark plug wires in the proper firing order. This is a very common mistake in putting an engine back together. 2: Did you remove the distrbutor? If so then you have not replaced it right and your firing order is offset. Remove distributor, find top dead center on number one cylinder, and replace distributor with the rotor in the proper position for number one cylinder as per manuals instr...

I agree that you have a timing problem. Its not that serious. 1: Find a repair manual for your autos make and make sure you have your spark plug wires in the proper firing order. This is a very common mistake in putting an engine back together. 2: Did you remove the distrbutor? If so then you have not replaced it right and your firing order is offset. Remove distributor, find top dead center on number one cylinder, and replace distributor with the rotor in the proper position for number one cylinder as per manuals instructions. 3: In replacing the heads you should not have touched the timing chain or belt, but if you did then you may not have replaced them in the proper alignment. Again consult your manual for the proper alignment of the timing gears.