Looking for ideas on 2 problems with 350 MPI bravo 3 (complicated)

[undercover]

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).

Findings/results:

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...

[undercover]

Update on problem #2 (vibration). I'm thinking now the problem is in the motor. When I rev it up in neutral, I get no vibration at all. If the coupler was off center, wouldn't it vibrate regardless of under load or not?

Also, I spent some time running just on the port motor and found that what always felt to me like a steady vibration could be made to change by doing things like stabbing the throttle versus slowly raising rpm. I guess it's time to go over the usual (plugs, wires, vac leak check, etc.) and get the rinda software...

[haulnazz15]

Revving it up in neutral doesn't usually prove much. With no load whatsoever on the engine, it will mask a lot of problems that occur once a load is put on it. Have you checked the u-joints for play/free movement? Check the timing/advance on the engine as well to ensure base is correct and it is advancing enough.

[undercover]

It's not in the u-joints because I swapped legs port to stb and the problem stayed port. I haven't checked base timing yet, but that's a good suggestion to ad to my list.

Could also be pulling timing due to a lean miss/knock, but I need to get my hands on some software to see the sensors are seeing.

[Fun Times]

I guess it's time to go over the usual (plugs, wires.)

Bet you find you're running vibration with one of the two. Those type of spark plugs can be pretty sensitive to install especially if you checked or changed the plug gap. But this sounds more like plug wires to me.

The engine coupler is usually not the case.

Good luck.

[undercover]

Just a followup post to close this out after finally solving the problem...

2 weeks ago, I got the 2 beep/min warning and that prompted me to get the diacom software. Error code was for the IAC. Replaced the IAC (autozone tv278 at less than half mercruiser's price) and cap/rotor/plugs at the same time. I went with NGK non-platinum plugs and gapped at .045" instead of .060". The motor's perfectly smooth now, but I'm not sure whether it was the failing IAC or cap/rotor/plugs that was causing the miss. The plugs I pulled looked great as did the cap/rotor, but I don't know if you can see burn-thru in these flat caps.