Did he "rebuild the engines" or just replace the heads?

A slight increase in compression should not be an issue, but that all depends on what "slight" means. If the original engine was designed/certified to operate at 86 or 87 octane and you're now operating at 89 or above, then a "slight increase" would likely be inconsequential to the operation. Should actually result in slightly better performance or economy, but you might never be able to actually measure it. Definitely shouldn't result in a dramatic LOSS in performance unless a lot of detonation is occurring, which will destroy the engine in short order.

Is this carb or FI? Has the ignition timing been validated or verified to be correct?

To answer your final question, "NO!" Definitely not acceptable.

 

Did he "rebuild the engines" or just replace the heads?


.........................................heads only

A slight increase in compression should not be an issue, but that all depends on what "slight" means. If the original engine was designed/certified to operate at 86 or 87 octane and you're now operating at 89 or above, then a "slight increase" would likely be inconsequential to the operation. Should actually result in slightly better performance or economy, but you might never be able to actually measure it.


..............................I hear you on slight,but I would never deviate from OEM



Definitely shouldn't result in a dramatic LOSS in performance unless a lot of detonation is occurring, which will destroy the engine in short order.


.........................my thought similar. Seems to me that a higher compression or reduced detonation chamber area/volume would stress the other components.


Is this carb or FI?

...................FI (sorry, meant to include that)



Has the ignition timing been validated or verified to be correct?

......................yes. Book is 8 to 10 degrees and he has it at 10

To answer your final question, "NO!" Definitely not acceptable.

Did he "rebuild the engines" or just replace the heads?

...............................................thanks.

A slight increase in compression should not be an issue, but that all depends on what "slight" means. If the original engine was designed/certified to operate at 86 or 87 octane and you're now operating at 89 or above, then a "slight increase" would likely be inconsequential to the operation. Should actually result in slightly better performance or economy, but you might never be able to actually measure it. Definitely shouldn't result in a dramatic LOSS in performance unless a lot of detonation is occurring, which will destroy the engine in short order.

Is this carb or FI? Has the ignition timing been validated or verified to be correct?

To answer your final question, "NO!" Definitely not acceptable.

Did he "rebuild the engines" or just replace the heads?

A slight increase in compression should not be an issue, but that all depends on what "slight" means. If the original engine was designed/certified to operate at 86 or 87 octane and you're now operating at 89 or above, then a "slight increase" would likely be inconsequential to the operation. Should actually result in slightly better performance or economy, but you might never be able to actually measure it. Definitely shouldn't result in a dramatic LOSS in performance unless a lot of detonation is occurring, which will destroy the engine in short order.

Is this carb or FI? Has the ignition timing been validated or verified to be correct?

To answer your final question, "NO!" Definitely not acceptable.

 

I don’t know, seems 1000 hours is not really a lot of hours on a big block imo. Maybe halfway through their life span? 4100 RPM sounds right for a stock motor. Before the rebuild did you try changing the props to a different pitch?

 

I don’t know, seems 1000 hours is not really a lot of hours on a big block imo.

...........................my previous small blocks had 1500 and ran ok



Maybe halfway through their life span?


.........................maybe. I asked around yers ago and many said 2k lifespan, but some said 3 to 5k can happen.


4100 RPM sounds right for a stock motor.

....................right on the motor on a sticker



Before the rebuild did you try changing the props to a different pitch?

.........................nope. Never a thought, given that it ran well

 

What casting number on original heads and what did he replace them with? Peanut, oval and rectangular are the 3 styles

 

...........I am sure he gave my original heads to the place he got my newheads from....a core? Have no idea to answer your question. As I am thinking about this travesty, it occurred to me that having the heads would be an ideal situation.

Let me ask you. Is there a Mercruiser part number for my heads that my engine serial number will lead me too?

Thanks.

 

How old is the fuel? Is it possible it is degraded and causing the timing to retard under load?

 

........................old. Possible, but I never had this as an issue before with older fuel.

 

Do a compression check. Any bets he got something wrong.

 

.......................ok. Good advice.

Is it possible that the compression can be......too high? I am familiar with loss of compression, but never heard of the other way around.

 

Probably not aftermarket heads if they took the old ones as cores. The new and old casting numbers relatively important to rule that out as your problem

 

You should get the casting numbers off the current heads, then you can figure out what they are. You could also look up the stock heads using the serial numbers on the engines so you have something to compare the two (old vs new).

I'm really wondering what the motivator was to do ANYTHING to the engines at only 1000 hrs, especially since (it seems) you rarely run it at or near its rated output. If a lot of your cruising/hrs is closer to 2000RPM and some 3100, those engines should last well over 2000 hrs before needing attention. And I don't think 4100 is "redline" on these engines, but rather the rated RPM. "Redline" is probably closer to 4400-4600.

