Lost rpm
 

I have an issue that is driving me crazy. Boat has twin 540's with Prochargers. Ran great two seasons ago but sometime last season we lost rpm in one motor. It was time for the top ends to be done so figured that was the issue. Did both motors this past winter and both were run extensively on a dyno. Made great power on the dyno. Put them back in the boat and we can still only run one motor up to 4,000 rpm - when they should be running 5300. The other motor runs to 4500 and starts making boost (2 psi) but the other motor will only run 4000 and makes no boost at that rpm so I feel that one is holding the other back. I've tried grounding the motor, checked voltage at the coil (12 volts +) and run off an external 5 gallon tank - nothing changes. I've pulled everything off the 4000 rpm motor's battery and am running it right from the + battery terminal to the starter and the - to the ground on the block. Nothing has changed on the boat, same props, same drives, etc. Boat used to run 85 and now I'm lucky to top 60. What can I possibly be missing? Any help would be greatly appreciated.

at this point,I'd almost be inclined to pull motors and switch positions and see if its really a engine problem or a hull problem.

Hull problem how? Like one side of the boat is heavier than the other?

Anybody within towing distance with a prop dyno

what parts where used on the dyno not used in The boat

Pretty much everything I've tried - fuel supply and electrical. I've checked throttle plates are fully open.

Old sc belts or low tension or other reasons for slipping or trailer still connected to boat :)

i’m assuming compression and fuel psi and timing correct

Haha didn't check for the trailer!
All belts are tight and in good condition. Planning to verify fuel pressure with a mechanical gauge - boats gauges show pressure is good - 50 psi. Compression was checked when doing top ends. The baffling part is everything ran great on the dyno.

Assuming Holley Carbs. Check the inlet fuel screens-filters in the carb, Also the water separating fuel filters.

Mark

no these are efi motors. Did not replace the water seperators but again, nothing changed from the dyno to install.

valet mode.

Well, things like fuel tank , gas in it, and supply line and fittings wheren't at dyno.

a cracked spark plug or two from engine installation not uncommon