Tuliped valve causes

[Profittakr]

I have Mercruiser 5.7l, Thunderbolt IV, S/N OD724908, 1992 24' Searay BR,

I have tuliped #6 exhaust valve twice, on 2 different motors. I have changed virtually EVERY part on this latest motor and still it tuliped a valve. What could possibly be the cause so that I don't do that again?

I have: new marine long block, professionally rebuilt carburator, new plugs, wires, ignition sensor, rotor, professionally rebuilt distributor, new fuel pump, new fuel lines, new fuel filter, new risers and exhaust manifolds, original intake manifold but has been magna fluxed TWICE looking for cracks and issues (all good).

I have no idea what else could be causing this issue. The ONLY thing I can think of is the Thunderbolt IV module although I have read that these are quite reliable,. The engine was running great, until it wasn't. That is when the tuliped valve was found. All other cylinders look fine. They do not look like they are running lean, but maybe they are. Can just one cylinder run lean from a Rochester 4bbl carb? I believe the 2 carb barrels feed 4 cylinders and 2 barrels feed the other 4 cylinders. If something was running lean, wouldn't 4 cylinders all experience the same problem?

Is this a timing issue? Carburation issue? or something else?

Please help. I don't want to destroy another motor...

 

[Rick Stephens]

Running lean. Professionally rebuilt doesn't appear to have been correctly rebuilt.

 

[alldodge]

Agree, running lean
All are lean its just #6 is first to fail

Increase main jets by at least to sizes. Rich is better and can be leaned out slowly after motor is broken in

 

[chs marine service]

one thing to check,the distributor has a washer/spacer between the gear and the housing. it has 4 pointy tips. parts 10 & 11., this was a brand new dizzy from standard that was assembled without #11 and caused the engine to throw the timing off. this small amount of movement caused the engne to detonate to a point it blew the motor. thank god SMP di the right thing and paid to replace it. just want to take a look at it

 

[Scott Danforth]

two primary causes on an SBC. either the valve is not closing, or your running lean, typically it will be cylinders 3, 4, 5, or 6 that go first as the siamesed exhaust ports on the SBC run hotter than the other 4.

if the valves ere adjusted properly, then do as AD mentioned, run a bit richer

if your base timing is off , it will accelerate the higher combustion temps

 

[louc]

Look at your spark plugs center insulator porcelain that can tell you a lot.
I always wondered if the 4.3 V6s were less prone to this because none of the exhaust ports are siamesed as on the V8s....yet my OMC is set up from the factory to run less spark advance than any of the V8s, go figure!

 

[muc]

Guessing your over propped for how you use the boat.
Do you use the boat for water sports?

 

[Scott06]

Guessing your over propped for how you use the boat.
Do you use the boat for water sports?

how Does over propping tulip the valves? Lugging and hotter Cylinder and exhaust gas temps?

 

[louc]

Yes that is the cause, laboring caused by low max rpm. That’s the main reason why you want to check that the engine will reach it’s specified maximum rpm at wide open throttle.
My boat came to me with a 14.5x19 when it should have had a 15x17, that got it closer but I found that a 15.5x15 was even better.

 

[muc]

how Does over propping tulip the valves? Lugging and hotter Cylinder and exhaust gas temps?

Yes.

Part of the problem is with how the boat is used. Bunch of people in the boat and pulling a tube or board while the boat is plowing thru the water. When I look at RPM history, I see a lot of hours between 2-3,000

But the machine shop usually wants to replace more then one valve. They say the other 3 are starting to go.

 

[kenny nunez]

Just one question, was the intake manifold re-used on the engine changes?

 

[Scott06]

Yes.

Part of the problem is with how the boat is used. Bunch of people in the boat and pulling a tube or board while the boat is plowing thru the water. When I look at RPM history, I see a lot of hours between 2-3,000

But the machine shop usually wants to replace more then one valve. They say the other 3 are starting to go.

Yeah better off being a tad under propped vs over propped, especially if you tow.
if you yank the heads and have tuliped valves would think it’s wise to replace them all especially on a SBC Where parts are relatively cheap