225 Excel drops rpm then back up

[Operaman]

Hey buddays. 1989 225 Excel at 4700 will drop to 4400 or 4300 then back up after a while. I went to my local, very talented guy in Amityville, Pearl Grey and he had a motor that we borrowed the TPS off of. It still did it.
I have kind of ruled out fuel since it comes back. I would think that if a pick up in the Carb was clogged why would it go back up like it freed. It could be a dirty Carb since I have learned that 2 cycle is all about fuel to get it running right. I ran it at night to look for spark arcs, but not under way, just at dock. No arcs that I can see.

Any thoughts? Thanks

[boats.net]

That engine does have a knock sensor that if picks up a knock will reduce timming and will continue to reduce timming until knock stops then will start to increase timming. Check compression and listen for a rattle in engine.

[Operaman]

That engine does have a knock sensor that if picks up a knock will reduce timming and will continue to reduce timming until knock stops then will start to increase timming. Check compression and listen for a rattle in engine.

Will do. Checked when I purchased last fall. Was excellent across all 6. Was wondering if a CDI (coil) was starting to fail. Or possibly a fouled Carb. I have the Yamaha Water Seperator installed this spring replacing a Racor. I checked the filter at the engine and there were slivers of black rubber in the filter. I never changed the fuel lines at the engine. When it happened the other night, my budday squeezed the fuel ball and no change.
Last night it slowed down over time, not suddenly as it did in the past and when it picks up it seems more sudden. This does not happen at idle and I am not at WOT long enough to have noticed it there. I have run 93 octane and my friend advised switching to 89. Something about lower Alchohol levels in the 89.

[Operaman]

Does the knock sensor ever fail?

[boats.net]

Yes the knock sensor can fail but make sure it's not the engine first. Carbs would be the next most likely suspect. That engine(Excel,F250 and F350) requires 89 octane gas and does seem to run better on 89 rather than 93.

[Operaman]

Thank you. It runs so good (knocks on desk as he types), doesn't make a sound nor skip a beat. I will listen and if all clear will do a nice carb cleaning. Should I be changing out the fuel lines as well?

[Operaman]

Gave a good cleaning, still does it. Runs a while at 4300 then wil drop for a while the go up.
Will get a Compression tester and do as recommended. I do not hear anything and it feels real strong so hoping for good compression.

[Operaman]

Compression is good. All very close to each other.

[jb123]

i still wouldnt rule out fuel delivery, use an aux tank , hose and ball right to motor filter.

then at about 12oo rpms on hose, pull each plug wire, ensuring a noticable sound/rpm drop.

sounds like you have one cyl cutting out then coming back.

run it at night, looking for any errant spark.

check spark from coils, ensuring you get about 1/4"' from tester...maybe you have a weak coil.

[Operaman]

i still wouldnt rule out fuel delivery, use an aux tank , hose and ball right to motor filter.

then at about 12oo rpms on hose, pull each plug wire, ensuring a noticable sound/rpm drop.

sounds like you have one cyl cutting out then coming back.

run it at night, looking for any errant spark.

check spark from coils, ensuring you get about 1/4"' from tester...maybe you have a weak coil.

Was thinking the same thing early on about the coil. Didn't know a coil could be weak. Thought they were good or bad, not firing. It is like what you say, a cylinder dropping out then coming back. The knock sensor apparantly retards timing as described above which also would do this lower rpm.
I am gonna look again at night and see if there is an arc. Also will see how the spark is with a test plug in each wire.
Thanks

[Operaman]

No arcs. Changed out knock sensor off another donor from Pearl Grey. Still did it, but one time quite a drop, like you pull the throttle back. When it happens, I still can WOT and get 5600. This tells me I am getting timing and fuel.
My budday and I lookd at each other and kept saying it is too much like throttling down. Engine is running great. This happens at 4200 to 4800. WOT will maintain. 3900, 3700 maintain all day long.
So, now gonna check for play in the cable. I see an arrow at the throttle cam. Is that to align with the roller in neutral or in gear or both.

Any thoughts? Thanks so much.

[Operaman]

Well, did the carb sync, feels better, notice the cable has play. Is it normal for the cable to move back and forth even at mid trottle?

Also, reading a similar problem a response recommends that it could be a fuel pump diaphram. I was told they were changed out right before I bought the boat last fall.
Do the pumps survive the Ethanol fuel? Is there a way to check the pumps? I have tried squeezing the ball when it drops, no change. No change until it does it on it's own.
I do have WOT when it happens. I feel it could be higher when it happens and get there faster, like when it doesn't drop.

Useing Ring Free and Startron everytime I fuel up, about 3o-35 gallons at a time. Using Cans. Thanks