Volvo Penta 4.3GL Power cycle

[bc2planker]

I have a 2001 Volvo Penta 4.3GL engine that I am fighting power issues on. It starts and idles fine, but when under load and I power up to WOT, it goes to about 3800rpm, then after 30 seconds or so, rpm and power start to drop to about 3200rpm. Then after another 30 seconds or so, it powers back up to 3800, then the cycle repeats (all without touching the throttle).
Its in a 23' SeaFox, so its a bigger boat for this size of engine, but I can't explain this power cycling. Sometimes it sounds like its missing a cylinder at high RPM.

I have checked and/or replaced the following:
- Fuel pump, lines, filter
- Fuel tank lifter tube and vent
- Spark plugs, wires, cap, rotor
- Ignition coil and module
- Engine timing
- Vacuum (its about 18 at idle)
- Carb rebuild (all new gaskets, etc)
- Engine compression (145-155 across all six)

Any other suggestions on where to go next?

 

[CaptnKingfisher]

I had an old V4 Johnson outboard that did this a long time ago. In my case I figured out I was having an intermittent spark issue. Sounds like your whole ignition system is new but I'd go through the manual and step by step verify each component is working properly. Start by checking spark plug gaps, resistance on spark plug leads and plug to distributer wire, resistance test on primary and secondary coil circuits, if this engine has points verify dwell set correctly and points gap, there's probably a way to bench test the condenser but idk it. When you have the high tension leads off you might as well put some dielectric grease on both ends. Maybe someone has a more specific solution for you but with my limited knowledge this would be my approach