1996 225STURU-Engine shutter at low RPMS

[zenoahphobic]

One other question. I want to make sure I have the proper prop on this boat. When I bought it, it came with a SWS M17 prop. Is that the appropriate prop for it?

How big is your boat? That pitch on a 225 implies a large boat. Your max Rpm's stated however suggest maybe a bit to high a pitch! Need to sort running first. Boat size?

[zenoahphobic]

Don't want to put a dampened on it, but how much use has this motor had? Your symptoms also can suggest it has reached its use by date. Internally thAt is - worn rings and pistons -due to suggestion that RPMs fall after a while and not optimum at start up. Too much blow by and pistons dragging creating local heat then increased friction.??

[jukes23]

Zeno,
Its on a 22'8 Grady White Seafarer. Not sure if that is overpropped or not.

[jukes23]

Motor runs fine for a while and then the rpms just drop off. Not sure how many hours the motor is on it.

[zenoahphobic]

Zeno,
Its on a 22'8 Grady White Seafarer. Not sure if that is overpropped or not.

Doesn't appear overproped but has it ever run 6000rpm with this prop?
I through the idea underproped because it may have run for lengthy periods at high rpm hence worn.
You mentioned a mates prop have you used that yet?

[jukes23]

Prop didnt fit. I am going to see if a local shop can help me out with it.

[bajakeith]

JUkes

Why would you be chaseing a prop ?? I thought you had running issue's ??

[zenoahphobic]

Why would you be chaseing a prop ?? I thought you had running issue's ??

Yes hard to see what to eliminate, it appears he has spent some time on that.
No harm changing prop, could eliminate "Shake" albeit doesn't appear at higher RPMs, and if overproped. Symptom loose rpm after sometime, suggest this if so because engine warms up under cowling making it less dense hence less power,
OP have you also tried cowling off running??

[jukes23]

Bajakeith,
It is possible that I am dealing with 2 completely separate issues so I want to try a different prop to eliminate an imbalance that is creating the low RPM shutter.

Zeno,

Most RPMs this motor has hit (at least since I have owned it) has been 5400. That is usually for the first hour or so until everything is warmed up, then the RPMS drop to 4800-4900.

When she is running first thing, it runs great. That's why every suggestion ive had of bad gas, vapor locks, bad vents, this/that doesnt compute. Could it be something that is not insync with the carbs? I mean there are 6 of them so I would assume that if one of them isnt dialed in just right, it could be causing problems (but using my own theory, that wouldnt make sense either.

What I dont want to do is just replace parts thinking that is the problem because Ive elimated the usual suspects at this point, whats left are sensors/parts that are fairly pricey.

One quick question, just what does the knock sensor do? Ive read the description, but if its not working (which I dont think mine is based on the test done), how does the motor run? This sensor is 150 bucks, I dont mind replacing it if its broke, but if thats not the issue, I dont want to throw money at it.

[zenoahphobic]

Bajakeith,
It is possible that I am dealing with 2 completely separate issues so I want to try a different prop to eliminate an imbalance that is creating the low RPM shutter.

Zeno,

Most RPMs this motor has hit (at least since I have owned it) has been 5400. That is usually for the first hour or so until everything is warmed up, then the RPMS drop to 4800-4900.

When she is running first thing, it runs great. That's why every suggestion ive had of bad gas, vapor locks, bad vents, this/that doesnt compute. Could it be something that is not insync with the carbs? I mean there are 6 of them so I would assume that if one of them isnt dialed in just right, it could be causing problems (but using my own theory, that wouldnt make sense either.

What I dont want to do is just replace parts thinking that is the problem because Ive elimated the usual suspects at this point, whats left are sensors/parts that are fairly pricey.

One quick question, just what does the knock sensor do? Ive read the description, but if its not working (which I dont think mine is based on the test done), how does the motor run? This sensor is 150 bucks, I dont mind replacing it if its broke, but if thats not the issue, I dont want to throw money at it.

You already advised that knock sensor, when triggered by detonation makes ECU retard ignition ( to stop detonation ).
A faulty knock sensor won't cause knocking/ shake.
Low octane fuel is usually the cause in a healthy correctly tuned engine.

[zenoahphobic]

I probably left lots of words out.
You don't need to change sensor to solve problem you describe just yet.
You are using the right fuel? What are you using?
Go try another prop for now, and report results.

[jukes23]

Would the knock sensor cause it not to hit top RPMs? I wish I understood more about the inner workings of the relationships between the sensors and the CDI play.

Fuel wise, I put treated ethanol gas in the tank. The gas gets cycled pretty quick as we run it typically 1-2 days a week.

[zenoahphobic]

Would the knock sensor cause it not to hit top RPMs? I wish I understood more about the inner workings of the relationships between the sensors and the CDI play.

Fuel wise, I put treated ethanol gas in the tank. The gas gets cycled pretty quick as we run it typically 1-2 days a week.

Your first point, only if the knock sensor does not detect detonation, and that detonation is actually occurring at high RPM. If detonating you must also hear this! Rattle! Or, sorry to confuse, if sensor faulty always outputting so that ignition is retarded and engine is not actually detonating, will cause loss of full power.

Second point: I am concerned about your fuel. Get hold of straight gasoline with Octane rating 90+. This will prove source of your problem if you have sustained high RPM (no dropping after a while). I don't think you tested the sensor correctly, you mentioned steady 2? mA. you should be measuring AC voltage. Change your gasoline first, run, then worry about sensor later.

Do one change at a time.
Either prop test or fuel change but one at a time and report results.

[jukes23]

ok will do

[frankjohnson]

If you want to rule out the knock sensor use a timing light, if timing retards back before tdc or jumps from after tdc to before tdc then your sensor may be intermittently failing. If the sensor is not working at all the timing can fall back to 7 degrees before tdc. Just a suggestion.