Mercruiser 5.7 won’t make power

[Oldpos]

Bear with me I will give all the specs first. Boat is a 24’ Tritoon with an I/o set up. Came factory with a 3.0 but I shoes in the 5.7 and everything as far as mounting was done correctly and is properly lined up with the drive.

The drive is a alpha gen ii , I bought a new upper unit from sei with a 1.47 ratio. Gimbal bearing was replaced along with bellows, I bought the bearing tool so everything is lined up.

Now the engine. I got everything from a donor boat but really only used the manifolds. I got. 1998 vortec 5.7 from a Tahoe, took it to a machine shop, he bored the block .030” and honed to match the pistons which are Keith black hypers, he squared deck and did the rods and stuff, I bought a new eagle crank shaft standard size. The heads were completely redone and pocketed the spring perches for the required heavier springs. The cam is a xtreme marine hydraulic flat tappet .462/.477 236-3 part number. I bought the recommended springs from comp cams also. I used 7.8” pushrods. Holly 600 marine carb, summit dual plane intake, thunderbolt v known good ignition system.

Prop is a 19p 13.5” stainless

So the problem- the engine will run great on the test stand and in the boat on muffs, but when under load I. The water it won’t go past 2600 rpm, it acts like it has no power at all, the secondaries are not opening but even if I manually open them the engine sort of bogs down.

So far I have replace the intake with a edelbrock performer thinking possibly a vacuum leak- no help

Bought a brand new edelbrock carb to see if the other carb was bad, no help

Replacing ignition system with delco est hei, no help

Compression check was 150psi all cylinders except nbr 3 which as just above 140 but within the 10% allowable.

Installed fuel pressure gauge by the carburetor fuel inlet, even when I manually open the secondaries I still have around 5 psi fuel pressure. Also it’s a Carter marine electric fuel pump.

Inspected exhaust flappers, the port side was banging a little at idle, neither were stuck, and both were present.

I have read all the threads on here I could find and tried everything everyone recommended in those threads. But obviously I’m missing something.

Does the pushrod length sound right? Timing I check about 10 times , I stabbed the distributor at 8 degrees btdc .along with making sure the plug wires were right. Someone please help !!!

 

[itsathepete]

Have you measured the timing advance? I would say 32 degrees total advance at 3000rpm. Could be lean. Do you see an even spray of fuel from the boosters? What is the rated flow of the fuel pump? 30gph minimum 40 is better. Can you pump fuel into a bucket and make sure it's flowing well and clean. What size fuel line are you using, 5/16 minimum 3/8 would be better. New fuel filter? check hoses and pickup from tank as well as antisiphone valve and vent.

 

[Oldpos]

Have you measured the timing advance? I would say 32 degrees total advance at 3000rpm. Could be lean. Do you see an even spray of fuel from the boosters? What is the rated flow of the fuel pump? 30gph minimum 40 is better. Can you pump fuel into a bucket and make sure it's flowing well and clean. What size fuel line are you using, 5/16 minimum 3/8 would be better. New fuel filter? check hoses and pickup from tank as well as antisiphone valve and vent.

The pump is 50 gph Carter p4389, 3/8 fuel line, the hoses to the tank are brand new, and the tank was pulled out and cleaned prior to being installed, and the flow is great , I used a bucket to catch it.

 

[Scott Danforth]

Try a 17p prop or a 15. Make sure you verify the drive ratio

Toons are a different animal.

Have you measured the SWL to riser height?

How is the cam installed? Straight up, 4deg advanced, or 4 degrees retarded?

Vortec motor and that cam should be closer to 10-12 degrees of base timing with 22-24 degrees of advance

The Tahoe motor was a roller motor, and you installed a flat tappet cam? You left a fair amount of power on the table

What Holley carb?

Are you running a carb spacer?

Secondaries won't open until you get the revs up

 

[Scott06]

The pump is 50 gph Carter p4389, 3/8 fuel line, the hoses to the tank are brand new, and the tank was pulled out and cleaned prior to being installed, and the flow is great , I used a bucket to catch it.

Try a lower pitch prop. did you check total ignition advance ?

 

[Oldpos]

Try a 17p prop or a 15. Make sure you verify the drive ratio

Toons are a different animal.

Have you measured the SWL to riser height?

How is the cam installed? Straight up, 4deg advanced, or 4 degrees retarded?

Vortec motor and that cam should be closer to 10-12 degrees of base timing with 22-24 degrees of advance

The Tahoe motor was a roller motor, and you installed a flat tappet cam? You left a fair amount of power on the table

What Holley carb?

Are you running a carb spacer?

Secondaries won't open until you get the revs up

I did try a 14.75” 15p prop and the rpms were identical which I thought real strange, I was expecting at least 600 rpm difference. Yes I am running a 1” spacer, the original carb was a 4160 spread bore 600 cfm. The edelbrock is Avs 2 1906, yes is a car carb but I wanted to throw something on there just to see if it fixed it. The risers are dry with no wash out or rust streaks. The cam was installed at center mark,

 

[Scott06]

I did try a 14.75” 15p prop and the rpms were identical which I thought real strange, I was expecting at least 600 rpm difference. Yes I am running a 1” spacer, the original carb was a 4160 spread bore 600 cfm. The edelbrock is Avs 2 1906, yes is a car carb but I wanted to throw something on there just to see if it fixed it. The risers are dry with no wash out or rust streaks. The cam was installed at center mark,

For a sanity check have you verified firing order ?

 

[itsathepete]

I was going to suggest a different prop too, but I see you've tried that. Check the total timing advance and check your tach to see if it's accurate. Did you get a gps speed on any of your outings and compare them

 

[Oldpos]

For a sanity check have you verified firing order ?

