Let's start by saying I'm NOT a wrench, so I could use some help here.

Just bought the boat used w/ 158 hours on the twin 1999 HP500 Carbs. Guy I bought it from took really good care of it. Had receipts for all maintenance (25 hour oil changes, 50 hour drive oil changes, etc.).

I had the valve springs replaced when I bought the boat just to be safe.

Was running the boat at WOT for longer than normal on Memorial Day weekend (maybe 5 - 7 mins at WOT). Blew a spark plug out of the port engine and when I shut the boat down, I noticed a "knocking" sound in the starboard engine. Towed the boat in and let it cool. Checked the oil in both engines and both were within 1/4 of a quart of full. Replaced the plugs in both engines and the port engine appears to be running fine. I should say both engines were running at normal Oil and Water temp when the plug blew.

The starboard engine is still knocking or ticking. Frequency of knock increases with RPM's. I thought it would be a lifter or something in the top end, but the local mechanic pulled the boat, put the ear muffs on and ran it. He says I have a rod knocking. Never heard of a rod knocking, but this mechanic is highly recommended by several local boaters who I know and trust. Is this likely? I thought the bottom ends on these HP 500's were pretty stout.

Also, I thought you typically blew a plug when the engine gets HOT, which didn't appear to be the case when mine blew based on water and oil temp. Only thing I can think of is they repainted the lowers on the outdrives when I bought the boat. Maybe the water intakes got restricted during the repaint? How much water PSI should I expect to see at 3500 RPM?

I would greatly appreciate any thoughts on the knocking rod or the blown plug.

Blew a plug out of the engine?? I've never actually seen that happen unless the plug itself was loose.

Were the threads on the head still in tact? Sounds like detonation, any aluminum on the plug?

Panther - thanks for the reply!!!

The ceramic portion of the plug blew out (and was still attached to the plug wire) but the metal portion (threads, nut head and electrode) were all still in tact, screwed into the block. I'd never heard of it either, but several people have since told me it is fairly common if the plugs get really hot. That may be BS. Like I said, I'm not a mechanic.

Wierd thing is that the engine that blew the plug now sounds fine (w/ all new plugs). It's the other engine that has this knocking sound. Totally wierd, especially considering normal readings on all the gauges, full oil, etc.

I'm really questioning my decision to buy used at this point.

A boat engine builder suggested I may have an exhaust flapper stuck rather than a rod knocking (he said they sound very similar).

Anyone know how/where to check for a stuck exhaust flapper on a HP500 Bravo I w/ Captain's Call exhaust?

There are exhaust flappers in the top of the "Y" pipe that goes down to your thru-prop exhaust. I have seen them make noise before, but they sound like a tick, not a knock. Although rare, I have seen plugs blow through like that before. I never thought that it was attributed to heat. I thought it was just a bad plug. In fact, just today I had a ticking noise in a customers Cavalier 2.2 liter. It ended up being loose porcelain in 1 plug that was causing a compression leak. I just put these plugs in about 1 year ago. No junk either, Ac Delco Platinums.

Thanks checkmate.

I would describe the sound more as a ticking, but the mechanic referred to it as a rod that was knocking. I'm no expert, but since the noise is definitely coming from the absolute back of the engine, I was at least hopeful.

So can I just disconnect the exhaust before the Captain's Call, put the ear muffs on, fire the motor and answer my question? I assume if the ticking/knocking goes away, it's in the exhaust?

NO YOU CAN'T DO THAT... You can't disconnect the exhaust and run it. Water will pour out all over the place. If you think the noise is coming from the flappers, disconnect the rubber hoses at the top of the "Y" pipe. Take a look inside to determine the condition of the flappers. But, do not run the engine.

If you just put springs in it it could be a loose rocker arm try to find out if noise is coming from one side or the other and pull that valve cover off. Take a couple feet of heater hose put one end to your ear and pin point the tick with other end.

The plug could have been cracked when it was instaled.

Your problem sounds like detonation. Running a motor 5-7 minutes WOT without knowing 100% that everything is perfect with the tune up is asking for problems. You may got both motors into heavy detonation and hammered rod or main bearings out of one of them. I hope I am wrong, but that is what it sounds like to me!! Many factors cause detonation from too hot of plugs, plugs being too loose or bad gas. Good luck!!

