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HP500 Carbs - blown plug and problem in bottom end at 171 hours?

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Old 06-08-2005, 12:36 PM
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Default HP500 Carbs - blown plug and problem in bottom end at 171 hours?

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.
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Old 06-08-2005, 02:40 PM
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Default Re: HP500 Carbs - blown plug and problem in bottom end at 171 hours?

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?
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Old 06-08-2005, 02:59 PM
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Default Re: HP500 Carbs - blown plug and problem in bottom end at 171 hours?

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.
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Old 06-08-2005, 05:21 PM
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Default Re: HP500 Carbs - blown plug and problem in bottom end at 171 hours?

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?
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Old 06-08-2005, 05:55 PM
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Default Re: HP500 Carbs - blown plug and problem in bottom end at 171 hours?

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.
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Old 06-08-2005, 06:26 PM
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Default Re: HP500 Carbs - blown plug and problem in bottom end at 171 hours?

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?
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Old 06-08-2005, 07:29 PM
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Default Re: HP500 Carbs - blown plug and problem in bottom end at 171 hours?

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.
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Old 06-08-2005, 08:17 PM
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Default Re: HP500 Carbs - blown plug and problem in bottom end at 171 hours?

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.
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Old 06-08-2005, 08:21 PM
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Default Re: HP500 Carbs - blown plug and problem in bottom end at 171 hours?

The plug could have been cracked when it was instaled.
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Old 06-08-2005, 09:24 PM
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Default Re: HP500 Carbs - blown plug and problem in bottom end at 171 hours?

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!!

Last edited by WETTE VETTE; 06-08-2005 at 09:28 PM.
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