Bouncing vacuum tells me work valve guides

Eeeww, that's not good. Definite intake valve issue. What's the intake vac look like on the other eng?

IMO you have a leaking head gasket.

Looked back for history in case there was some and found it. Here is a refresher for everyone on awol's engines/boat, and performance last year when it was new to him:

Estimated HP output - Build Sheet attached

Please read it so you/we can get a better understanding of it all.

Note for those after reading that and involved with that last year:

It is not just a basic rebuild
It does not have a 'like' HP500 cam and etc
It has a 230/236 .598/.610 114LSA Crane cam.
The HP500 had the 721 which was 222/230 at .050" 110 LSA cam.
The Hp500 had GM cast iron heads upgraded from 2.19/1.88 valves to 2.25/1.88 valves. the OP has Dart heads. Which ? I don't know. The part# on his build sheet is for Dart Iron Eagle small block heads. So that's wrong of course.
The HP500 has a Dart 4150intake. The OP's has a newer designed Weiand 4150 singleplane intake.
The HP500 cae with Gill exhaust manifolds/risers. The OP's seems to have Stainless Marine exhaust. I don't know if dry or not.

The OP got 89+mph out of his 33 PowerPlay with his engine's. Def faster than stock HP500's !

BTW, beautiful boat Awol !!!

Vacuum is rock steady on the other motor.

SB has the link to my progress on the boat since I bought it. It's come a long way. 89 was it's best last year, it's usually an 87 mph boat. I was hoping for 90 this year.

As far as the current dilemma, I've tapped out and contacted a shop and will be bringing the boat in this week. He's a week out, so I'm thinking I wont get any info until 2 weeks or so. I'd love for it to be an intake manifold gasket leak or something simple, which would cause a misfire and vacuum fluctuation. The compression being solid across the board has me doubting a head gasket, but you never know.

If its something really serious, I may bail on this season and work up some blower motors and get this thing running 100+.

If you have any more ideas, Im all game. At the very least, I'll update this thread if a diagnosis/repair is reached.

The intake vacuum jumping around (seems a bit low too) plus passing the compression test makes me think that one or more of the intake valves are intermittently sticking open. At cranking speed , the valve/s may be un-sticking fast enough but when running it doesn't close in time.

Another possibility would be lifter problems or improper valve lash adj. I would try adjusting the valves paying particular attention to how much the adjusters need to be backed off before they start to tap (assuming hyd lifters) . On a SBC, tapping should start at 1/2 to 3/4 of a turn. Not sure if big blocks are different. This may be more than you want to get into but thought I would throw it out there.

it could be valves adjusted to tight,that can cause water reversion because the exhaust valve doe,s not close all the way.even if only one exhaust valve is set to tight a small amount of reversion can happen.

water reversion is easy to figure out - if you have risers and manifolds pull ur risers off and look for water and or rust in the exhaust portion down in the exhaust manifolds - especially at the lowest exhaust portion of the manifolds. With reversion the spark plugs should show some water on them or rust itself. Also with plugs out crank over the engine and see if water comes out - before doing this disable the ignition for NO spark.

If you have headers / NO risers pull them off and look for water in the exhaust portion and in the exhaust ports of the heads. Rust will be in the exhaust ports of the heads (cast iron heads ) and will be able to see whats on the exhaust valve stems as well. rust / pits or carbon or whatever

I cranked the motor over without the plugs (and ignition coil pulled) and no water came out.

Agreed, and If adjusted too tight they will only get tighter over time.