Another blown head gasket - Fuel Mixture?

[Joshto]

Hey folks,

I searched around but didn't find the answer to my specific question.

I/O: Merc 3.0 4L (1998) - Alpha one Gen 1
Boat: Cadorette Nuova 170

I replaced my engine with a used one about 3 years ago. The used engine had blown the head gasket, so I replaced it. I have put about 60 hours on the engine so far. The engine ran great; it had plenty of power, enough to hit 45-50 MPH @ 5500 RPM.

Last week, out of the blue, the engine started running rough on idle. I adjusted the carb's fuel mixture back about 1/4 of a turn. Idle came back to normal @ approx 700 rpm in the water.

Yesterday I went out cruising at about 4500 RPM, and 10 min later, without warning, the engine started losing power, to the point that it wouldn't go back on plane. On my way back to the ramp, I noticed a hissing sound coming from right below the manifold. I figured it had blown the gasket. I checked the temperature and oil (looked for water), everything was normal.

I removed the head, and indeed, the gasket blew on the third cylinder's port side. I'll have the head resurfaced, and the block tested with a straightedge at the engine shop, but my question is, could the 1/4 turn fuel mixture adjustment cause it? I've read that preignition/detonation can blow the gasket; I guess those are extreme cases. And if you think it could, why the 3rd cylinder only?

Looking for advice as I'm planning to pull everything back together this weekend. I wouldn't want to blow another gasket out of a bad fuel mixture.

I wonder if I should rebuild the carburetor before installing the head again. Not sure if I want to introduce another variable.

Leave you the pics here for your delight and insights. Thanks.

 

[achris]

5500rpm is too high for those engines. Max rpm, 4800. Go up in prop pitch 4"

Unlikely to be fuel mixture, more likely too much timing advance. What was the timing set to?

Chris...

 

[Scott Danforth]

at 4600 RPM, the motor starts go come apart (literally, the deck starts to flex and the bolts start to loose clamp force)
at 4800 RPM, the deck moves a bit more, the head bolts stretch a bit more, the head warps from the crappy design in cooling and the motor spits the head gaskets
at 5500 RPM, you are lucky you didnt get hit in the head by the pistons coming loose as the cast crank and rods usually fail about 5200 RPM

the motors were designed to run 4400-4600 RPM, 4800 RPM at the absolute top end.

the stock head bolts wont hold the head down enough running higher RPM

I suggest decking the motor, surfacing the head and switching to better head bolts such as ARP (and yes, regardless if its in the catalog, there are ARP bolts you can buy for the head.)

 

[Joshto]

at 4600 RPM, the motor starts go come apart (literally, the deck starts to flex and the bolts start to loose clamp force)
at 4800 RPM, the deck moves a bit more, the head bolts stretch a bit more, the head warps from the crappy design in cooling and the motor spits the head gaskets
at 5500 RPM, you are lucky you didnt get hit in the head by the pistons coming loose as the cast crank and rods usually fail about 5200 RPM

the motors were designed to run 4400-4600 RPM, 4800 RPM at the absolute top end.

the stock head bolts wont hold the head down enough running higher RPM

I suggest decking the motor, surfacing the head and switching to better head bolts such as ARP (and yes, regardless if its in the catalog, there are ARP bolts you can buy for the head.)

Thanks for the tip. I thought the WOT was 5800. In any case I've brought it to that speed maybe twice in perfect mirror lake conditions. I normally cruise at 4500 max. So you don't think the fuel mixture caused it? I'll look for those bolts

 

[Joshto]

5500rpm is too high for those engines. Max rpm, 4800. Go up in prop pitch 4"

Unlikely to be fuel mixture, more likely too much timing advance. What was the timing set to?

Chris...

I don't know. It's been the same since I bought the engine. Never had to adjust it...as said the engine always ran "perfect". How do I check that?

 

[Paintman1960]

Thanks for the tip. I thought the WOT was 5800. In any case I've brought it to that speed maybe twice in perfect mirror lake conditions. I normally cruise at 4500 max. So you don't think the fuel mixture caused? I'll look for those bolts

Might want to check RPM's with a different tach, seems extreme to me..
I have a 383, and never run over 5k or so ,Can't even think of pushing it that far , In fact i don't think I could

 

[Joshto]

5500rpm is too high for those engines. Max rpm, 4800. Go up in prop pitch 4"

Unlikely to be fuel mixture, more likely too much timing advance. What was the timing set to?

