1998 Props on 405

[Viper]

I agree that it's unlikely you'll see that much improvement on rpm from repairing the one prop unless there's something we can't see in the pics. It's more likely a case of incorrect propping, engine issues, fuel delivery to the engines, or a weight problem.

You may have posted this before but can you let us know the engine brand, model, and serial number. Sounds like you've had a lot of hardware replaced but what diagnostic tests were performed?

[Serendipity]

Mjk1040

I agree with you completely because after all the work was done. I drove the boat and one engine was still 3000. Next day on a longer trip to have the props removed I got 3600 on both engines.
I am having the props dropped down to a 22/19 to get on plane quicker. Cost me a 1000 to have both props reworked which sounded high to me based on other post/forums for 2 four blade props, but maybe it isnt.
I do agree and feel the boat will get 4000 with time. I would actually be okay with the 3600 for what I am going to use it for.
Happy boating and ya have a great weekend

[Serendipity]

Viper
Diagnostics performed was I wanted compressions on every cylinder and they checked 160 to 165 on each.

Everything I can think of has been replaced to do with maintenance. Caps, rotors, risers, elbows, manifolds, fuel filters, spark plugs, oil and filter changes, fuel regulator even though I was told it rarely goes bad, fuel water separators, fuel pumps, impellers and more.

I really feel the boat simply hasnt been driven which could be good or bad depending on what issues develop.

Engine are the standard 454 7.4 XL MPI 310 or 320 HP if I remember on my survey. Listing said 454 HP and I knew that was wrong.

Clean engine compartment as I haven't touched it yet but will clean it to my standard. I hope the 22/19 props gets me on plane quicker and maybe a couple hundred more rpms for the hell of it.

Regardless I am pretty pumped about the boat.

[Serendipity]

Happy boating

[Serendipity]

Fuel update..... The front tanks have 1/8 to 1/4 in both of them but the rear mid tanks have almost 3/4 in them. So I will add fuel additive to all the tanks and top each off for the trip.
Sure this saves a few dollars on gas but I was hoping for almost empty tanks so I could put fresh back in. Do ya feel like I will use the 342 gallons the tanks hold on this trip I have planned from Galveston Texas to Mississippi? Naturally, I want run them dry but I would like to run them all to 1/8 and replace all the fuel filters and water separators again for my piece of mind. I hate doing maintenance on the water when I am trying to have fun.

[g36]

I get about 3/4 -1 mile per gallon at about 2200 rpm apx 10mph. don't know how far your trip is but that might give you a ballpark depends how hard you want to run it

[tomschauer]

Unless you are running some old bad fuel through the engines, which very well may be the case, they will not get better as you run them, or "blow them out". If its not old fuel, its most likely over propped or perhaps a couple bad injectors.

[Serendipity]

I would agree with you Tom

[Serendipity]

So here is the update on the rpms issue.

Changed the props from 22/21 to 22/19 to get on plane quicker. Now my baby is running 23 mph as shown and 4300 rpms. That is good enough for me because I didnt purchase a speed boat. Lol

No reason for the rpms to change other than the prop pitch change, the engines have been run more now than they have in a while and I did go ahead and have the bottom hull waxed and polished at the last minute. I guess it could be a combination of the 3, but isn't 23 mph pretty good for 7.4 Mercs MPI's on a 405?

Regardless I went in planning to or needing to replace both engines if needed because of age. Still might need to at some point but I am happy for the moment

[Serendipity]

Side note, I am sure all the maintenance made a difference as well because I replaced pretty much everything I could think of due to age. I installed a new Garmin GPS/Chartplotter/Sonar as well so hopefully the fun starts now. Pics taken after top part was color sanded and waxed. Hull still needed to be done.

I am looking forward to the trip next week and crossing my fingers the boat is ready for a 400-500 mile trip