Maybe someone has had this problem: My Merc 525SC (Gen IV block, roller, Weiand 177, 1050 Dominator, Thunderbolt IV) will run great (WOT at 5200) for 15 or 20 minutes. Then it cuts out. It's not sudden, it's like I pulled the throttle back. Then she won't start - not even a pop - for 20 minutes. Then she starts, and I putt home slowly. So far I have:
Installed new ground to terminal block in dash, installed new ignition switch, installed new purple ignition wire from switch to coil, replaced coil, replaced tach (the tach used to jump around, now does not), I have fuel as the accel pumps squirt when pressed. Cap and rotor are new last year. I checked every connection I could find. The battery is new. I have good charge off the alternator, I have good oil pressure.
Anybody have any ideas?

Sounds like its running out of fuel. It could have a crack in the tank pick up tube or???
Do you have fuel pressure gauges????
BTW 15-20 mins at WOT is not a good way to make an engine live a long and happy life.

I am going to install a fuel gauge tonight. I don't run WOT for any more than a few minutes. What I meant was the boat runs strong for 20 mins. If I run at 1200 RPM, she does not die. But that's not much fun!

Sounds like another post that was on here recently. They seemed to think ignition module. Does not sound fuel related.

I think the final diagnosis in that thread was the hall effect sensor in the distributor was failing.

Thanks for the response!! I was thinking it is ignition related. I will replace the sensor.

I replaced the ignition sessor. It was badly corroded, however, I am still having the same problem. Ran fine for 15 to 20 mins...slowly cut out, then after 30 minutes started and ran at low speed fine for a while. I will install fuel pressure gauge next, but the sensor did not fix the problem. Could it be the ignition module? Rev limiter?....I am going nuts!

seems elect to me...if were fuel would surge before shuts off I would think...the fuel pressure will show that...all the 525SC's I've seen had a factory fuel pressure gauge...I would try the module...heating up then shutting off....seen that before..also not sure what coil you used but some don't play well with others...MSD sells some drag race only coils that will overheat after longer running...

I just had this problem on one of mine. The best way to diagnose it is to use the Mercury flow chart. That is how I found that the pickup in the distributor was bad in mine.

Remember, you have to work your way through the flow chart when the boat is not running. If you wait until the boat is running again you are trying to diagnose a problem that is no longer a problem....if that makes sense.

Thank you. I will take this out and follow the chart.