mercruiser 7.4 mpi can't figure out the ratio

[alldodge]

Yes prop are good I would have a vibration if they were bad. Also I since I was having this issue I tried going full throttle at once shot and the boat was more slower getting to plane but eventually reach to plane. I remember when I bought the boat and went for a test drive the previous owner was like hold on and he gave it full throttle and it made your head go back. It was faster than.

This is where things don't add up with WOT.
Using the same prop pitch but a different ratio drives.
The higher ratio drive will take off quicker but will over rev.
The lower ratio drive will take off slower but will not over rev

They cannot equal the same WOT RPM

 

[Scott06]

Not getting a hold of them

Googling a spec sheet for your boat I was able to find one but it didn’t show a 7.4 engine option only 496 merc and 8.1 Volvo engines, assumed it was wrong year but wondering now if someone repowered it with a locally available 7.4 ?

If with the higher numerical drive maybe you don’t have enough a$$ to swing the props and that why two drives give same rpm? Grabbing at straws did u verify the tach yet ? Screwy tach makes most sense of data you’ve given

 

[Cruis]

Googling a spec sheet for your boat I was able to find one but it didn’t show a 7.4 engine option only 496 merc and 8.1 Volvo engines, assumed it was wrong year but wondering now if someone repowered it with a locally available 7.4 ?

If with the higher numerical drive maybe you don’t have enough a$$ to swing the props and that why two drives give same rpm? Grabbing at straws did u verify the tach yet ? Screwy tach makes most sense of data you’ve given

The tach is working I also think I’ll notice if the engine is revs higher than usual. Do you think the new drive is paired up with a different ratio lower unit seen in the chart I posted previously can it be a faulty drive Sometimes I get a hard clunk with changing gear to forward and reverse? Also I’m trying to get a hold of someone with the same about and engine and see was outdrive ratio they have saw a couple on the market but now I’m seeing Volvo.

 

[Cruis]

This is where things don't add up with WOT.
Using the same prop pitch but a different ratio drives.
The higher ratio drive will take off quicker but will over rev.
The lower ratio drive will take off slower but will not over rev

They cannot equal the same WOT RPM

I know what you mean it really doesn’t make sense

 

[Fun Times]

Are you able to read any of the 2 stern drive serial numbers? usually found on the upper housing, https://forums.iboats.com/threads/w...nd-engine-serial-numbers.608167/#post-4434267

 

[Cruis]

No luck finding the number tired removing the paint too also.

 

[Cruis]

This is where things don't add up with WOT.
Using the same prop pitch but a different ratio drives.
The higher ratio drive will take off quicker but will over rev.
The lower ratio drive will take off slower but will not over rev

They cannot equal the same WOT RPM

Hi, I did a compression test on the engine the
Piston
#1 -150 psi
#2 - 150 psi
#3 - 150 psi
#4 - 90 psi
#5 - 140 psi
#6 - 160 psi
#7 - 130 psi
#8 - 150 psi
Not sure if because piston 4 has 90 psi making the boat not getting the speed. what did you think?

 

[Scott06]

Hi, I did a compression test on the engine the
Piston
#1 -150 psi
#2 - 150 psi
#3 - 150 psi
#4 - 90 psi
#5 - 140 psi
#6 - 160 psi
#7 - 130 psi
#8 - 150 psi
Not sure if because piston 4 has 90 psi making the boat not getting the speed. what did you think?

90 psi if accurate is a dead cylinder so yes would affect performance significantly. Would do a leakdown to see where you are loosing compression. Might be a head gasket or valve related vs needing bottom end rebuild. Have you had issues with exhaust leaking water into chambers ?

 

[alldodge]

With one dead cylinder you've lost 12% power (310-12% = 37 hp).

That said, if the cylinder just went dead between old drive and new one, this could be the issue, but that would be a heck of a coincidence. My guess is it was already dead and the reason you only was getting 25-26 mph.

 

[Scott06]

With one dead cylinder you've lost 12% power (310-12% = 37 hp).

That said, if the cylinder just went dead between old drive and new one, this could be the issue, but that would be a heck of a coincidence. My guess is it was already dead and the reason you only was getting 25-26 mph.

Agreed original numbers looked low. Hopefully just a valve job to fix

 

[Cruis]

90 psi if accurate is a dead cylinder so yes would affect performance significantly. Would do a leakdown to see where you are loosing compression. Might be a head gasket or valve related vs needing bottom end rebuild. Have you had issues with exhaust leaking water into chambers ?

Haven’t had water leaking in the chamber. Wouldn’t the spark plug look a bit rusty? They are look the same.

 

[Cruis]

With one dead cylinder you've lost 12% power (310-12% = 37 hp).

That said, if the cylinder just went dead between old drive and new one, this could be the issue, but that would be a heck of a coincidence. My guess is it was already dead and the reason you only was getting 25-26 mph.

When I hit the old drive got caught up with do object in the water it was sudden causing the upright lower shaft to snap. Can that be a cause of why I’m getting low compression on cylinder 4 ?

 

[Scott06]

Haven’t had water leaking in the chamber. Wouldn’t the spark plug look a bit rusty? They are look the same.

I think normally the spark plugs look really clean as the water cleans the carbon off it. One common way water can get in and hang up valves is via the exhaust manifolds and risers. need to find reason for low compression.

 

[alldodge]

When I hit the old drive got caught up with do object in the water it was sudden causing the upright lower shaft to snap. Can that be a cause of why I’m getting low compression on cylinder 4 ?

The drive breaking wouldn't cause a cylinder to go bad

 

[Cruis]

I think normally the spark plugs look really clean as the water cleans the carbon off it. One common way water can get in and hang up valves is via the exhaust manifolds and risers. need to find reason for low compression.

They were not that Clean very all of the had a little bit of carbon on them. And the water would go in the chamber if the head gasket is blower? And If the value it stuck I would I be able to see it after removing the head ?

 

[alldodge]

If head gasket was blown 1 or 2 plugs would be white from steam cleaning. The motor would be running rough, and the only water would be those cylinders not all of them

 

[Cruis]

If head gasket was blown 1 or 2 plugs would be white from steam cleaning. The motor would be running rough, and the only water would be those cylinders not all of them

So, my opinion on why I’m getting Low compression is either a value is left stuck open or the pistons rings are bad.
How can I figure it out?

 

[alldodge]

So, my opinion on why I’m getting Low compression is either a value is left stuck open or the pistons rings are bad.
How can I figure it out?

Leak down test
Without a tester you can run the piston up to TDC on the low cylinder. Connect a compressor to spark plug hole and pressurize around 50 to 90 psi.

Listen for the air escaping and determine if its leaking out the intake/exhaust or if its going into the block

 

[QBhoy]

Surely not a mechanical issue. More drive train or tachometer.

 

[Cruis]

Hi, I bought a bravo 3 2:1 and the carrier bearing had a bit corrosion so I’m want to change it. I already took it out and have a new one coming in do I need to redo shimming I’m reading on online that I do but, one other Mechanic say no I don’t. What do you think