92 Yamaha ProV 175- low power issue

[Telstar_2112]

We have a 92 Yamaha ProV 175 on a 21' Grady. Ran fine for years. Suddenly it lacks the power to get up on plane. The motor runs fine in neutral and spins right up to 5k. Under load, we can get it to around 4200 rpm but it takes a very long time. Not like it used to be where it would jump up pretty quick. We changed the filter/water separator and spark plugs. We also tried pumping the fuel bulb while under load- no help. Different than most of the posts I've looked at is that during this time the motor is still running smooth, not kicking or sputtering or anything, but also not pushing or spinning up like it should. Early last season the fuel pumps were replaced.

Any ideas what to try next?

I am thinking:
1 Check compression.
2 Try pulling plug wires off one at a time under load to see if all cyls firing.
3 Trying an Octane booster in case it is bad fuel, or would this show up when not under load?
4 Cleaning carbs.

Any suggestions greatly appreciated.

Telstar

 

[Telstar_2112]

Re: 92 Yamaha ProV 175- low power issue

OK, We worked on the boat last night and I am completely stumped!

Compression is good on all cylinders, 115-125.
We pulled each plug wire off under load and found that the bottom left cylinder is not contributing. All others had a noticeable slowdown and speed up as we pulled the wires off and put them back on.

We checked that the plug was firing, it was, but we tried replacing it any how. No-change.
Swapped coils, no change. Same cylinder bad.
Note, that one plug from the bad cylinder was shinny and spotless clean like new, while the other 5 were a little dark and a little oily. (they are relatively new as well since we replace them two weeks ago and only ran a little bit.)

Removed that carburator and took out all jets and pretty much any brass parts and although they looked spotless clean, blew them out with carb cleaner. All ports seemed to blow through. Ran it again and no change, cylinder still not contributing.
Checked that that float bowl on that carb was full of gas, it was. Cleaned carb again, out of desperation, no change.
Clear bowl on engine always had plenty of fuel in it.

I have pretty much used up my limited knowledge of boat engines so if anyone has any suggestions, please, please help!

TIA,
Steve.