BajaKevin

iTrader: (1)

Ok so I bought this Scarab about 2 months ago with a built 454 (bored to 496).


Right away I noticed it wasn't turning the RPM's I knew it should. Was maxing out around 4700. I chalked it up to being wrong prop.

So aside from some short local test runs I took her out on her first long run on the 3rd of May and aside from the, what I believed, low RPM at WOT she ran great. It was roughly a hour and 15 minute ride which I was at cruising speed for 90% of the time. The only issue I had was on the way home when I opened her up she had dropped RPM from around 4500 to 3500 slowly but then picked back up on her own all without me ever touching the throttle. Ran fine the rest of the time.

I took 2 more short local trips after that, one time it did the drop in RPMs and picked back up again twice on a 10 min run.

Headed out this past Tuesday to go see the Blue Angels fly. Heading out of my creek I noticed a misfire in the engine but she got up on plane and I ran for about a 1/2 hour at cruising speed to meet a buddy at a lighthouse. Came off plane and pulled up next to him. Went to leave to go to our raft up destination and she wouldn't go above 2200-2400 rpm and wouldn't get up on plane. Got towed home.

Starting checking things out. Changed fuel filter and checked fuel for water. Filter was questionable but fuel from tank looked clean. While idling on hose I started pulling wires from cap to check for single cylinder issue. No change in idle from #5 cylinder. Pull spark plug out of #5 and find this....


Do a compression test on #5. Zero compression. Initial thought was stuck valve. Pull the valve cover and turn engine by hand. Everything moved as it should... Time to pull the head. Once the head is off I discovered the melted piston ....








So like the title says obviously there was a detonation issue that melted the piston, broke the top ring which got up on top of the piston and burned the head gasket.

My problem is I don't know where the detonation issue came from. I always ran 93 octane fuel (or at least payed for 93 lol), I am using a Fram HPG1 fuel filter and Holley electric fuel pump, -10 fuel line from tank to -8 line at carb (Holley 750 double pumper). Base timing was set 12 degrees with a total of 31 at 3500RPM. NGK BR6FS plugs (I just put in at beginning of the month, there was AC Delco plugs in there but I didn't take note of the part number). New cap and rotor at same time as plugs but reused the wires as they appeared in good shape.

Obviously this was an existing issue but I'm just curious as to what was causing the detonation. Is there a way to determine that? Or is it just a guessing game?

Thanks for any input and sorry for the long post.

Molson

Looks like cross detonation to me

MILD THUNDER

iTrader: (3)

I'd take a look at fuel supply issues. Like JETS.

MILD THUNDER

iTrader: (3)

I would lose the fram HPG1 filter, as that is a car fuel filter. I would also lose the holley electric fuel pump. Which pump model is it? Hopefully not the red pump.

Black Baja

iTrader: (2)

The plug was definately way to hot and I highly doubt the timing was 31 degrees total. When they melt across cylinders and break ring lands you can almost bet the timing was turned up.

BajaKevin

iTrader: (1)

I'm not sure which pump it is. I can find out next time I uncover the boat.

Which fuel pump and filter would you recommend replacing it with.

Timing was set with a snap on advance timing light. Now it could of been different before we set it (remember I just got the boat) and the distributor had been loosened and moved before checking initial timing.

TomZ

iTrader: (2)

Wrong piston for the application too. A hypereutectic piston has no business being in a performance marine engine. Count yourself lucky that the piston stayed together.

Some bad news.... you have a .060 over 454 (468 cid) not a 496. The part number (H661CP) indicates a low compression standard stroke replacement piston. With an open chamber head you're probably at 8:1 or less compression-wise. I hope it wasn't sold to you as being a 496 because if it was, then you were lied to. I wouldn't be surprised if it had a cast crank and 2-bolt block at this point . Since you're looking at a rebuild anyway, I'd look at some offerings in the Swap Shop. There a few engines for sale there that could get you back on the water.

I would say that the engine was just plain built incorrectly for the intended usage. Hard to say without knowing the cam's specs, etc, but this looks like it wasn't built for the marine environment.

TomZ

iTrader: (2)

I forgot to mention the part about the plug being too hot in my last post... I thought the same thing.

TomZ

iTrader: (2)

Nice looking Scarab btw! I hope you get back out on the water quickly.

sleev-les

Kev, I'm recruiting some womenz for a bikini boat wash so we can pay for our motors this year ........j/k


Keeping my fingers crossed that you can get on the water this season. Like Tom said and my text yesterday, there are some good buys in the swap shop right now.