Engine bogs down -- comments appreciated

[Gary4731]

2001 OX66 on a 25" walk-around . . .motor starts, idles, runs great. . .just tuned, plugs, caps, cleaned O2 sensor, VST tank, fuel filter, water separator, etc. . . .while speeding along at 5500-5700 rpm, the engine just starts to slow down . . .no missing, coughing, etc . . .just like starving for fuel . . .BUT when I touch the trim button up or down, the engine picks back up and runs strong again . . .when I put it in neutral and rev the engine, it goes up in rpm quickly and smoothly. . .my local Yamaha dealer speculated that there may be bad electrical connections at the battery causing momentary lack of electricity, and when I touch the trim the surge of electricity corrects the motor.

I replaced the batteries last summer . . they are charged and I get 14.4volts while running.. .checked the connections and all seem OK. Your thoughts?

[pstephens46]

That’s a good clue. Electrical “power loss” somewhere. I would clean all grounds first. The ones bolted to block.

[99yam40]

I agree, bad connection somewhere .
also reving a motor in neutral does not do anything except possibly harm it,
proves nothing without a load on it

[Whaler9113]

How far of an RPM drop and how old is the O2 Sensor?

[TownsendsFJR1300]

When you "checked" the battery connections, did you remove the cables?

Did you clean the terminals and cable ends with a brush as well as check for any corrosion inside the end of the cables?

Lastly, do you have an on /off main battery switch installed?

[rejesterd]

Hi.

If you don't touch the trim switch, how far do the rpms drop?

Also, is this a 200hp?

[Gary4731]

Whaler9113: RPM drops to about 3000 and seems to go lower before I trim it up or down; don't know age of O2 sensor but it was very clean when I took it out for cleaning --- TownsendsFJR1300: Yes, removed cables, but I did not remove cowl and check motor connections yet, will do today once the rain stops. Yes, I have a new A-B switch. ---Rejestered: it is s 250 Saltwater Series

[rejesterd]

Does it slowly drop to 3000? Or does it immediately drop to 3000 then slowly get lower after that? The reason I ask is because there might be multiple issues going on, where the engine puts itself into a control mode and you have an electrical connection issue on top of that.

[Gary4731]

rejesterd: it slowly drops to 3000 . . .but I know what you are meaning by going into control ode . . .doesn't seem to do that. Thought initially it may be a filter problem in the VST, but I haven't run it much since deep cleaning and thorough maintenance last summer . . .not much use since the hurricane Michael in October. It does not seem to be missing, coughing, sputtering, etc. It just drops RPM and then goes like heck again when I touch the trim button and move the motor up or down a bit.

[rejesterd]

Agreed.. if it was the VST filter clogging up, it probably wouldn't just drop smoothly like that.

I think you ruled out a bad battery or rectifier/regulator, because you're seeing 14.4V when running. So I would do as someone suggested previously and check all the grounds on the engine itself.

[Gary4731]

Update: Took the boat to the dealer who changed water separator filter ( which was for a 100hp motor versus mt 250hp---I did not know there were different sizes); cleaned VST; replaced in line filter; replaced plugs; took the boat out and got up to 5700 rpm and got the same result that happened to me -- the engine slowed down. Now, we are both puzzled. Someone suggested that the rectifier was bad . . .can someone explain to me how a rectifier work and how this would cause this type of motor reaction I am experiencing? I'd like to avoid this $631.00 expense...

[99yam40]

did they monitor fuel pressures?
or anything else while running the boat when the problem showed up to see what was changing?

[Gary4731]

99yam40 - -not sure what tests were conducted while on sea trial, but dealer called me ans said they would pursue until it got fixed, yikes! My question about the rectifier stems from many folks suggesting that there was an electrical ground issue . . .I fail to see how a ground issue would cause a motor at high rpm to suddenly slow down...fuel, air, spark ....kinda simple, huh?

[Rappin Capn]

My 2001 250 ox66 was bogging down, I changed the 3 fuel pumps and put a new vst filter and all was well. Plus I use a 10 micron fuel separator filter, and all is well. I believe you said you cleaned you vst filter, but those vista's are very finicky, bet to replace the filter with a new one, not just clean it. Hope that helps.

[Gary4731]

RappinCapn: I cleaned everything and performed lots of maintenance last fall . . .the dealer just cleaned filters, replaced plugs, etc., and took the boat on sea trial . . .same problems as I was having . . .brought the boat back to the shop to check more stuff, and took it back out . . .am waiting for more info when they reopen Monday.