Chris...

Forgot to mention...it runs a 21 pitch prop.

 

[Paintman1960]

21 " on a 3.L?
That's what I run my 23 ft , 383,
Am I thinking wrong here ? or am I missing something ?

 

[Joshto]

21 " on a 3.L?
That's what I run my 23 ft , 383,
Am I thinking wrong here ? or am I missing something ?

I don't understand. What's wrong with running a 14.25/21 on a 3.0?

 

[nola mike]

Thanks for the tip. I thought the WOT was 5800. In any case I've brought it to that speed maybe twice in perfect mirror lake conditions. I normally cruise at 4500 max. So you don't think the fuel mixture caused it? I'll look for those bolts

The idle mixture affects idle, once off idle the idle ports don't do anything.

 

[achris]

I don't understand. What's wrong with running a 14.25/21 on a 3.0?

Post the rest of your numbers (speed, max rpm, drive ratio) and I'll run them through my prop program and see what they say...
(Also, how big is the boat?)

 

[santino44]

Post the rest of your numbers (speed, max rpm, drive ratio) and I'll run them through my prop program and see what they say...
(Also, how big is the boat?)

Chris, take a look for me too. Boat Yamaha 19 feet, weight approximately 1300 kg. with a full tank of gasoline. Alpha 1 gene 2 reduction 2.0. 3.0 TKS engine. The screw is installed 3x14.25x21. Maximum speed 63 km / h at 4800 rpm. 37-39 km / h at 3000 rpm. Loading 2 adult passengers and 1 child.

 

[Scott Danforth]

my guess is the tach is way off and you are actually lugging the motor

 

[achris]

Chris, take a look for me too. Boat Yamaha 19 feet, weight approximately 1300 kg. with a full tank of gasoline. Alpha 1 gene 2 reduction 2.0. 3.0 TKS engine. The screw is installed 3x14.25x21. Maximum speed 63 km / h at 4800 rpm. 37-39 km / h at 3000 rpm. Loading 2 adult passengers and 1 child.

Please don't hijack someone else's thread. If you have a question of your own, ask it in your own thread.

 

[Joshto]

my guess is the tach is way off and you are actually lugging the motor

Thanks. As soon as I install the head back, I'll find another tach and run it parallel with mine...I hope that explains why I thought I was doing 5500.

 

[Joshto]

Post the rest of your numbers (speed, max rpm, drive ratio) and I'll run them through my prop program and see what they say...
(Also, how big is the boat?)

The boat is 17 feet; 2200 pounds (According to nadaguides).

Max rpm is 4800 (now I know)
Drive ratio...not sure it appears to be 1.94
Max Speed on my GPS Speedometer ~47 mph. I normally cruise at 30-35 mph.

 

[Joshto]

An update here. I had the head resurfaced.

The engine shop guy said the head was in pretty bad shape. Also, he's 90% sure the head's imperfections blew the gasket. The block is as straight as it could be.

I guess if the engine went 60 hours with a messed up head and rev abuse, it should last way longer this time...

 

[Searay205]

I had a very similar boat. 47mph is impressive. Mine was 16' and 2100 lb bow rider. most I could get was 44 mph at 4600 rpm. 30 years on motor when I sold, ran perfect.

 

[Joshto]

To close this topic up. I put together the engine without messing with the carburator nor the timing. The engine runs like a champ now (Pat myself on the back for adjusting the valves first time right ). No more rough idle..it's super smooth and quiet. Power is back. At this point I strongly believe the engine began to run rough when the gasket started going out slowly.

Lessons learned: retorque the head after 20 hours of operation (I never did that and it's clearly stated in the manual) and retorque the manifold after the first start.

Regarding the tach, I ran another one in parallel and oddly enough they both show the same rpms. I can't explain how that engine could have possibly gone to 5500 rpms...it'll remain a missTERY.

 

[tank1949]

Might want to check RPM's with a different tach, seems extreme to me..
I have a 383, and never run over 5k or so ,Can't even think of pushing it that far , In fact i don't think I could

Not unless you want a hand granade! I have had tack problems as well.