 

You should get the casting numbers off the current heads, then you can figure out what they are. You could also look up the stock heads using the serial numbers on the engines so you have something to compare the two (old vs new).

I'm really wondering what the motivator was to do ANYTHING to the engines at only 1000 hrs, especially since (it seems) you rarely run it at or near its rated output. If a lot of your cruising/hrs is closer to 2000RPM and some 3100, those engines should last well over 2000 hrs before needing attention. And I don't think 4100 is "redline" on these engines, but rather the rated RPM. "Redline" is probably closer to 4400-4600.

I am guessing the original heads are long gone to wherever he got my current heads. Can always ask.

 

At the age of those engines there are more factors in play than just hours. Aftermarket heads won't have serial numbers but there should be some markings on the exterior to tell you what they are

 

Got it

 

So….just to recap… your “mechanic” decides to replace your heads with a rebuilt set and now the engines flatten out at 3400.

So that definitely is in the category that he got the wrong heads for your engines or they were poorly rebuilt. Additionally he may have used a thicker head gasket that reduced compression.

The next step is a compression test to see if anything is addressable without replacing the heads. We had another thread where the rebuild “mechanic” screwed up the valve job so they didn’t seal.

Sorry you are going through this but there are a lot of incompetent people out there who think they are mechanics.

Do the compression test and post the results. Also shoot a picture of the spark plugs.

 

Will do.

Thanks for the offer to look the results over.

 

Have you paid him?

Imagine trying to explain this when you go to sell the boat. No way should you accept and pay for a hack rebuild...

 

Oh boy, do I agree.

 

Why did you rebuild engines that were performing somewhat close WOT spec?

 

Best question asked so far. X10!

 

As others asked, what was the purpose of the rebuild?

Something is wrong.

Your engines should reach their maximum rated RPM, which will be somewhere in the 42-4600 range. This is important, and a boat should be run to its maximum RPM at least once a season. Doing so establishes the motors are running correctly and you are not overloading the vessel.

Those that think they are babying their engines because they never elevate the rpm's, might find they are in fact not babying them at all. Load on a motor is way more important than rpm (so long as you are not over-revving). A guy running big blocks all day at 2800 with a max WOT of 3400, is being way harder on them than a guy running 3600 with a max WOT of 4400.

As you correctly state, running at 3100 when the engines won't turn past 3400 is a recipe for disaster. It's a great way to burn holes in pistons, blow head gaskets, etc.

If the engines ran at or close to rated rpm range, and will not now, something is wrong. There are a lot of things that could be, but the old adage I've found very useful is, "If you worked on the motors, and a new problem comes up, it is almost certainly something you did."

 

As others asked, what was the purpose of the rebuild?

Something is wrong.

Your engines should reach their maximum rated RPM, which will be somewhere in the 42-4600 range. This is important, and a boat should be run to its maximum RPM at least once a season. Doing so establishes the motors are running correctly and you are not overloading the vessel.

Those that think they are babying their engines because they never elevate the rpm's, might find they are in fact not babying them at all. Load on a motor is way more important than rpm (so long as you are not over-revving). A guy running big blocks all day at 2800 with a max WOT of 3400, is being way harder on them than a guy running 3600 with a max WOT of 4400.

As you correctly state, running at 3100 when the engines won't turn past 3400 is a recipe for disaster. It's a great way to burn holes in pistons, blow head gaskets, etc.

If the engines ran at or close to rated rpm range, and will not now, something is wrong. There are a lot of things that could be, but the old adage I've found very useful is, "If you worked on the motors, and a new problem comes up, it is almost certainly something you did."

A good adage and an easy one to agree with............

 

It almost sounds like a set of open chamber heads were installed where closed chamber heads were original. The change of compression will be significantly downward and so will power. It would be very anemic and hard to start. But, Sheesh - any engine mechanic knows this.

Like said earlier - do a compression test and verify you are showing at least 110 psig on all of the cylinders. If you are down around 60 or 70 PSIG then you may know the issue is in the cylinder heads used. Also verify the casting numbers against what was original Mercruiser; the numbers are under the valve covers if Chevy or usually stamped on the front boss if aftermarket.
Another member had valve train issues where the heads were rebuilt and the OEM valve train didn't correctly adjust causing the valves to not fully close which will result in major performance issues and engine damage. This can be checked with a compression test but more accurately diagnosed with a Leak-Down test.
Or, ignition timing is incorrectly set.