Lol yes I finally did today, I sucked up my pride and verified rotor cap plug wire placement

 

[Oldpos]

Try a 17p prop or a 15. Make sure you verify the drive ratio

Toons are a different animal.

Have you measured the SWL to riser height?

How is the cam installed? Straight up, 4deg advanced, or 4 degrees retarded?

Vortec motor and that cam should be closer to 10-12 degrees of base timing with 22-24 degrees of advance

The Tahoe motor was a roller motor, and you installed a flat tappet cam? You left a fair amount of power on the table

What Holley carb?

Are you running a carb spacer?

Secondaries won't open until you get the revs up

How much would the cam placement effect the performance?

 

[itsathepete]

How much would the cam placement effect the performance?

Advancing a cam can improve low end torque and retarding can help top end. Some cams have advance built in to the grind. Setting it straight up is generally recommended.

 

[Oldpos]

I was going to suggest a different prop too, but I see you've tried that. Check the total timing advance and check your tach to see if it's accurate. Did you get a gps speed on any of your outings and compare them

Yes I used gps, top speed is 17mph, the 3.0 engine I got 23 mph out of the same boat.

 

[itsathepete]

17 mph at 2600rpm with a 19p prop and a 1.94 gear ratio (from the 3.0) is 30% slip
17 mph at 2600rpm with a 15p prop and a 1.94 gear ratio is 11% slip.
I'm assuming the 30% slip is due to not planing out and not due to a bad prop.

 

[jimmbo]

deleted
I was going to ask if the Centrifugal Advance Weights were free to move, but it has an electronic Advance. However, is that working?

How does the cam you picked compared to the Stock Mercrusier Cam? A lot of people figure they need a different Cam, thinking they are going to get gobs more power, and on a light boat they might, but on a Barge, it can't get going fast enough to take advantage of the Grind

 

[Oldpos]

17 mph at 2600rpm with a 19p prop and a 1.94 gear ratio (from the 3.0) is 30% slip
17 mph at 2600rpm with a 15p prop and a 1.94 gear ratio is 11% slip.
I'm assuming the 30% slip is due to not planing out and not due to a bad prop.

I have a 1.47 ratio drive how does that calculate?

 

[Scott Danforth]

I did try a 14.75” 15p prop and the rpms were identical which I thought real strange, I was expecting at least 600 rpm difference. Yes I am running a 1” spacer, the original carb was a 4160 spread bore 600 cfm. The edelbrock is Avs 2 1906, yes is a car carb but I wanted to throw something on there just to see if it fixed it. The risers are dry with no wash out or rust streaks. The cam was installed at center mark,

not changing RPM means that something is limiting RPM

either fuel supply, such as the fuel tank anti-siphon valve, etc. or ignition.

with the engine under load, what is your fuel pressure at the carb inlet?

try running the engine from a 5 gallon outboard fuel tank if you have one

holley spread bore is a 4175. 4160 is a single feed square bore. what are the carb numbers on the air horn?

the AVS carbs kind of suck... they are shiney. however from a performance stand point...... well, they are shiney. they need to be tuned.



your cam is fine, and should be installed straight up.

I installed this cam in my 1988 350 in the Rogue

Operating Range: 1300-5500 RPM
Duration Advertised: 262 Intake / 268 Exhaust
Duration @ .050'' Lift: 218 Intake / 224 Exhaust
Valve Lift w/1.5 Rockers: .462'' Intake / .477'' Exhaust
Lobe Separation Angle: 112

 

[dubs283]

Four + times in this thread people ask OP, "Have you verified timing advance?", with no response from OP, not even an acknowledgement of the question. I know the ignition system was replaced and issue is still there but just so that very simple check can be performed/verified I'll ask.

OP: Have you verified the timing is advancing properly?

 

[Oldpos]

Four + times in this thread people ask OP, "Have you verified timing advance?", with no response from OP, not even an acknowledgement of the question. I know the ignition system was replaced and issue is still there but just so that very simple check can be performed/verified I'll ask.

OP: Have you verified the timing is advancing properly?

I am about to check it again this morning to make sure it is still advancing

 

[Oldpos]

not changing RPM means that something is limiting RPM

either fuel supply, such as the fuel tank anti-siphon valve, etc. or ignition.

with the engine under load, what is your fuel pressure at the carb inlet?

try running the engine from a 5 gallon outboard fuel tank if you have one

holley spread bore is a 4175. 4160 is a single feed square bore. what are the carb numbers on the air horn?

the AVS carbs kind of suck... they are shiney. however from a performance stand point...... well, they are shiney. they need to be tuned.



your cam is fine, and should be installed straight up.

I installed this cam in my 1988 350 in the Rogue

Operating Range: 1300-5500 RPM
Duration Advertised: 262 Intake / 268 Exhaust
Duration @ .050'' Lift: 218 Intake / 224 Exhaust
Valve Lift w/1.5 Rockers: .462'' Intake / .477'' Exhaust
Lobe Separation Angle: 112

The edelbrock was the only new unmolested carb I could get that day, once I figure out this issue I will slap the Holley marine carb back on since it appears nothing was wrong with it

 

[Scott06]

Four + times in this thread people ask OP, "Have you verified timing advance?", with no response from OP, not even an acknowledgement of the question. I know the ignition system was replaced and issue is still there but just so that very simple check can be performed/verified I'll ask.

OP: Have you verified the timing is advancing properly?

Agreed but he also said he replaced the TB ignition with a delco est to no avail so would assume two different systems would not not advance...

To OP- might be worth verifying balancer hasn't slipped verify indicated TDC is actually tdc ...