1996 225STURU-Engine shutter at low RPMS

[jukes23]

I have been dealing with a real doozy of a problem on my 1996 225STURU. Problem was first the fact that the motor would run great in the morning when we first fired it up. After running for about an hour or two, the motor would start to bog out when at WOT. We Rebuilt the carbs over the weekend...the bogging is gone. However a persistent problem is still there. I should have also mentioned it earlier. At low RPMS (say 1500-2500), it has this horrible shudder and literally shakes the boat. once you power it up, it goes away.

Could this be the knock sensor? What symptoms should I expect if the sensor is bad? I did test the sensor, but have some questions regarding it because the manual wasnt really clear on testing it. Manual says:
1. set your voltmeter to mV (millivolts).
2. hook one lead to the sensor wire
3. hook the other lead to the body of the sensor
4. tap the sensor with a screwdriver
5. (this part wasnt clear) Replace sensor if several millivolts (2-10 mV) are not displayed on the meter when tapping. Does that mean when I tap the sensor, the number should change? when hooking it up, the meter continually read around 2 mA. If I tapped it, nothing changed.

I am trying to isolate just exactly what is causing this problem because its really really bothering me at this point.
Thoughts?

Joe

[zenoahphobic]

Bad shudder

Knock sensors don't have anything to do with bad shaking.only you know how bad, because you need to rule out imbalance somewhere and weAr at mounts, pivots and including loose bolts. Check prop and wobble in crankshaft (bearings) if you have to

[zenoahphobic]

Knock sensor

The knock sensor is simply a switch . It makes contact when given enough of shake, and sends this occurrence to the ECU

[rodbolt17]

actually.
its simply NOT a switch.
its simply a piezoelectric device.
shock it and it shakes.

shake it and it shocks.
same as your grill lighter.

post the complete model number.

there were some differences based on the prefix to the 225.

[zenoahphobic]

Oh Ok, hence voltage reading. Thanks

[boscoe99]

Knock Sensor
The knock sensor is a piezoelectric device (it generates a small AC voltage when subjected to certain vibrations). The ECU provides a 5-volt reference signal to the knock sensor. If a knock occurs, the knock sensor produces an AC voltage that “rides” on the 5-volt signal. The amplitude and frequency of the AC signal generated by the knock sensor are dependent on the level of knock. When the knock sensor detects detonation or any other vibration with the same intensity as detonation, a signal is sent to the ECU. Depending on intensity and duration of detonation, ignition timing will be retarded in steps until detonation stops.

[zenoahphobic]

Thanks I oversimplified , I was told , by mechanic that installed LPG early knock sensors in cars were (I don't know what the internals were) made contact, several times with earth latching a check engine display on dashboard - to dismiss this display as anything to worry about.(happens because ECU designed for petrol and use of LPG sometime parameters outside range it can handle
I never checked this out- so in fact the 5 volt voltage signal is added to (5v plus) when detonation "triggers" this sensor. Thanks for the clarification , and I digress from OP's problem, sorry .

[jukes23]

Here is what I know: The motor would run fine when you first fire it up and take off. She will hit 53-5400 RPMS with no issue. After running it for an hour or so, if you would put it in WOT...it would make this bogging sound and would run at 46-4800 RPMS. We rebuilt the carbs and the bogging is completely gone. The low RPM shutter has always been there. As far as something being out of balance my only retort would be that at higher speeds, wouldnt it shake worse? Ran it today and it ran great...coming back in, wouldnt go past 4800, but no bogging. Not sure what's left to check/change out.

Rodbolt: the model number is S225TURU. The rest is a little fuzzy as the serial number is about gone. 62J SU(maybe 3U its hard to tell) 850235

Thanks,

Joe

[bajakeith]

shudder

Its hard (for me) to understand what your saying !! have you tried a portable tank test run ??? the way your describing problem is a problem for me.... could be anything from a plugged tank vent to a dirty/plugged filter some where..and /or a coil that shorts as it gets hot...need more (better info

[jukes23]

Not sure how else to put it. Tank vent is fine

1. Motor has had the shutter since we purchased it
2. We've had numerous issues with the carbs on this motor from stuck floats to fuel pumps to the bogging at WOT. To date, we have rebuilt the carbs, changed out the fuel pumps, we treat the fuel for ethanol issues. The only filter I dont think we have changed is the fuel filter inside the motor. After all of that, there is no more bogging in the motor, but the shutter is still there at low RPMS and after running it for a while, WOT is approximately 4800 RPMS. Something is going on, just not sure what

3. To recap:
Shutters at low RPM (up to about 1800 then it goes away)
In the morning it will pull 5300 RPMS, but by end of day, wont go past 4800.

Does that help?

[bajakeith]

Hi Jukes

I would try a portable tank with new fuel line and connects first...that's a boat mech trick that has found many,,many problems in the tank,,fuel line,,connecters and is easy/smeasy....been in the biz around 40 yrs

[jukes23]

the problem is this problem doesnt show up right away. It typically doesnt start happening until the motor has been ran for over an hour or more...that is the only common denominator. Portable tank wise, I'd need something in 10-15 gallon range as this motor drinks gas.

Not discounting the option, just explaining the situation.

[bajakeith]

Hi Jukes

all ya need is a small 2/3 gal. tank

[jukes23]

so is your suggestion to run it for an hour or so until the RPMS start dropping, then switch to the small tank?

[bajakeith]

Yes !!

that will rule out the anti sipon valve,,vent line,,feed line,,primer bulb,,2 connects.......