5.7 mercriuser surging problems

i have a 5.7l v8 mercruiser carbed with thunderbolt iv ign.whilst skiing the boat suddenly started surging between 2500-4500rpm a made it back to dock then engine would not restart. changed distributor sensor and engine started and reved in neutral really well.once towing the same surging problem returned i have tried fuel lines , pump, pick up ,seperator,choked carb to pull through any blockages but still the same.engine runs well upto last quarter of throttle then surges i am beginning to think ign module but have been told they rarely go could these symptoms suggest this or something else?

Ayuh,.... Sure sounds like Bad gas, or Bad fuel delivery, probably the Carb...

As Bondo says, definitely a fuel issue.
You can check the module with an advance timing light.
Look on your module and it will have the degrees of advance marked on it. 22 or 23 degrees. Start your engine and warm it up. Check your base timing. Now add the base timing number to the number you found on the module to get your total advance. Set your timing light to the total degrees of advance. Run the engine up to about 3500 RPM's in neutral and turn the distributor to set the timing marks on the balancer and tab to zero.

You can set your engine to TDC and mark the balancer, measuring off the tab, where 30-32 degrees would be. Restart the engine and re check by setting your light to zero and running the engine up to 3500 rpms.

Do this as to not waste your money by throwing parts at your engine for a fuel issue.

anti-syphon valve...check the fuel pump's diaphragm using the sight tube...fuel being contaminated...