5.7L Black Scorpion running rough after warmed up

[Caddmann2266]

I have a 2007 Sanger V215 wake series boat.
It has the 5.7L Black Scorpion 350 MAG MPI motor.
After being out on the Delta all day yesterday running it pretty hard it started acting up at the end of the day. Ended up getting towed back to our launch site.
Symtoms: Cruising about 35MPH the motor started revving up and down quite considerably. I cut the throttle back slowly and it continued to do this till we were stopped.
At idle it kept the same for about a minute then started idling normal. I was then able to get going up to 30MPH for about 5 minutes running great and it started the same symtoms all over again. When it was running rough I would try and throttle it up a little bit and I would get back firing sounds.
Summed up: it runs great for a few minutes then acts up, wait about five minutes runs great again then acts up.
There is more history behind this but I wanted to keep this some what short for now.
Sooooo.....any and all questions or help would be greatly appreciated.

 

[alldodge]

Thinking its starving for gas. Being able to let it idle for a short time and run good again for a short, I would start with the gas tank vent. Could be the tank is drawing a vacuum because the vent is clogged, or partly clogged. When it does this remove the gas cap and listen for air being sucked in.

Could also be the antisiphon valve connected to the tank. If it gets clogged the pump has a hard time drawing fuel

 

[Caddmann2266]

Thinking its starving for gas. Being able to let it idle for a short time and run good again for a short, I would start with the gas tank vent. Could be the tank is drawing a vacuum because the vent is clogged, or partly clogged. When it does this remove the gas cap and listen for air being sucked in.

Could also be the antisiphon valve connected to the tank. If it gets clogged the pump has a hard time drawing

 

[Caddmann2266]

Thinking its starving for gas. Being able to let it idle for a short time and run good again for a short, I would start with the gas tank vent. Could be the tank is drawing a vacuum because the vent is clogged, or partly clogged. When it does this remove the gas cap and listen for air being sucked in.

Could also be the antisiphon valve connected to the tank. If it gets clogged the pump has a hard time drawing fuel

Thanks, I'll check those out

 

[alldodge]

Could also put a fuel pressure gauge on the fuel rail and see if its holding at 43 psi

 

[ThetaChi12]

Caddmann, my 89 Merc 5.7 started acting up over the weekend with the same symptoms. I had an issue like that a couple years back on which I found bad spark plugs to be the culprit. Now I've got inconsistent bogging at high power settings and occasional chugging / cutting out at idle. I was just about to start tearing into the carburetor thinking it's a fuel issue, but reading alldodge's post gives me some pause.

Interestingly enough, I also have a hard time filling up my gas tank. I'm thinking I might have a bad anti-siphon valve. I changed the vent line recently.

 

[QBhoy]

Crap cap distrutor issue possible.