Loss of top end RPM, 1996 Yamaha 150 SW2

Search had great info, but just not exactly my setup or no resolution.

I've got a Grady White Escape 209 with a 96 Yamaha 150 SW2. I've lost about 600rpm off my top end. It's used to put out 5300-5400rpm and now tops out at 4700/36mph. Starts, idles, and runs strong right up to 4700 then stays there. No sputter or misfire. No slow degradation from 5300. The last 30 degrees of throttle throw seems to have no effect where it used to.

I've checked the throttle rigging, and all looks normal there. Inspected and replaced all the spark plugs. All looked normal there. Replaced the fuel filter which had a little debris, but this had no effect. I'll replace the fuel pump diaphragms tomorrow and check pressure/vaccuum. If still no luck, it's onto the carbs. Fuel is new. I'm going to get a compression tester this weekend, but I doubt its that since it didn't gradually degrad and seems to run normal through the low to mid range. Is that a good assumption?

Would a clogged high pressure jet in one of the carbs cause this drop in RPM? Can't imagine all the WOT jets would clog at the same time...but possible. Any sensor to check that might be limiting the system? I welcome any other suggestions?

Re: Loss of top end RPM, 1996 Yamaha 150 SW2

Update. Fuel pumps look worn. A little bit of crud in the back diaphragm. Replacing tomorrow. Also pulle d the carbs to see if any crud made it there.

On the lower carb there is a rod that actuates a device on the port side below the fuel filter via a ~12 inch rod. Move with the throttle...what is this and what does it do?

Re: Loss of top end RPM, 1996 Yamaha 150 SW2

That is the oil pump link rod. Start with the basics...

Re: Loss of top end RPM, 1996 Yamaha 150 SW2

Thanks for the reply. Makes sense. More speed more oil. I read about the possibility of a clogged fuel pick up. My fuel line connects to a welded elbow coming out of the top of the tank. Pulling and inspecting that doesn't look like and option. Any risk to blowing compressed air in the opposite direction of flow to clean things out? I could then pull the sender and suck out any crud from the bottom of the tank...

Re: Loss of top end RPM, 1996 Yamaha 150 SW2

Will not work if boat is up to USCG standards equipped with anti syphon valve.

Re: Loss of top end RPM, 1996 Yamaha 150 SW2

Why not test things to see if the pick up is even a problem.
measure fuel pressure and vacuum, that will tell the story

Re: Loss of top end RPM, 1996 Yamaha 150 SW2

Update2. New fuel pump diaphrams show up today. While waiting on parts I pulled the carbs and cleaned. I picked up the boat in Feb and figured I see how the carbs looked to at least establish a baseline there. They looked good. The look better now, but don't suspect that as the problem. Reassmbly on Friday. In the mean time I also puchased a vacuum/pressure guage and associated clear tubing. Truly, I admit, what I should have done from the start. So if the pumps don't fix it, guage test will follow. Also changed out the fuel water seperator filter. Not sure how long that had been in place.... More to follow. Thanks for the replies.

Re: Loss of top end RPM, 1996 Yamaha 150 SW2

Update3. Looks like I'm back in action. 5200rpm @ 42 Knots. I believe the new diaphragms in the fuel pump did the trick. I'm still curious about the fuel press/vacuum and clear tube tests. I have all the gear now I'll measure that soon to verify it's correct and at a minimum set a baseline. Pulled all the new plugs and all appear normal - as did the old ones.
1995 Escape 209 w/ 1996 Yamaha 150 SW2.
Norfolk, VA

Re: Loss of top end RPM, 1996 Yamaha 150 SW2

Sounds like a good plan, that way you will have something to compare to if you start having problems later on