BTW you should occasionally run those motors on the pins just like the diesel engines to exercise sensors, regulators, and linkages. Those bulwarks are good for 5500 RPM so fully loaded at 4800 RPM won't hurt them at all. The only thing hurt is your fuel budget....

 

As others have said, verify compression and even leak down to rule out the heads.

Second thing I'd look at is the timing... Timing had to be set because the distributor had to be removed. It may be slightly off. Also verify the advance system is working correctly (I don't know what type of ignition setup you have).

If all those things check out, start looking at fuel pressure under load.

It's likely something simple.

 

Agree with compression and possibly even a leak down to verify the mechanical side.
Older posts indicate your fuel was old last season. Have you tried running off of a secondary fuel source with known good fuel? You can't overlook the simple things.

 

I may have missed this in the replies, have you talked to the mechanic about this to ask them to do engine compression checks as well as a leak down test? Have you paid them in full yet?

 

Initial discussion with mechanic was loaded with excuses by him for rpm deficit.

The guy has been around awhile. A local marine supply merchant validated my guess that this guy was one of the bet around.

Sadly, I paid him as we went..... because I trusted him to do a good job. I am into him about 400 - 600.00 (currently owed), a fraction of what I've paid him so far. Nevertheless, this remaining balance ain't going to him uintil my engines are right.......or never.

 

Rivertender, is this the same mechanic/job that you posted about in May 2022?

 

It should still rev out above 3500 rpm even with open chamber heads . Street raced for years and any big block would rev to 5000 easily, possibly timing or fuel

 

Even pulling a trailer up a hill? I don't think an auto with a transmission is a fair comparison to a heavy boat trying to get on step.
@Rivertender, I read some of your earlier posts on this and another forum. It sounds like you replaced the distributors with EST Voyagers before the head swap. Is that the case?
Your symptoms as others have mentioned could easily just be a timing problem.
The Voyager distributors work great in jet boats (my experience with them), but I have my doubts with an i/o application. They do not hit their full advance until 3600 or something like that. Jets don't care as it is just a pump in front of a nozzle (restriction). The original Thunderbolt distributors can achieve full advance as low 2000 rpm.
Have you checked the timing with your engines loaded?
We are all trying to help.

 

It is cheaper to start with the routine maintenance items first to see what effect that has on issues such as running a little on the warm side. Impellers are wear items that need replacement on a schedule. Otherwise you have overheating when they wear out and breakdown.

 

Can you post the pictures he sent you along with any explanation of what they are pictures of?

 

If your full advance isn't coming in until 3600 rpm, well that for sure isn't helping your situation. It needs to be in much much sooner for that motor to make peak power and get that boat on plane.

 

It should still rev out above 3500 rpm even with open chamber heads . Street raced for years and any big block would rev to 5000 easily, possibly timing or fuel

I think he is talking about when the engines are under load. They simply wouldn't make enough power with changing the heads from CC to OC. Compression ratio will drop too much.
He doesn't say if he is carbureted or fuel injected in order to comment on the ignition.
He also doesn't know if the valve train is adjustable or non-adjustable.

 

This was the 3 year old post that mistakenly made me think you replaced your distributors with Delco Voyager EST's.
Marine Voyager engine parts - The Hull Truth - Boating and Fishing Forum
It sounds like you didn't so fair enough.
Another post that threw a flag for me was from last spring when you commented that the boat wasn't used in 2021 and the fuel in it was old. It sounds like that wasn't corrected and it can be a big problem when trying to make power with your engines.
Information that you may not consider to be important can easily mislead others when left out and send them in another direction. It was the reason I looked into your earlier posts. Nothing more.
Hoping for a simple resolution for you.

 

This was the 3 year old post that mistakenly made me think you replaced your distributors with Delco Voyager EST's.
Marine Voyager engine parts - The Hull Truth - Boating and Fishing Forum
It sounds like you didn't so fair enough.
Another post that threw a flag for me was from last spring when you commented that the boat wasn't used in 2021 and the fuel in it was old. It sounds like that wasn't corrected and it can be a big problem when trying to make power with your engines.
Information that you may not consider to be important can easily mislead others when left out and send them in another direction. It was the reason I looked into your earlier posts. Nothing more.
Hoping for a simple resolution for you.

copy.

Thanks.

 

It is cheaper to start with the routine maintenance items first to see what effect that has on issues such as running a little on the warm side. Impellers are wear items that need replacement on a schedule. Otherwise you have overheating when they wear out and breakdown.

Agreed

 

Can you post the pictures he sent you along with any explanation of what they are pictures of?

I went to look for them and my new phone texts don't go back fr enough. Hoping there is a way to capture my older texts.