525 SC Head gasket Or??
 

Back round... Motor is 97 model 525 SC. Started popping thru carb under acceleration (trying to get on plain).
Checked valvetrain first. Everything moving fine.
Swapped carb - same problem
Swapped distributor, coil, cap & plug wires- same problem
New plugs - same problem
new fuel pump - same problem

Then (finally) stepped back and ran compression check. #1 cyl 138 psi, #2 cyl 135 psi...... #5 & #7 cyl 0 psi. Double checked valve train and valves "appear" to be getting normal full lift. At this point I am thinking it's a head gasket blown between #5 & #7 cylinders. Can anyone think of either a way to verify this, or some thing else that would cause this, or other things i should look for while replacing the head gasket.

How many hours?

You have an obvious no compression problem but for what its worth mine started running bad like that before I tore it down and it ended up being a bad sea water pump...the cam inside that drives the fuel pump got flat and I was losing fuel pressure...

I bring this up because hopefully you didn't have the same issue and run it lean and the no compression is actually 2 bad pistons or bad valves.

With the valves closed pressurize #5 and see if air come out #7 spark plug hole

a leakdown test will solve your mystery

Winner, winner, chicken dinner....... Blowing out as fast as it goes in. Thanks a ton for the tip!

About 250 hours. I will look at pistons also when i pull the head off.

Might as well do a cam swap, up the compression, add a chiller, bigger blower, swap the 454 for a 540, upgrade ignition, aluminum heads.....see how that snowballs hahahahaha

In all seriousness tho...check the deck surface and head mounting surface for degradation from the head gasket being bad. Doesn't take much to start eating away at those surfaces. Hopefully it'll be a quick head gasket change and back on the water in no time!! Good luck!!

Oh, believe me, I know that snowball..... And would if it wasn't a twin snowball. That and I've only found one chick that will ride with me at 90+...... Rest are booze cruise speed. Might as well put a 25 honda on the back as far as they concerned. LOL
Yeah, the heads were just done right before I bought it, so hoping maybe just an oops oversight kinda thing. Definitely gonna check it all out when I get the head off

If its that bad, I think you are going to have head or block issues. Hopefully not, but be prepared.

Ugh.... Fingers crossed. Will find out this evening.
On another note, during my marathon of chasing down the problem, I checked the timing and it was way off.... Only 8 degrees advance at idle and around 20 @ 4000 rpm. Question now is would the blown head gasket cause the timing to be off or did the advance module go out and cause the blown head gasket? Hate to get it put back together only to have the same thing happen again.

that's because the timing curve on the 525sc's takes forever to get to total. So, human error like most things.

I've seen two 525SC's with blown headgaskets people have adjusted timing to total specs around 3500rpm. Tisk tisk ! I was just done at a boat only repair place.

Do some digging,I and others have posted what the timing curves are on these engines.

Here:
total is 35* at 4500rpm
Initalis 13* at idle.
Tbolt IV module is 22A

https://cimg9.ibsrv.net/gimg/www.off...4c9e59eb08.jpg

Torched the head between 3&5&7. Amazingly the block looks fine.
Gonna change the timing module just to be safe before I put it back together.

Not exactly good news but it could have been worse.

https://cimg9.ibsrv.net/gimg/www.off...30a13a59ce.jpg
https://cimg7.ibsrv.net/gimg/www.off...74134cab85.jpg
https://cimg4.ibsrv.net/gimg/www.off...12bc151aa7.jpgAt least you got your problem tracked down... I need to find out how I got water into the number 5,7 and 6 cylinders on my 525sc motor... All the gaskets appeared ok. Next step will be checking head and block for cracks...

And your exhaust, why are those valves stuck open ? or were the springs pulled

exhaust leaking appears to me...have engine down for same issue, looks way better than that but killed a piston

Do you still have the Gil exhaust?? If so, that would be my guess. I had to replace my Gils on my 525SC at 250 hrs. Hairline cracks in the dividers just below the risers.