NHGuy
Captain
- Joined
- May 21, 2009
- Messages
- 3,631
Re: Replace Mercruiser 5.7 Carb with 4.3 EFI
Hi, I'm late to this discussion. But I'd suggest replace or repair with 5.7 stuff. The 4.3 has less torque even though it's close on total horsepower. It's torque that moves boats and 23 footers have some considerable weight. I think a 5.7 has about 20 percent more torque. That's your holeshot power.
You say the mechanic told you it damaged a rod or maybe more than 1 rod. That tells me there was water in the combustion chamber when the piston came up. Water won't compress like air. So it damaged the rod. If it also broke the piston, the water in that chamber would then drop into the motor oil.
You could either have the motor repaired or replaced once it is inspected by someone qualified.
Once you have a motor, check that the cooling jackets on the exhaust and the intake are perfect.
A lot of the 5.7 carb motors have a water passage in the intake manifold that, if it leaks, could allow water into your motor oil. I don't see any reason that caused your problem, but while the motor is out that needs an inspection.
You said the risers are new, that's very good. Replace the exhaust manifolds if there is any possibility they are bad. They can be tested like this. http://forums.iboats.com/mercruiser...s/testing-exhaust-manifolds-leaks-288629.html If the exhaust manifolds are from salt water, or if you don't know how old they are, those are also reasons to replace them. Remember we want the next motor not to have the same problem the broken one has. So this is a way to eliminate the possibilities.
ps. My 1989 freshwater Mercruiser 5.7 had an exhaust manifold fail 2 years ago causing water in the combustion chambers,. I recognized the problem before I bent a rod, but it still needed the things I am writing about.
Make sure you have a perfect seal from the exhaust manifolds to the risers. It's critical to keep the cooling water out of the exhaust tract. Use new factory manifold to riser gaskets every time. They are steel with gray graphite coating. To clean the mating surfaces I use a little cheap air powered die grinder with surface conditioning discs like these Pack of 5 2" Fine Grade Fiber Grinding Discs If you can't get the discs there just use a narrow strip of Scotch Brite pad to clean the surfaces. Rub the centers of the surfaces the long way so you don't round off the edges. The mating surfaces are flat and the gasket fills the space between them.
When putting the risers on, start out by getting the rubber exhaust hoses into the down pipe and the riser, then put the riser down onto the manifold and gasket.
And make sure you don't overload your boat. If the rear of the boat is so low the riser tops are below the waterline, outside water can flow backward into the exhaust manifold when the motor is off. So if you bought new risers make sure they go above any waterline the boat will see. There are various thickness spacer blocks available for this.
I'd suggest checking around where the boat is for a boat engine technician. You could even use a regular engine shop if you could then have a boat person install the cooling stuff afterward. Greece is in a recession and you might find a great deal. Or do it yourself with our input.
The engine is nothing more than a Chevrolet 350 with stainless head gaskets and brass core plugs. The carburetor is a very slightly modified Rochester Quadrajet. It's straight out of a Chevrolet pick up or delivery truck. It's the most common engine ever over here. It's not exotic and it's easy to find information. You just need the right guy or a motor that's all ready.
Too bad the boat's not in Italy, some of those Italian motor guys are geniuses!
Hi, I'm late to this discussion. But I'd suggest replace or repair with 5.7 stuff. The 4.3 has less torque even though it's close on total horsepower. It's torque that moves boats and 23 footers have some considerable weight. I think a 5.7 has about 20 percent more torque. That's your holeshot power.
You say the mechanic told you it damaged a rod or maybe more than 1 rod. That tells me there was water in the combustion chamber when the piston came up. Water won't compress like air. So it damaged the rod. If it also broke the piston, the water in that chamber would then drop into the motor oil.
You could either have the motor repaired or replaced once it is inspected by someone qualified.
Once you have a motor, check that the cooling jackets on the exhaust and the intake are perfect.
A lot of the 5.7 carb motors have a water passage in the intake manifold that, if it leaks, could allow water into your motor oil. I don't see any reason that caused your problem, but while the motor is out that needs an inspection.
You said the risers are new, that's very good. Replace the exhaust manifolds if there is any possibility they are bad. They can be tested like this. http://forums.iboats.com/mercruiser...s/testing-exhaust-manifolds-leaks-288629.html If the exhaust manifolds are from salt water, or if you don't know how old they are, those are also reasons to replace them. Remember we want the next motor not to have the same problem the broken one has. So this is a way to eliminate the possibilities.
ps. My 1989 freshwater Mercruiser 5.7 had an exhaust manifold fail 2 years ago causing water in the combustion chambers,. I recognized the problem before I bent a rod, but it still needed the things I am writing about.
Make sure you have a perfect seal from the exhaust manifolds to the risers. It's critical to keep the cooling water out of the exhaust tract. Use new factory manifold to riser gaskets every time. They are steel with gray graphite coating. To clean the mating surfaces I use a little cheap air powered die grinder with surface conditioning discs like these Pack of 5 2" Fine Grade Fiber Grinding Discs If you can't get the discs there just use a narrow strip of Scotch Brite pad to clean the surfaces. Rub the centers of the surfaces the long way so you don't round off the edges. The mating surfaces are flat and the gasket fills the space between them.
When putting the risers on, start out by getting the rubber exhaust hoses into the down pipe and the riser, then put the riser down onto the manifold and gasket.
And make sure you don't overload your boat. If the rear of the boat is so low the riser tops are below the waterline, outside water can flow backward into the exhaust manifold when the motor is off. So if you bought new risers make sure they go above any waterline the boat will see. There are various thickness spacer blocks available for this.
I'd suggest checking around where the boat is for a boat engine technician. You could even use a regular engine shop if you could then have a boat person install the cooling stuff afterward. Greece is in a recession and you might find a great deal. Or do it yourself with our input.
The engine is nothing more than a Chevrolet 350 with stainless head gaskets and brass core plugs. The carburetor is a very slightly modified Rochester Quadrajet. It's straight out of a Chevrolet pick up or delivery truck. It's the most common engine ever over here. It's not exotic and it's easy to find information. You just need the right guy or a motor that's all ready.
Too bad the boat's not in Italy, some of those Italian motor guys are geniuses!