Checkmate - guess I should have figured that out, bad suggestion. Thanks for the sanity check!

Rush - Thanks for the advice. The mechanic is going to check the exhaust flappers and then go at the engine with a mechanic's stethoscope if it isn't in the exhaust.

WetteVette - Thanks for the reply. I hope you're wrong too for my wallet's sake. Pier57 tuned the motors after they replaced the springs, and they ran great for the other 12.9 hours since purchase, so hopefully it wasn't detonation. But you're right, I didn't think it sounded good either.

All - I just remembered that there was a small amount of water coming out of the "hinges" around the Captain's Call flappers in the thru transom exhaust when I ran the starboard motor after it cooled off (with the exhaust routed through the prop). Could this be a sign of an exhaust problem in the Y pipe???

If the knock is noticeable at idle you may pull one plug wire at a time and see if if lessens. Theory being that the knock will reduce when spark is removed if it is just one rod.

the small water leak is not uncommon at all and not really a sign of any kinda problems inside.

If the knock is noticeable at idle you may pull one plug wire at a time and see if if lessens. Theory being that the knock will reduce when spark is removed if it is just one rod.
This is correct,
If it is a rod knocking the sound will be lower and may disappear when the plug wire is removed. I would recommend getting a stethoscope and listening to the engine in various locations to pinpoint the sounds location.

Thanks for the help BadDog, Hillbilly and Nobody.

This is why OSO is such a great board!!!

Did you do lifters when you did the plugs?

I still want to know why the plug flew out? was it loose? Did it take any threads with it? Detonation can bend a rod and kill the bearings but as Rush stated after a spring job I would suspect a rocker first. Generally speaking if you have a bearing knock it is out of rhythm and inconsistent but gets worse with engine load. A rocker will be constant and it will be loudest on the valve cover. The exhaust flapper I don't have any experience with.

Roby

I still want to know why the plug flew out? was it loose? Did it take any threads with it? Detonation can bend a rod and kill the bearings but as Rush stated after a spring job I would suspect a rocker first. Generally speaking if you have a bearing knock it is out of rhythm and inconsistent but gets worse with engine load. A rocker will be constant and it will be loudest on the valve cover. The exhaust flapper I don't have any experience with.

Roby

Roby,

Sounds like the ceramic portion of the plug only came out.

1stfntn,

It is also possible that you are hearing noise from the out-drive. Remember that the drive shaft and U-joint are always turning.

Loose rocker arm seems likely given that you just had springs changed.

Good luck

When they replaced the valve springs, did they have to take off the exhaust manifolds? An exhaust leak makes a ticking noise. I always check the simple stuff first.

The only way to do this with the head installed is to use compressed air running through the spark plug hole via a disconnected compression gauge hose. Then the springs are removed using a specialty spring compressor. The exhaust manifolds as well as an intake can stay intact.

Roby

Your problem sounds like detonation. Running a motor 5-7 minutes WOT without knowing 100% that everything is perfect with the tune up is asking for problems. You may got both motors into heavy detonation and hammered rod or main bearings out of one of them. I hope I am wrong, but that is what it sounds like to me!! Many factors cause detonation from too hot of plugs, plugs being too loose or bad gas. Good luck!!

After reading your first post I said detonation. I have destroyed more motor over the years to detonation. The first time I blew a spark plug out was my first trip out with new gale banks twin turbo setup running 15lbs of boost. That was my first experience with detonation. Since then I have done a lot of research on detonation. As Wette has stated detonation will destroy motors. On the one motor you may be lucky that the spark plug blew out being the weak link before that motor was destroyed. If you still have the plugs that you had when you ran the boat hard you might be able to find alum. specs on the plugs. If that is the case then detonation is what ate them up. As Wette also said detonation is cause from a number of things, such as too much timing, too lean, too high compression, too low octane gas, high engine temp, or too hot/wrong spark plug. My suggestion after you check the valve train (could be bent push rod) I would pull at least one motor & check it out.

What you will need to find out is what caused the detonation. If you do not find out what is causing it you will tear up the next motor just as quick.