95 Yam 150 SW2 lost top end RPM

[Claxton971]

Hi - New to the board. Search had great info, but just not exactly my setup.

I've got an Escape 209 with a 95 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. 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?

 

[ahill]

Is it fuel injected like OX 66?
If so check O2 sensor.

 

[Claxton971]

No fuel injectors. V-6 w/ 3 double-bowl carbs. Typo above: the engine is actually a 1996 Yamaha 150 saltwater series II.

 

[Claxton971]

Update. 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.

 

[gw204]

You might also try asking on www.screamandfly.com. Those guys know absolutely everything about outboards.

You might also try having a buddy drive while you manually manipulate the carb linkage to see if you get full RPM that way. Calm water only of course...

 

[Fish Tank]

Bent prop?
How long has it been in the water? Any "stuff" growing on the hull?

 

[enfish]

Had that happen 1 time on our motor. We normally topped out at 5300 but one day suddenly during the middle of the day we topped out at 4400. Same exact symptoms with the throttle you describe... would get up to 4400 with the throttle control in the normal position but the last part of the throttle throw did nothing. In our case, it turned out we had a bad spark plug and were only running on 5 cylinders. It still ran very smooth at idle and operated normally up to 4400 RPM, even on 5 cylinders. Since you've done the simple things first, you're on the right track with a compression check.

 

[Claxton971]

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.