5.7 Mercruiser Wont Plain

[Iowaboatguy]

I have a 1991 Rinker Festiva Cuddy Cabin with a 5.7 Mercruiser Alpha One.
Not sure why but the boat will not plain now. I have the throttle all the way down and just stays up with its nose in the air. You can tell its trying but just doesn't gain enough speed.
I've done a tune up. The boat repair shop isn't sure what to do, they have exhausted their efforts. Recommended it to someone with more knowledge of older sterndrives.
Anybody have any ideas what the problem could be?

 

[alldodge]

Has anyone done a compression test?
What is the drive ratio?
What pitch prop is on the drive?

 

[kenny nunez]

IN addition to AD’s suggestions, has anyone recently done any carburetor repair or replaced it? Is the flame arrestor clean? Are the rams completely retracting ? Does this boat stay in the water ? Are you sure there is no water trapped in the hull?
What about the fuel pressure, it should be 4-6 lbs at cranking speed.
These are a few things that can cause this kind of headache.

 

[wellcraft-classic210]

Whats going on when you try to get on plane -- do you have a tach showing motor rpms ?? Does the engine runs steady as in no bogging or stuttering ? How fast can you get it to go ?

 

[Scott Danforth]

do a compression check as first step.

 

[QBhoy]

Agree with the above entirely...but I’ll add..I’m familiar with a couple of these actual boats. They are heavy buggers and if full of fuel (the big tank is at the rear of the boat fwd of engine)...the 2 I know of both need trim tabs full down to help get over the hump.
One has a 5.7 and the other was reengied with a 5.0 mpi. Both have the same issue.
They were both running an Alpha one and I think a 17” black max prop. They have both now changed to a 16x16 big ally prop. Massive difference in getting on plane. Only a slight loss in top end.

Quick comlression check, check the prop and try the tabs. See how she goes.

 

[tpenfield]

When the repair shops are stumped, it is always good to turn to the iBoats forum for answers. It is plain to see that your boat won't plane.

When was the last time it did plane?

Was it all of a sudden or a gradual change?

All the stuff you & the repair shop have done . . . you can eliminate those as the problem.

I would say it is either the boat or the engine.

Has the boat gained any weight? Does it float lower in the water? Anything changed about where weight is distributed throughout the boat? Got water in any of the compartments/bilge down below?

As suggested, a compression test would be good. Is the engine reaching its rated maximum RPM? Any different than before?

 

[jakwi]

Is the flame arrestor clean?

This was exactly my thought also. I had a boat that wouldn't plane, I took the flame arrestor off and it would pop right up. After I cleaned it everything was fine again.

After this I second compression, it's directly related to the power your motor can produce, and if it's uniformly low the motor will sound like it's running normally, but simply not make as much power.

Also I sometimes have to ask passengers to move to the front of the boat if I am fully loaded with a full water tank, and a full gas tank, both of which are in the rear on my boat. Moving several hundred pounds forward will usually make a big difference.

 

[Bondo]

Ayuh,...... Welcome Aboard,...... I donno what the problem is with yer boat,.....

But,...... I can guaranty the answer is in this list, Right Here,.... Which is found in Here,...