Full force, I am following this very closely. Very interested to see what you find. Do you think maybe tight lash, or a lifter pumping up at high rpms/weak valve spring?

I've read the whole thread just thinking of other ideas.

July 2015.... Has issue.. https://m.youtube.com/watch?v=j_pb2olpIss


October 2015 has issue.... https://m.youtube.com/watch?v=ie_MVroPW9w


Dyno same in February-2016 (see paperwork posted )

Aug 2016 has issue...https://m.youtube.com/watch?v=iurnf_hS8tw


Also seems now even with less jetting the engine will not lean up at all... Really looking at wiring/ignition now

I am pretty confident it's not engine mechanical issue, I may be wrong but I just don't see it.

Possible it's reversion? Maybe check the risers for carbon wash?

No reversion dry tails.

I am having a similar issue. Port motor. I've gone through almost all the same stuff as you. Both run perfect, but port motor down 2 to 300. They were pretty much even early summer.

Anyway it could be a fuel flow issue... After the reg... Or where ever your reading pressure? And maybe the load difference from the dyno and the boat makes the problem occur? Sorry if this all sounds dumb.. Just sounds like you covered most everything... So just trying to think of the oddball things. Do both motors feed from same tank? Maybe anti siphon?

Check the filters/screens in the car inlet fittings.

Mark

If it was that, wouldn't it show lean on the Afr gauge at wot?

Again, different motor stuff but a thought.... How warm did you get the motor on the dyno?? full heat soake operating temp like sustained running?

I have seen things manifest themselves, airleaks, casting pourosities etc only after the motor is completely and totally at operating temperature.

Something like that might not show up after a quick warmup and dyno blast, but after 10~15 minutes of actually running with a load would show up.