I have a 07 250 Mercury Pro XS. This week I had a complete tune up done to the motor.I also had a brand new cranking battery installed.When I took it out for the first time today I could not get it above 4200 rpms.Also when I would trim the motor, while on plane,it would bog the motor down below 4200 rpms.At one point it went up to 5200 rpms but my depth finder started cycling through the dim to bright light screens.Can anyone tell me what is the problem with the motor? Thanks.

check to make sure your connections are tight, and make sure they put actual nuts on the post instead of the wingnuts. It might be a bad battery, but that is where your problem is.

I checked the battery,both connection was tight with regular nuts and not wing nuts.I purchased the battery new yesterday.Its an Interstat 1000mca and 800cca.

check the plug on the motor harness and make sure it is making a good tight clean connection, and all of you spark plug wires are snapped firmy on. also make sure all of your plugs are in there tight.

How long has the engine sat?

No more than 2 months,and I do add Stabil ethonal treatment in the fuel when I fill it up.

Sounds like ignition coils.

Would check coils first. I had to have my high pressure pump replaced when it was doing the same thing.

sounds like maybe you got a bad "new battery"
Have it checked , "new does not make it good "
thats why they have a warranty

Your voltage while running ( seen on an in dash voltage meter or on your sonar graph ) should be 13.3 to 14.1 volts being produced from the alternator . If the voltage is lower than 13.3 then either the alt or the voltage regulators have a problem. Maybe

I say this because I had an alternator belt break and my motor acted the very same way. Just a suggestion.

I would also read this link on the required battery for the big Mercs
http://www.bbcboards.net/zerothread?id=346429-batteries-that-meet-the-requirement-this-just-in