No blower(yet), got my hopes that it will be a routine rebuild. The more I research and read about the MPI, it will likely end up on ebay. For sure if it ends up being the cause of this.

Anyone know what make of headers these are? I really like them and would go with them again. They lasted 25 years, so they cant be too bad.

Yep, for sure. CMI Etops with rope seals.
I have the same headers on my boat, except dry through transom. Pressure testing them is fairly easy. Cap the outlet that goes from the header to tailpipe and then put about 40psi of air into the other water inlet hose. Drop the header into a plastic tub of water and watch for bubbles to find leak. Depending on where the leak is, it could possibly be welded.

https://cimg1.ibsrv.net/gimg/www.off...286ee8adbf.jpg
Tested headers, no leaks found yay! Got engine out- not fun. Back seat was scum adhered in place, must have been 15 ground wires( one on every hidden bolt lol) Put it on my brand new 1000lbs stand, the stand folded in half- top notch china quality there. Looks like something is missing?

https://cimg7.ibsrv.net/gimg/www.off...4af9634474.jpg
https://cimg8.ibsrv.net/gimg/www.off...437c8ac3ac.jpg
https://cimg2.ibsrv.net/gimg/www.off...3f7e973ee2.jpg
https://cimg4.ibsrv.net/gimg/www.off...309c94f849.jpg
https://cimg6.ibsrv.net/gimg/www.off...b01eeb3364.jpg
Roller rockers(crane), Ported heads, and Domed pistons. Not what I was expecting to see

https://cimg3.ibsrv.net/gimg/www.off...60ec3e75f6.jpg

Light scoring
https://cimg4.ibsrv.net/gimg/www.off...32bec7c5e4.jpg

Head gasket
https://cimg5.ibsrv.net/gimg/www.off...16f867f03c.jpg

Head gasket 6 and 8
https://cimg6.ibsrv.net/gimg/www.off...822acf9555.jpg

Head 6 and 8
https://cimg7.ibsrv.net/gimg/www.off...0a487c8be2.jpg

5 and 7
https://cimg9.ibsrv.net/gimg/www.off...1166ffd8f0.jpg

5 and 7

So who said blown head gasket? WINNER

GasketS actually

https://cimg2.ibsrv.net/gimg/www.off...54de1a43e0.jpg

See the ridge? Is this some kind of marine finish? Very pronounced around all cylinders.
https://cimg5.ibsrv.net/gimg/www.off...7b3a0bdb78.jpg

Blown? not bad enough for 0 psi I dont think, and why was #5 reading over 100?
I cant find any other reason other than a blown head gasket, Anyone have thoughts?

JE pistons!

Ok, research done. The block was "o ringed" for use with copper head gaskets. The person that assembled the engine this last go round used standard marine felpro gaskets.

Static CR calculated with the .042 copper head gasket I will be using. 11.5-1, Opinions on if I will be able to run 91 octane pump gas? Its getting aluminum heads(320cc 2.250) The cam make and numbers were intentionally ground off. I degreed it and found the specs to be 232/[email protected] and .612/.634". The cam had a 4 degree bushing in it that I dont know if it was originally installed adv. or retarded. I would think with that kind of compression maybe it was retarded? Since I dont have a timing card I am kinda at a loss. I am definitely going Carb, just have not decided on an intake yet. My thinking is this cam may be a nightmare to get the MPI to work with. Wondering if that Daytona sensors can make use of the knock sensor, that would be cool. Someone spent a bit of coin on this engine, kind of a bonus, I had no idea when I bought it. I just thought it was a stock 502 that needed a tune up.

11.5 to 1 is way to high for 91 octane in a marine application. I have a similar cam with a bit more duration and I’m running 10.1:1 compression. If you can get a knock sensor with contact output it will work with the Daytona Sensors ignition. I have the retard input tied to a switch so when I have to buy gas at a marina, I flip the switch and pull 4 degrees out. I have come up with some unique programming for the Daytona Sensors to smooth out the idle. It works great.

Buying parts. 850 dbl Pumper "marinized". SCE copper head gaskets. Marine "lower set"(Felpro). ARP complete headstud kit. Teague block off plate. Clevite 77 P bearings. Still wondering on intake manifold "500" single plane or Performer RPM air gap? Have questions about "dry" tailpipes and if they are even a possibility with the rope style t clamps. Still need Daytona Sensors box/coil, JE piston rings, Flame arrestor, Header bolts and intake bolts, Carb gaskets(for tuning), carb linkage plate/fitting and/or new cable, new oil/ steering cooler or a better set up ? Hoses, intake and header gaskets, water pump and new impeller. And anything else that I read about and think I gotta have to make this thing reliable and enhance performance. Thinking of getting new spiro locks and cutting the pistons to about a 17cc dome to get the compression down around 10-1. Also thinkin of an electric fuel pump and a return setup. Cooler fuel and consistent pressure.

850 is too small. And you want a single plane intake. Dual plane will be choking it out at higher rpm.

I think you’ll be way past safe with iron heads and 10:1 compression.
The last couple years I’ve been setting engines up more for the fuel available, not necessarily squeezing every bit of power.
In my area only 1 marina has 93, so unless it’s solely a short trip trailered boat I lower the compression and put a lazy timing curve in so it runs on 87/89.

It is getting aluminum heads. Most of the marinas have 91, and I would have burn 20 gallons of diesel to get to any 93 or better as selection is scarce out here. I have a needy motorcycle, so I keep a 5gl can of VP around. Definitely do not want to have to feed the boat that stuff. I always wondered why the factory High Performance marine engines had such a low CR, I am understanding this now.

I ran 10.5 to 1 on my 509's on midgrade with no knock. Dart alum heads, 741 cam, lightning headers. The engine had high quench though. the pistons were 4 corner zero decked.
I would think that you could run 91 on 11 to 1

I went with Performer marine heads. I dont know what inconnel exhaust valves are, but it seemed like a good idea. I got some 11/32 keepers just hoping the retainers on my cast iron heads are 7 degree. 1400 bucks for old rope seal style tail pipes, so I guess the captains call will be staying for this engine overhaul. Holy smokes that seems high. I know a great welder, I may look into modding my existing pipes. It is a complete working set up, but I guess the rope styles are not worth much. I am wondering how all you tinkerers are running your boats out of the water? Valves? troughs?

Valve springs- inner spring on about half of them are broken. Ordered new springs, matching 10 degree retainers and keepers. Then realized I need cups for the aluminum heads. What an ordeal, the last time I did a high performance build, you could buy a spring kit to match the cam. Not anymore, tried to get a kit for a "741" cam, retainer fitment issues with crane springs. Some springs have different ID on both the inner and outer took about 3 hours with several vendors and shops. I guess I could have got the heads fully assembled and saved money and TIME. Did not seem like the springs included with the heads were up to the task. All other parts are here. The build begins now! Gapping the rings is first. Since the sea water pump(impeller) is easily accessible, I am using a 2 ft piece of hyd, suction hose from work with a garden hose fitting. I will just pull the hose from the drive, and install my hose and hook to a garden hose, so I can run it out of the water.