Powerboat Forums at SpeedWake banner

Ideas on Burned Valve?

1810 Views 18 Replies 13 Participants Last post by  Blue Thunder
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 numbers)

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.
1 - 19 of 19 Posts
Could be a leaky exhaust manifold. Water touching a super hot exhaust valve will ruin it.

Detonation? Its a blower motor so this could be quite possible.

Was it a intake or exhaust valve? Could have been running lean too.
Oil getting in can burn up stuff too. It creates some extra heat. 5.0 Ford motors all burned up a rear cylinders exhaust valve because the PVC was drawn into that cylinder more than the rest. Once blow by started, they ALL burned that one valve...maybe a situation like that. Keep in mind. Good Luck with it
Running lean was the 1st idea, but when the carbs were rebuilt, we used the same jetting with the theory that it was OK before. We also figured that if anything, after the rebuild, it would be richer.

Detonation. I always use high test and usually run several cans of octane booster for a poker run just to be safe. Have never heard detonation before, but it is kinda loud.
What jets are you running in the carbs? The carb itself seem pretty small. I run a 177 blower with a 1050 dominator. Ran on the dyno the air fuel mixture was right on the money with 100s for jets. I no expert Just my experience. Good luck with the rebuild
Good point.
With the 850 there is a good chance, both at WFO and especially when throttle is backed off just a touch after WFO, that the vacuum under carb raises enough to shut down the PV and burn baby burn. This is assuming PV is not boost referenced.

The 525SC's I worked on (with baby blowers too) came with 1050's from Merc and 98's in all 4 corners. After the upgrades I ended up at 102's all around. I actually have to look at my notes, it may have been left with 104's. I used both during tuning. Just having a senior moment right now.

Also, the Merc 525SC's used a very cold plug. MR41T's.

Basically, 'tuliping' happens when the valve is subjected to higher temperatures than it's material limits.

Could be that your valves aren't made for this application. Many will suggest Iconel or Ferrea Super Alloy. Or possibly not enough margin cut into the valves ? Or etc?

Could be your engine would eat these too, so You need to do some failure analysis (take a long look at plugs and top of pistons) looking for material transfer of piston tops to the plugs. Will need magnifying glass and bright light to look down into plugs to look for shiny crystal like deposits. These are from your pistons. :shocked:

Then, figure out your component/tuning issues to correct this.

My edit: My edit: Note: I'm not suggesting that as a jetting reference to your 850 as they are very different carbs.
See less See more
Thanks guys, all this gives me ideas. The motors have been blowers for several years running those carbs. I wiil need to check the jets for size.

The port engine (non-damaged) has been run 3-4 years. The Stbd engine (burnt valve) was just rebuilt before I got the boat last winter. I put maybe 50 hours on it last year.

It does have a boost reference.

A friend is coming over and we will be removing the valves this evening.
A few things based on observation and tear-down...

1. During testing and the poker run (I was not there for initial testing), the starboard engine was suffering a lean back fire upon hard acceleration and re-entry. While riding in the back, I think I could hear detonation, but with the blower whine and 5" open pipes, it was hard to really tell. And because the intake valve looked fine, it leads me to believe that it was NOT detonating.

2. When we first encountered the dead cylinder after the first stop, we limped back to the local marina and changed plugs. #2 was welded shut!

3. The exhaust valve DEFINITELY got hot. It had the tell-tale tulip but also showed a lighter color than the rest of the valves.

4. The pistons look good. Burned fuel deposits across all four on the even bank (the odd bank is still together) but no sign of damage. Actually, based on what I see, it could be rich (at least at idle and low throttle).

I think that:

1. The carb is NOT jetted correctly and for two reasons... a) the blower is now essentially new and creating almost 2 PSI more boost than last year, b) and the lean back fire.

2. Valves are incorrect for the application. Looking at them in my photographic memory, they appear to be a stock-GM valve (well, the intakes look stock to me). And now that we've added boost AND have an unknown in the way of the carb, I think we uncovered the weak link, and thankfully all we had was a tuliped valve (instead of a broken one or holed piston).

I'm not too sure I believe that the carbs are too small. Remember, they only add more air. We have enough air. We need more fuel. I think jetting her fat would be a better first step.

I say we repair the damage, and then get ready to tune. I can pick up a jet kit and we can go from there. The plugs were unreadable last night.... we need to see them right after a WOT run.

I also think that the plugs may be too hot.

Anyone want to chime in on my theory here?
See less See more
Probably NOT inconel valves. Probably Manley HD valves - if so - common problem.

Marine engines MUST have Inconel exhaust valves.
What was the valve seat width??
you need a wide valve seat especially on the blower motor. Should be around .060
I think the machine shop out in smithfield built those motors didint they? Keens or Keenes? it's next to Napa on Rt10 They might know something about them.
Newly reconditioned blower? That is what is giving you more air. Lighter color would signify the leaner condition. Intake valves should be extreme duty stainless and the exhaust should be inconel.
good luck
If it's burnt, it's a lean condition, check your plugs for color..it's safer to stay on the richer side than lean..
did you do a prop change latley? common problem is over propping, higher loads cause higher combustion chamber temps resulting in tullip valves.
No prop change. Engine spins to 5200
A friend of mine has 250 blowers with single 850 carbs. He has no problems. I don't think the carbs are too small. Jetting is always important on a boat engine and a supercharger makes it critical. I would think about removing the rear power valve and jetting up the secondaries 6-8 sizes. What kind of octane boost are you using? Most are useless. 1 can to 40 gallons probably doesn't make any difference. I mix in race gas for poker runs. 10 gallons in each 100 gallon tank Good luck, Ian
Hollowpoint said:

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.

Just my opinion, but this is running very hard at 3 minutes of WOT. You maybe can get away with that on a stock 330hp merc engine or a full race engine regularly, but what you have I don't think so. Unless it is built like a race engine which sounds like maybe not. You should keep the WOT blasts under 30 seconds for reliability sake. The valves need time to cool and this happens when the are closed. The longer and higher rpm you run the less seat time they get and the hotter they get.
Re: Re: Ideas on Burned Valve?

Blue Thunder said:
The valves need time to cool and this happens when the are closed. The longer and higher rpm you run the less seat time they get and the hotter they get.
This is where your extreme duty stainless intake and inconel exhaust valves come into play. fuel pressure is also a huge factor do you know what it is at wot.

Look at the obvious.

both motors have run well for 4 years
both carbs rebuilt
one motor rebuilt
one blower rebuilt. (most likely new teflon strips were added, more air being pushed due to more efficient running blower)

rebuilt motor broke.

Make sure you have the valves stated above These are a must with a blower application. and a fuel pressure gauge. Cheap insurance. Spend a day with the motor on the dyno. It will tell you what you need to know for fuel, air mixture, fuel pressure at wot . why would you want to chance doing this all over again?
See less See more
I believe you want severe duty manleys on the intake and extreme duty manleys (innconel) on the exhaust. Or ferria on the exhaust which is what I run.
1 - 19 of 19 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.