Mercruiser 350 struggling to get on plane

[mwalker4484]

Hello, First post but from what I’ve read I will be as thorough as possible. I have a 2005 Crownline 270 CR with a Mercruiser 350 and Bravo III. Engine only has 280 hours. Since putting the boat in the water this year I can not get the boat to plane. I have used engine trim and trim tabs to no avail. Engine is running around 4,000 rpms max under load and sounds and looks likes it’s running strong. Replaced fuel filter and fuel pump. Still no help. Boat starts no problem idles fine and runs fine until I get on the throttle and it will not get above 15 knots nor plane out. Recently replaced spark plugs still no help. I am at a loss. Let me know if you need any more info from me.

 

[Bt Doctur]

a 27 footer with a single small block is underpowered even if it has a bravo III
Did it ever run right for you or is this a new purchase?
Is this carb`d, EFI, or MPI?
Compression test numbers?
Correct spark plug firing order?
If a flat cap, replace the dist cap.

 

[mwalker4484]

This is a new purchase but it passed the survey. Was hooked up to a compression tester I don not have the numbers though. and passed. I’m aware that it’s underpowered but I should be getting on plane and doing more than 15 knots. It’s MPI

 

[Scott Danforth]

to pass the survey, the compression numbers simply all need to be above 100 psi and within 10psi of each other.

however a healthy motor will have compression about 150 psi in each cylinder

run compression numbers

check your fuel for water

during the sea trial, did it get on plane?

you should be engine trimmed all the way down and tabs near all the way up to get up on plane

 

[Bt Doctur]

Dimensions
LOA: 28 ft 8 in
Beam: 8 ft 6 in
Minimum Draft: 1 ft 9 in
Maximum Draft: 3 ft 0 in
Bridge Clearance: 6 ft 8 in
Deadrise: 18 ° at Transom
Dry Weight: 7200 lbs

Engines
Total Power: 300 HP bull

Engine 1:
Engine Brand: MerCruiser
Year Built: 2005
Engine Model: 350 MAG MPI Bravo III
Engine Type: Inboard/Outboard
Engine/Fuel Type: Gas/Petrol
Engine Hours: 267
Propeller: 3 blade propeller
Drive Type: Stern drive
Engine Power: 300 HP to this I say bull

 

[mwalker4484]

Engine trimmed all the way down tabs all up. Was able to get to 18 knots today against a current. Could it be bad gas?

 

[Scott Danforth]

run compression numbers

 

[mwalker4484]

Will do hopefully in next couple of days

 

[QBhoy]

Hi

the diz cap is a well known issue on these but that aside it could be a number of things. First thing I would do is get it plugged in and see if you have a guardian mode active and or any faults causing this.
Also bare in mind if you have a full tank of fuel in this model and others similar. It’s a hell of a weight right at the rear of the boat. That said. Usually this engine and B3 props (if correct) punch well above their weight at getting big boats out the water. I’d be at the diz cap and plugging it in though. You could be in a 90% guardian mode or worse and not know it.

 

[mwalker4484]

Will do hopefully in next couple of days I’ll have it plugged in. Only have 1/4 tank of gas I was trying to run it down in case it was bad gas.

 

[tank1949]

Will do hopefully in next couple of days I’ll have it plugged in. Only have 1/4 tank of gas I was trying to run it down in case it was bad gas.

do yourself a favor and research what rpms that motor is supposed to churn at WOT and trimmed properly and empty or full. If your tack is CORRECT and you are only turning 4000 rps, you may have clogged fuel filter, water leak in hidden compartment adding too much extra weight, poor motor compression due to previous owner overheating motor and bribing surveyor (incompetence or malice) , distributor NOT advancing properly (electronically or mechanically), and as others have said compression should be around 150. Keep in mind as other have also stated, your boat is under powered. But, it should have prop pitched to achieve needed rpms.

 

[Rick Stephens]

4000 RPM WOT is not high enough. If the engine mechanically checks out it needs to be propped for higher RPMs.

 

[Hydrojacked]

You mentioned putting your boat in the water this year... has it been splashed long enough in salt or brackish water to have growth on the bottom or on the prop? Even with bottom paint, ( my boat is kept in the water year round) I lose the ability to plane between 6-10 months with nothing else wrong mechanically.

 

[mwalker4484]

I took it out the other day and noticed some growth on bottom. Didn’t seem like enough to slow her down that much but def worth diving into. I cleaned up most of the prop and outdrive but didn’t get to the hull as much as I would have liked to. Starter just went yesterday so once starter arrives I can get back to this problem. One thing at a time lol. Thanks for responses

 

[QBhoy]

Dimensions
LOA: 28 ft 8 in
Beam: 8 ft 6 in
Minimum Draft: 1 ft 9 in
Maximum Draft: 3 ft 0 in
Bridge Clearance: 6 ft 8 in
Deadrise: 18 ° at Transom
Dry Weight: 7200 lbs

Engines
Total Power: 300 HP bull

Engine 1:
Engine Brand: MerCruiser
Year Built: 2005
Engine Model: 350 MAG MPI Bravo III
Engine Type: Inboard/Outboard
Engine/Fuel Type: Gas/Petrol
Engine Hours: 267
Propeller: 3 blade propeller
Drive Type: Stern drive
Engine Power: 300 HP to this I say bull

It definitely has 300hp. It’s a 350 mag mpi.

 

[mwalker4484]

Update**** worst news I could receive today. Looks like a cracked piston. As I’m getting intermittent smoke puffs coming from engine when oil cap is removed. From bad to worse. Looks like I’m shopping for a new engine. Mine as well upgrade to the 6.2 I guess. To the bank I go.Thanks for all the input.

 

[mwalker4484]

Update**** worst news I could receive today. Looks like a cracked piston. As I’m getting intermittent smoke puffs coming from engine when oil cap is removed. From bad to worse. Looks like I’m shopping for a new engine. Mine as well upgrade to the 6.2 I guess. To the bank I go.Thanks for all the input.

 

[Kola16]

Sorry to hear about your motor. When you go to repower, know that your boat is pretty heavy so if there is room I would be putting the 7.4 over the 6.2. It has a lot more torque to get you up on plane.