5.0 Mercruiser prop question NEED HELP

[Francin728]

I recently purchased a Donzi 19 LXR with a Mercruiser 5.0 and an alpha one stern drive (boat weights approx. 3000lbs).

The engine runs great I opted to rebuild the caurberator, replaced the plugs, wires cap, and rotor.

The problem I am having is when I accelerate the boat takes forever to get on plane. The engine revs to ~2800 rpms until it finally gets on plane at which time I can get up to 4000 rpms if I trim the drive. The drive has a 1.47 ratio (I am seeing 1.65 as standard for the 5.0L) and a 15 1/2X 15P prop.

I have tried contacting Donzi and Mercruiser for help with either a prop or any ideas and neither has been able to help.

Would a more aggressive prop get me on plane sooner so I can get up to RPM's or should I try a shallower prop to increase RPM"'s?

Any help is appreciated.

 

[Robbabob]

Re: 5.0 Mercruiser prop question NEED HELP

Hi Francin and welcome to the forum!

Seems your boat should be jumping outta the water. Meaning, that pitch prop with your motor should be performing very well. Something not right with HP i am guessing. You say it runs great... you did a carb rebuild, etc. etc. etc. Check the timing and make sure the throttle is opening all the way. Next, check compression.

You should be seeing up to 4600 to 4800 RPM at WOT.

Check out this thread at the top of the page and fill in the data.... http://forums.iboats.com/showthread.php?t=287818

 

[Bondo]

Re: 5.0 Mercruiser prop question NEED HELP

Would a more aggressive prop get me on plane sooner so I can get up to RPM's or should I try a shallower prop to increase RPM"'s?

Ayuh,.... Welcome Aboard,... What prop is on it Now,..??

In the meantime,... Read this,...

 

[Francin728]

Re: 5.0 Mercruiser prop question NEED HELP

I stated in the original post it is a 15 1/5" X 15p prop. Would going down to a shallower prop (given the increased drive RPM's) improve my hole shot?

 

[Robbabob]

Re: 5.0 Mercruiser prop question NEED HELP

I don't think your engine is performing fully. Changing props will not fix this, making adjustments on the engine is where you need to be looking.

 

[Bondo]

Re: 5.0 Mercruiser prop question NEED HELP

I stated in the original post it is a 15 1/5" X 15p prop. Would going down to a shallower prop (given the increased drive RPM's) improve my hole shot?

Ayuh,... Sorry, I missed that....

That prop is as shallow as they go...
yer gonna have to go to the 1.65:1 gear ratio you should have...

 

[Francin728]

Re: 5.0 Mercruiser prop question NEED HELP

Solved the problem. The previous owner had two of the ignition wires crossed. Boat runs great now and I am playing with different props.

I would like to check the timing. Are the mercruisers required to be put into timing mode like the old cars or do you just hook up the light and verify?

 

[Bondo]

Re: 5.0 Mercruiser prop question NEED HELP

Solved the problem. The previous owner had two of the ignition wires crossed. Boat runs great now and I am playing with different props.

I would like to check the timing. Are the mercruisers required to be put into timing mode like the old cars or do you just hook up the light and verify?

Ayuh,... Hard to say,... Ya haven't given Us any Info....

If it's old enough, the T-Bolt IV ignition is a point, 'n shoot system...
The newer T-Bolt V needs to be put in base timing mode...

 

[steelespike]

Re: 5.0 Mercruiser prop question NEED HELP

If you give us your present max rpm and speed reminding us of the prop size we may be able to tell if the setup is working and what solution may be needed if any.

 

[Francin728]

Re: 5.0 Mercruiser prop question NEED HELP

The engine is a 97 carb thunderbolt V I believe. How do you put this engine in timing mode?

As far as the prop is concerned I am getting ~36 mph at ~4000. From 4000-4700 I am picking up RPMs but I barely pick up any speed beyond that(39-40 tops). The speed was checked with a GPS speed app on my phone.

The current prop is a 14'5" X 19p. Would having a prop shop add cup to the prop help prevent slippage in the 4000+ RPM range?

The boat originally came with a 1.62 ratio outdrive turning a 14.25" X 21P priop. Currently it has a 1.47 drive with a 14.5X 19p prop. I figured the 19p prop would compensate for the slight increase in prop speed from the different gear ratio.

Any advice is appreciated.