Please Help!!! 1991 Mercruiser 7.4 Bluewater not running on all cylinders

I need someone with Mercruiser 7.4 experience. Port engine not running on all cylinders. Here is the history. Had a frozen valve on cylinder 4 had the head machined and new valve guides installed. Reinstalled the head 2,4,6,8 side. Not my first rodeo with this, my boat has had reversion problems in the past. Adjusted valve as per merc manual, Mark IV big block. New manifolds, wires, rotor, cap, plugs oil change, the usual tune up rebuild. Compression test 120 on all 8 cylinders. She started right up after rebuild. Here is were my problem is and im not sure where to go from here. With a timing light she wont even run unless im at 16 Degrees BeforeTDC. In order to make power, When i turn the distributor counter clockwise she runs better moving the time mark well past the markers up toward the water pump but still doesnt run on all cylinders. I check TDC and it lines up on the balancer and piston 1 is all the way up so i assume it is correct. You can pull the wires on cylinder 2 and 5 there is no difference while running. (again i have 120 compression across the board) I have checked my wires 50 times and im getting spark.
I dont know what else to do at this point. I cant get the timing correct and she isn't running on all the cylinders.

HELP HELP HELP!!!!

Try swapping wires 2 and 5 in the cap.

I know...but it's worth a try, yes?

Jeff

Hi

Just wanted to mention that sometimes the cylinder bank which has issues from water intrusion can often contaminate the opposite cylinder bank if the exhaust passage in the intake manifold is open.

You may want to pop off the opposite valve cover and look for bent pushrods etc.

Good Luck

Misfit

Also 120 psi is low for that engine or any GM marine engine, normal comp test results are between 150-170 lbs. You might recheck the hydraulic lifter adjustment to make sure they are not too tight. All cyls really the same? Is it pushing a lot of blow by out of the valve cover breathers?
I did a top end overhaul on my ‘88 4.3 in ‘17 after it blew both head gaskets from an overheat a few years earlier. When I checked the compression after installing reman heads, new exhaust etc I got 160-170 on all 6 cyls.

All fixed runs like a top. turns out didn't have cylinders 2,3,5,8 running which is the left bank of the quadrajet. Seafoam and and some carb cleaner and with in a few minutes she started running perfect perfect perfect!! first time experience for me but another lesson learned

120 dry and cold is fine for a bbc.

Ah you didn't say cold comp test, they are never accurate.

Actually, Mercruiser endorsed GM's revised procedure for "cold" compression tests almost twenty years ago...

..."Ah you didn't say cold comp test, they are never accurate. "

But what else can one do if the motor won't run

Jeff