I have a mercruiser 7.4 300hp and when idle i can get it to 4600rpm but when cruising only 3000rpm?

RPM is determined by load. So, it's normal to be able to rev the engine a much higher rpm in neutral. The real question is is cruising 3000? This engine on most boats should have a full throttle max of about 5000. There could be many reasons your not getting this. First, Make sure all 8 cylinders are firing. (spark) you can do this with a timing light. Then check timing. Next check the fuel. Is it fresh, filters clean etc. Then look at the prop. no nicks bent blades, etc. Now look at the boat. Is there a lot of water in bilge, or heavy load, like a lot of gear. pick the boat up out of the water and check the bottom carefully. It would be a good idea if you have someone familiar with this boat, make and model, look at it. He might know the history of this boat and know where to look for problems. Good Luck.I have a mercruiser 7.4 300hp and when idle i can get it to 4600rpm but when cruising only 3000rpm?
The first thing to check is your prop.Is it the proper one for your boat size and weight?Did you just purchase this boat?When did it have a tune-up last?Check and change all your fuel filters.You should also check your timing.It would be a good idea to check your compression too.Hopefully you only need to drop your prop pitch.Good luck.