I just did a leakdown test and found a dead cylinder. Upon removing the head today we found a bad valve. My question is what could have caused an exhaust valve to tulip badly? I have owned the boat less than 2 years and don't know how the engine was put together since they have never been torn down. Engine info discovered thus far is:
B & M 250 Blower on a 454
Single 850 Holley
Roller rockers
Flat Tappet cam
LS7 heads ported/polished

My mechanic tells me on a quick look that the engine looks like it was probably put together well but we have no details on engine parts yet. (need to look up part number)

Rebuilt carbs with same jets were done over winter as was a rebuilt blower by the Blower Shop. Engines run about 6 lb boost.

It went during a poker run and I was not running that hard. Maybe had full throttle less than 3-4 minutes at the start before backing down to 4000-4500rpm. The miss started after that. Engines spin to 5200.

Fresh hi test and 4 cans of octane booster in about 160gal.

Valve is not too expensive to get fixed, but my BIG QUESTION IS WHAT COULD BURN THE VALVE IN THE 1ST PLACE. I want to fix the source of the problem so that it doesn't eat a piston next time.

Any theories? Please post anything that can help.

How much old gas was in the boat ? Sorry to hear about your boat only one valve it want take much , Check the valves to make sure that they are at least Inconel valves . Stainless valves with a blower will not hold up.

Did you check ignition timing before disassembly? Detonation will even damage inconel valves in a NA engine...

Yes timing will KILL a motor.

Had Stabil in the gas. about 50 gal. Ran the boat 1st time for an hour. Pulled it up and filled it with high test and 4 bottles octane boost.

Timing was OK.

Is it over propped? Lugging the motor can cause similar problems. Is timing severely retarded? Sometimes you think you're safe with a blower motor by backing the timing off but it's a double edge sword, retarding the timing to prevent detonation can send exhaust gas temps through the roof, tuliping the exhaust valve in the process.

...just my .02

Good luck,,

Kurt

Haven't changed props yet. Still spinning 27P 3 blade Mirage and can spin it to 5200. So I don't think it's lugging. I'll try to get some valve out to look at tonight and maybe get some part numbers.

Valve quality is very important but good valves won't hold up either if the machine work isn't done correctly. Make sure the valve to seat contact area is sufficient for your application. I always pull an exh valve from the head once the machine shop is done and use lapping compound to mark the valve and measure.

Does the carb run a powervalve and if so is it boost referenced? Full throttle you would be at a very low vacuum, powervalve remains open. Cutting back to 4000 could have allowed the power valve to close, which could represent 10 jet size reduction, instant lean fuel mixture. I prefer to eliminate power (jet up accordingly) valves in blown applications and use plug checks and a pyrometer to jet the carb.

Do you run a pyrometer in the exhaust? That is a great way to see if you're doing damage or not.

Excess heat will allow the valve to tulip, which will eventually eliminate the valve lash, eventually leading to the valve being held open and being burnt, etc....

The best thing to do with last years gasoline in a boat is burn it in your lawnmower. It doesn't belong in a blown boat.

HP,
You say you have had the boat 2 years. Did it run ok all of last year ? If so, I doubt that the quality of the parts or the machinie work is an issue. If it ran ok last year, then something that happened over the winter is most likely the cause. A valve tulips because of excessive heat in the combustion chamber, or water coming in through the exhaust for some reason.

You said that you rebuilt the carb ? Was anything changed as far as jetting, power valves etc ? Also said you had the blower rebuilt ..... any possibility that something was done there that could cause a problem ? Is the boost the same as it was last year ? You get the idea, I think something changed, or happened over the winter that caused the problem.

Another thing is .... did you fog the engine real good before parking it for the winter ? We are seeing more and more engines with sticky valves after sitting for a while. Personally, I think the new fuels just do not like sitting for very long. I know you had stabil in it, but was it enough ? The fogging oil will get into everything and protect it pretty well s I think it is a must now days. I would definitely fill up ( ouch ) to dilute the old fuel with new before running it the first time of the season.

Just thinking out loud,

Bill Koustenis
Advanced Automotive Machine
Waldorf Md