Anyone experienced I6 400R's lose RPM (on both motors)- but everything checks out?
Had multiple shops check them out.

Boat is a Fountain Thundercat 34 with 400's
Props are Merc CNC 34x15x15R
Boat was bought last summer, ran 6900-7000 with max speed of 117. Was Dyno'd before purchase -250 hrs.

Winterized and changed the water pumps and gear-lube (merc extreme)/engine oil.

This summer the boat came out of storage and only runs 6300-6400, with a max speed of 107 with the same setup, lake, temps and trim. Takes forever to get to speed (couple of mins to hit max RPM) sometimes the boat would struggle to get over 100. Motors now have 300 hrs. After all of the things listed below still runs the same.

Had it into multiple shops and changed/check:
Replaced all coils/plugs
Checked Props at BBlades, ran some signature props with similar speed/rpms
Cylinder balance checked, confirmed 100% throttle coming from the DTS controls
Water Separators replaced
T-Stat housing upgraded to new style
No Codes, and not in limp mode
Ran on Marine fuel and 93 pump gas

No shaft Dyno in lower Michigan that I could find to determine the motors lost top end power. They did a cylinder balance test an both checked out ok, but no compression test - is cylinder balance good to determine if the cylinders are worn? Is there anything electronically that can limit power/rpm?

Are you sure the same props are on it from when you purchased it?

Same props, confirmed. There was a small repair the bblades made and indicated they straightened some waves they found in a previous repair - but nothing that would account for 600+ rpm. The originals owner (the boat was bought through an estate sale) also confirmed RPM/Props were as sold

What can't be explained is the boat came with 32's which was confirmed by Merc Racing, so not clear how 34's would hit 6900-7000 as the 32's would have to be 200-400 rpm about the limiter. Makes one think they possibly had a tune, but how did the tune go away over the winter? This is a crazy issue, as nothing seems out of the ordinary.

if someone flashed the motors, could that have removed a possible tune? i'd prob vote for a compression & Leak down if possible - loss of a hole or two could account for the missing RPM if all else has remained constant (props)

Since my last post we tried smaller props, dropped from 15x34x15r to 15x32x15r. The boat ran the same RPM’s with both props?? I think it’s something electronic, or in the plastic plumbing for the SC? The boats been to multiple shops, no one seems to use software to diagnose any electronic issues - other then checking codes. This is a buddies boat, with props it’s already a $20k investment, It seems to me the motors communicate to each other or one is pulling the other down. Both have a 16 psi boost reading, 145 deg temps and 20+ psi water. At 23 deg of trim (max speed sweet spot) both sets (he actually tried 3 sets - 15.25x34x15r) max out at 6300-6400.

did you do any compression or leakdown testing? or boost system testing?

just cylinder balance to date, on both - 287 hrs