Mercruiser 5.7 won’t make power

[jimmbo]

I've seen many wiped Lobes that looked very nice and Shiny

 

[Oldpos]

Ok I took the intake off and resealed it, measured the cam lobes by rocker movement all intake and exhaust within around .005” of each other, obviously distributor re stabbed, I have to wait until tomorrow for the sealant to cure before running any more tests, piston stop tool will be here tomorrow from Amazon.

 

[Scott06]

24 degrees roughly, 8 degrees base. Both still occasionally jump around

If you end up with nothing else to try I would set total advance at 30 and see what that does. I tink the delco kit states 8 intial 25 total for a 350 but I think merc for a vortec is 10 initial and 30 or so total .

 

[Oldpos]

If you end up with nothing else to try I would set total advance at 30 and see what that does. I tink the delco kit states 8 intial 25 total for a 350 but I think merc for a vortec is 10 initial and 30 or so total .

Your right on the timing, I will try bumping it a bit also, can’t really hurt

 

[Oldpos]

If I am going from a 14” 19p to a 16” 13p prop does anyone know how many Roma I will gain? I got a 13p being delivered tomorrow to see if anything I have done has changed anything. In the beginning I tried a prop change with no rpm difference

 

[Scott06]

If I am going from a 14” 19p to a 16” 13p prop does anyone know how many Roma I will gain? I got a 13p being delivered tomorrow to see if anything I have done has changed anything. In the beginning I tried a prop change with no rpm difference

1000-1200 rpm depending on how much cup it has and if the engine can swing it

Be interested to see if that works maybe the cam doesnt have enough low end torque for this application

 

[jimmbo]

Adding Diameter, while subtracting Pitch can result in the engine seeing the same overall Load, and thereby no change in RPM. A 2" increase in Diameter is substantial

 

[Oldpos]

I know is a rough estimate but I went to the Mercury prop selector site and this is what it came up with for my current set up, when it had the 3.0 I used the same prop, and the calculator recommended the same prop. I have a ton to learn about props.

 

[jimmbo]

I have always found the Prop Calculators a bit optimistic on the amount of Pitch chosen. Then again, I do my boating between 1950 and 2400 ft above Sea Level.

 

[Scott Danforth]

1000-1200 rpm depending on how much cup it has and if the engine can swing it

Be interested to see if that works maybe the cam doesnt have enough low end torque for this application

I ran the same cam in the SBC on the Rogue. spinning VP DuoProp B6 props (about 23p) and pushing 4200# out of the hole almost instantaneously. I dont think its a prop issue. however toons are a different animal. the lower the drive in the water is does load the prop up a lot more, however having thru-prop exhaust should help spin the props

 

[Oldpos]

I ran the same cam in the SBC on the Rogue. spinning VP DuoProp B6 props (about 23p) and pushing 4200# out of the hole almost instantaneously. I dont think its a prop issue. however toons are a different animal. the lower the drive in the water is does load the prop up a lot more, however having thru-prop exhaust should help spin the props

Since you had the same cam, we’re you happy with it?

 

[Scott Danforth]

Since you had the same cam, we’re you happy with it?

Yes, then I needed more power and built a BBC

 

[jimmbo]

Given a choice of a 300hp SB, and a 300hp BB, I will always choose the bigger engine, just for the extra Torque at low rpm.

 

[Scott06]

however toons are a different animal. the lower the drive in the water is does load the prop up a lot more, however having thru-prop exhaust should help spin the props

thats why i mentioned it toons dont plane out , also had similar issues with my brothers Twin Vee power cat , been hard to hone in on optimal prop. Im sure ops boat is more tail heavy with a 5.7 than 3.0.

about all he hasnt checked yet is if cam is retarded or degreed it in.

 

[Oldpos]

thats why i mentioned it toons dont plane out , also had similar issues with my brothers Twin Vee power cat , been hard to hone in on optimal prop. Im sure ops boat is more tail heavy with a 5.7 than 3.0.

about all he hasnt checked yet is if cam is retarded or degreed it in.

Agreed but I’m at the point of either pulling the motor ( would need to to deal with a cam anyhow,
Or the other thing I am pondering
Removing the center toon.
The original engine pod a very long pointy nose to it that I cut off to fit a third toon, problem was the transome only hung down about 20 inches from the bottom of the floor, the new center toon hangs down about 25 , I did have the log modified to taper the last 18 inches of the bottom tapered up about 3 inches. Maybe the difference is too much putting a heavy load on the prop?

 

[Scott06]

Agreed but I’m at the point of either pulling the motor ( would need to to deal with a cam anyhow,
Or the other thing I am pondering
Removing the center toon.
The original engine pod a very long pointy nose to it that I cut off to fit a third toon, problem was the transome only hung down about 20 inches from the bottom of the floor, the new center toon hangs down about 25 , I did have the log modified to taper the last 18 inches of the bottom tapered up about 3 inches. Maybe the difference is too much putting a heavy load on the prop?

Dont know but you certainly have a reasonably smart group here stumped. If you pull the engine maybe go back in with a big block…

at least now with adding a third toon it isnt an apples to apples comparison. so you have a 25” center tube and what size outer tubes?

 

[dubs283]

The original engine pod a very long pointy nose to it that I cut off to fit a third toon, problem was the transome only hung down about 20 inches from the bottom of the floor, the new center toon hangs down about 25 , I did have the log modified to taper the last 18 inches of the bottom tapered up about 3 inches. Maybe the difference is too much putting a heavy load on the prop?

Your problem is obvious. Judging from the first and only picture you've posted you have a tri toon wIth an sbc v8.

You've added the middle full tune in lew of the original tapered front motor well. The new mid toon had a 5" lower transom than the original motor well. You've modified the aft portion of the new mid toon with a dead rise to be now only 2" lower at the transom, yes?

Dunno much about pontoons, other than they're a pontoon but seems like you've changed the dynamics/geometry of the planing surface a bit and based on how low the transom is now with the added surface of the mid toon I think you're trying to push too much with to little, or at least too low in the water to even have a chance of planing

And, is that an electric fuel pump ahead of the water sep? Curious as to why it's there

 

[Oldpos]

Your problem is obvious. Judging from the first and only picture you've posted you have a tri toon wIth an sbc v8.

You've added the middle full tune in lew of the original tapered front motor well. The new mid toon had a 5" lower transom than the original motor well. You've modified the aft portion of the new mid toon with a dead rise to be now only 2" lower at the transom, yes?

Dunno much about pontoons, other than they're a pontoon but seems like you've changed the dynamics/geometry of the planing surface a bit and based on how low the transom is now with the added surface of the mid toon I think you're trying to push too much with to little, or at least too low in the water to even have a chance of planing

And, is that an electric fuel pump ahead of the water sep? Curious as to why it's there

The two side toons are 25”, the center toon is a 24”. It the rackets to mount it were a lot longer to it causes the center toon to sit about 2” lower than the side toons, which from everything I read about Titoons that is desirable.
Yes it’s a electric fuel pump, the vortec engine has no provision for a mechanical pump, and I had the center toon modified where the fuel tank is inside of the toon about half way up the boat. So it’s the only place I could put it.

 

[Oldpos]

Here is a picture , you need to zoom it in a little to see the height difference,

 

[Oldpos]

With a mild cam what vacuum reading would you expect?