engine knock

Last saturday I went out for a ride, did some barefooting, and then hanged in the river looking the city from the water. Then headed back to the marina...
engine performed flawlesly as always, but from the last stop of it I started hearing a strange noise, like a blown exhaust gasket, so I lifted the doghouse while underways, and there it was this knock coming from the engine...
WTF, boat idles fine no sound, I pumped the engine w/o gear up to 2500 rpm and no knock, only knock in gear starting to plane, around 1500-1800rpm and stays there.

Engine starts fine without hesitation.

No loss of power, no miss, no overheat...can´t say about oil pressure as the gauge broke a couple weeks ago, I grounded the sensor cable to the block and it didn´t peg. But after this I might think I had no pressure and maybe I toasted a rod bearing...oil pump was new when I rebuilt the engine.

Other thing in play here, I started using fuel from a new gas station, this was my third outing using this fuel (put around 8 gallons in those fill ups)...so also made me wonder if this could be engine pinging due to bad quality fuel? advance is set to 10º.

engine has only around 300 hours from rebuilt....so as you can imagine i´m really upset with this issue.

I will go with my mechanic and friend who did the rebuilt to investigate some more, and let you know how this thing turns on...

You need to KNOW if you have oil pressure before you run it again new fuel or not. I would also check compression before running it just because it is quick and easy and gives you a look at the plugs. IF you have good compression readings, good oil pressure, and your oil isnt full of metal filings then run it.

I took a quick look at the oil that day, and seemed to be fine, I rubbed it between the fingers and no metals there..but you never know...
I´ll keeo you guys posted...

How old is your distributor cap?

Take it off, look at it very closely. You might see a very faint mark where it is arcing between 2 cylinders. If you have a new cap....you can throw it on.

Happened to me...thought my motor was toast!

Check your flex plate...

I'm hoping for the best outcome, after having 3 bad engines in a row I know the feeling all too well. Hopefully it turns out to be something minor.

take a long piece of pipe or a broom handle or long screw driver or a mechanics stethescope to listen and locate the source.

Good luck.

Well we will know when we know but heat/detonation/bad gas type issues is my best guess. You could have stuck a valve, dropped a seat, or cracked/blew up or melted a piston. A little too much timing and/or less than ideal gas can cause/facilitate any of those things. They can happen pretty quick or overtime at barefoot speeds and loads.

Knock under load, = rod bearing.
With low comp on #2 and bad plug, rip the haed off - fix what ya find.
Hard to guess, but when you pull the head you'll know.

        Good luck, hope it don't cost too much!

looks like my back deeps will be put on hold until this is solved...

wow, auto bad words deleting!

kapla wrote: Engine ran sooo good, guess *************** happens...

Hay Kap,

   The 30 seconds before it lets go is always the best it EVER ran.
Keep your head up, ya know if we could wrench on it with you we would be there. Thats what makes this site that Keith created sooo cool.

(Thanks for bringing us together Keith - get well soon!)

   The 4 wide nats at Z Max dragway was last weekend.
Google "Mat Hegan explosion" hard to believe you could
live through that!!!

That sounds like a good plan. That may tell more than just tearing into it. Just the fact that it is still rotating is a good thing.

We had a Mastercraft way back when and it was bought brand new, it had a 200 hour warranty, at 211 hours we blew a hole about 1 1/2 inches in the center of a piston, had to replace the piston and we got it back up and running again. The theory was a lean condition. Later the whole top deck worked the pop rivets loose and detached from the bottom/hull. That was real special. That was when we went to a BFN. When the BFN had somewhere near 3000 hours we rebuilt it, on teardown the piston skirts were detached/broken off and being held in place by the rings. Willies Malibu detonated at 5200 rpms at 800 hrs and had major damages, hole in block from the rod into the water jacket...when I pulled the head there was no piston left the rod looked like a hammer had beat it for hours. Roy was driving and he said the sound was something he would not forget.

I've got my fingers crossed that yours will be a fairly minor repair, you may have caught it before the real bad stuff has happened.

Man! Kap you sure have had your share of problems this year. Cheer up..you will get it figured out. In the meantime I quess it is sun-bathing on the bow of your freinds cruiser. Seem to remember some good pics from past outings on the cruiser!

Kap,
It could have been bad gas!!
Last year I had the tips blown off 3 plugs from bad gas. Luckly my motor has a knock sensor and wouldn't let it go above I think it was 2,800 rpm.
Long story short I would hate to think what would have happened if I was able to run it at full rpm.

Oh by the way I was running Startron in the fuel at the time. I WON'T EVER BUY THAT CRAP AGAIN back to Stabil marine and no more problems.