Johnson J150VXSIG Speed/rpm mode

[Grinna]

I have almost exhausted all logical causes of the issue of a maximum 3300rpm 25knots when it drops 3xcylinders. Alarms dont sound. I swapped on another powerpack last week and it ran fine to 5200rpm 47knots with no issues. Yesturday we're back to the previous issue.
I have
replaced cartridge filter
cleared/cleaned connical engine filter
checked correct rubber fuel lines to carbs
checked fuel lines are 6mm id.
replaced primer bulb
drained ,removed and cleaned 300litre fuel tank
Checked fuel tank vent is clear
replaced powerpack
removed and tested thermostats
checked main fuse holder and fuse
load tested coils

any ideas and opinions appreciated.
engine hours are origional 290hrs...............cheers

 

[racerone]

V-6 motors require fuel line of at least 9 mm.------Run with a timing light hooked up to see if there are ignition issues.---Check shift interrupt switch if equiped with that item.