5.0L VP I/O with 244 hours, but 20 years old. engine starts like new, idles forever if you're in shallow water, but open the throttle and when it gets to about 2,500RPM, a slight Burp, then immeciate bog back to idle. The engine never shuts down, just goes to idle. No other symptoms that I can see. Happens in the water or on the hard, so the outdrive is not an issue, IMO.

Any ideas about how to diagnose this?