Engine rebuild or replace

[Blake Davis]

Hi everyone, I am new to the site. I have a 1986 340 that is in good shape, but the port engine has been a problem for a while. I was willing to pay to have a marine mechanic to look at it, but trying to get a marine mechanic at this time of the year in my area is impossible so I have brought in a automechanic that works on some of our company equipment. There was too much fuel getting pumped into the carb, and flooding it, so I had the carb rebuilt, and replaced fuel pump, wires, cap, and rotor. Once this was all done we attempted to start engine which created an explosion that blew off the top of my port side muffler with enough power that it sent the top of the muffler through my canvas, and over to the next dock(Thankfully no one was hurt). This engine has always ran rich, spewing out gas in the water, and at this point I am thinking of a bebuild, or after market. It's a good boat, and I don't mind spending some money on it. The original motors were 350s that were replaced with 454s. Apparently the 454s only have 600hrs. on them, but I am not confident in this. The 454s make the boat sit very squat in the water. We will be doing a compression test this week. What are your thoughts?

 

[Timvail]

Hi everyone, I am new to the site. I have a 1986 340 that is in good shape, but the port engine has been a problem for a while. I was willing to pay to have a marine mechanic to look at it, but trying to get a marine mechanic at this time of the year in my area is impossible so I have brought in a automechanic that works on some of our company equipment. There was too much fuel getting pumped into the carb, and flooding it, so I had the carb rebuilt, and replaced fuel pump, wires, cap, and rotor. Once this was all done we attempted to start engine which created an explosion that blew off the top of my port side muffler with enough power that it sent the top of the muffler through my canvas, and over to the next dock(Thankfully no one was hurt). This engine has always ran rich, spewing out gas in the water, and at this point I am thinking of a bebuild, or after market. It's a good boat, and I don't mind spending some money on it. The original motors were 350s that were replaced with 454s. Apparently the 454s only have 600hrs. on them, but I am not confident in this. The 454s make the boat sit very squat in the water. We will be doing a compression test this week. What are your thoughts?

You need to have a marine mechanic to have a look and check for other damage. Raw gas dilutes the oil which causes friction on the walls of the cylinders. Icluding all internal parts. good luck.

 

[scoflaw]

Depends if you can live with the "squat" or not.

 

[Skip Boyer]

I take it your squat is due to adding the extra pounds difference between the small blocks and big blocks. Probably added at least three hundred pounds to the stern.

 

[89P270]

My two cents - First my experience - I paid a dealer to replace two 350 Mags and Alphas (called bobtails when you include the outdrive with the engine) in a 270 Pachanga. I replaced with 357 4V Alpha rebuilt Mercruiser factory sets and 2 new Gen II alpha drives. When we threw in the added work for custom stainless exhaust (OEM wouldn't work with the new engines manifold spacing) I had about $34K in a 1989 boat in 2012 (see my Pachanga 1989/2012 thread). So, experience I have. What I would say, if you plan on keeping your 340 (which I love BTW) for a decade more, replace with new. Cant beat the peace of mind new engines will give you and improved performance you can get. And, replace with Mercrusier 383 scorpions that will give you more torque and HP than the 454 and they are small blocks which gets you the reduced stern squat you originally had. Expect to pay $14K for twins and installation $$ if you dont do the work. HOWEVER, if like me, you sell the boat three years later, don't even think about the repower, do a rebuild for 7K and be done with it. Some of this is qualified whether yo do work yourself or not. Saving $$ doing the swap yourself goes a long way. The other thing I did is when I swapped the outdrives I went to counter-rotating set up (factory was both right hand drive). I thought this was going to be an improvement and stop chine walk and such. It did support reduced chine walk but I don't think there was any performance improvement.

 

[hughespat57]

Some additional background would help as well.

With pleasure boats "engine hours" is an age old myth.
600 hrs over ten years? (60hr/yr) or over five years (120hr/yr) More per year is "better"; sitting is hard on engines.
Even a small commercial boat can easily put several thousand hours a year on an engine. (8hr x 5days x 52weeks)
That's why you'll here an engine should last thousands of hours, but it's apples and oranges.

freshwater or saltwater? big difference.
And the biggest is how the PO maintained it.

My 94' 300 is twin 350 I/O with 400 hrs, original engines, all freshwater lake boat, engines look almost new.
And compression and performance are still very good, even risers and manifolds are good.
But I wouldn't be surprised if one dies catastrophically at there age.

Then you are stuck with the PO upgrade to big blocks so its fix what you have or swap out both as 89P270 points out.
When were the 454 installed?
Did the PO replace the drives with Alpha Gen 2 at the same time? If yes then at least you only need a pair of recent 350s.

ASE Master Heavy Truck Mechanic
USCG Machinery Technician, retired

 

[NorCal Boater]

You don't say if its an Express or a Sundancer. Because of the squat I will assume its a Sundancer. From '87 to '89 you would be hard pressed to find a 340 with small blocks. And in fact, I think 454s were the only engines offered in '88 & '89. The squat was normal. With v-drives the engines are as far back in the boat as they can get. I had an '89 and it sat and rode ass-heavy but trim tabs helped. So while the BB motors are contributing to the stern-low attitude, it was/is not unusual in this boat.

As said above, if you are going to keep the boat for a long time do the swap. There are a lot of ways to go. You can go all new but with the 383s or 6.2s, nothing from the old motors can be reused and the motor mounts will need to be re-engineered as well as exhaust and you may need to change the gear ratio in the transmissions. In my '89 one of the 454s ran in reverse to achieve counter-rotation. I believe starting in 1990 it was done with the gearbox.

You can also buy 454 short blocks or long blocks and some items from the old motors can be used such as exhaust manifolds, water pumps, motor mounts, etc. For me, I would go with the new blocks (short or long) as regardless of how much you spend or how nice it is, at the end of the day it's still a 34-year old vessel.

Good luck and let us know what you decide.
Shawn