Help? Power issues?

I have a 1984 Charger inboard/outboard with an OMC 305 engine. We have been having power issues and would like to know if anyone has encountered the same thing or may have suggestions on what the issue may be. Here is the issue: the boat starts and runs great. When we are in the water and engage the throttle to full power the RPM's go up but it takes a long time for the boat to actually catch up to the RPM. It seems like its about a 5 minute delay before it picks up speed. When it does it takes off and runs great. After its been throttled down for awhile it does it all over again. We recently had a new impeller installed recently but it was doing this before it was installed. Anyone have any suggestions?

Sounds like a spun prop. Pull your prop and look at where the round rubber bushing meets the metal prop casing. You will likely see that the rubber where they meet is raised and distorted. Look carefully, because it can be hard to see. So, find another prop that fits with the same diameter and pitch, and send the old one off to a specialty prop shop and have it refurbished.

Really? Wow, never even thought of that. We are pretty new to the boating community. Thanks so much for the help. I will definitely pull it off and check. We are to go out to the lake next weekend. Will let you know if that fixes it. Appreciate the help!

That old 305 is a fine engine, you just need to keep it maintained. Keep the rpm's reasonable. Change the oil and filter religiously at its specified maintenance interval. Occasionally add a can or two of Sea Foam to the fuel to keep your carb clean and break up carbon that accumulates on the valves. One weakness they have is a thin oil pan, so every once in awhile, work your hand down under a cold engine and see if there is oil leaking there. If so, it is possible that the oil pan has rusted through, so the engine will need to be pulled and the oil pan replaced. If you need to replace it, find an aluminum or stainless steel replacement. Also, it probably has a canister-type fuel filter. Change that out every two oil changes, or so. If you ever need to change the canister cover itself, an old Ford canister is a perfect fit. One other thing, you should have a Recor fuel/water separator installed in the fuel line between your tank and the engine. That will keep water from getting into your carburetor and on-board fuel filter. Remember to check and empty it at every oil change.