Background: new-to-us twin-engine boat with 350 MPI Horizons mated to B3 drives (2004). The boat has just shy of 500 hours and has impeccable maintenance records. Port side clunks into forward and reverse (no hesitation) and has a slight vibration/roughness coming up on plane (btw 2 and 3k rpm). Stb side has similar clunk going into reverse, but you can't even tell it's shifting into forward it's so smooth. Stb side is also silky smooth throughout the RPM range.
Here's what I've tried so far:
1) First swapped prop sets left-to-right. Vibration stayed on port side.
2) Pulled outdrives, checked alignment, checked shift cable adjustment (upper and lower), replaced port gimbal bearing and reinstalled outdrives on the opposite sides. Also changed drive lube (old lube looked great with at least the 70 hours we've put on the boat).
Problem 1 - clunking into gear moved from port to stb with the swapping of the legs. So, I know this is inside the drive somewhere and not in the shift cables which were dead-on. I haven't had any mechanics on the boat but have spoken to a few in the yard, and they say some bravos do this, some don't. Am I being too picky here? It just annoys me to have two identical drives behave differently.
Problem 2 - vibration seems better, but the port side still feels a little rougher than stb between 2 and 3k rpm. Stb side is still silky smooth. This leads me either to the coupler or engine itself on the port side and rules out anything from the gimbal back. The alignment was off on the port side as it took a good tug to get the tool in/out. I corrected the alignment and didn't notice any significant difference when checking the alignment at each 90 deg of engine rotation. I didn't change the alignment on the stb side because it was smooth and I didn't want to disturb anything on that side in case the problem traveled with the leg, which could then leave me wondering is it the leg or my monkeying with the aligment on that side that caused the vibration to change sides. All-in-all I feel like I achieved a better alignment on the port side than the stb.
I can't rule out the coupler, although it didn't seem off-center to me and was well greased. Other thoughts are something in the engine itself like a clogged or leaking fuel injector, bad spark plug wire, loose harmonic balancer, etc. that's causing a roughness under load. Plugs were changed at winter layup 2 seasons ago and compression tests were done at that time as well with good results. Are there any other tests I can do on the coupler other than checking alignment at different engine rotations? Any other checks I can do on the engine itself? I'm running out of ideas on this one!
Sorry about the long post, but as you can see, I've already done a lot and am still stumped...