Engine RPM of 1200 when cold and gradually rises to 1500 when warmup

I pull off the cylinder and replace the head gasket with timing marks march up correctly on Nissan Qr20 30t. when i start the engine cold the rpm goes to 1500 and start to come back to 1200, and as the engine warms up, the rpm will rise back to 1500 when idling. I have change the MAF sensor but still the same.
Kindly help with any advice. thanks