Strange tach behavior on a Penta 5.0L

[aggiedave98]

Ok, this thread crosses over from prop to engine/tach. But I think I ruled out the prop.

I’ve got a 2004 Starcraft Stardeck 21ft with a Volvo Penta SX 5.0L. I was driving my boat the other day and my rpm showed much higher than expected under way on the lake. Like it was showing 3000rpm and only going a little over 10mph. It was a rough lake and into the wind. When I started testing it by backing off the throttle and then giving it more, the rpms would sometimes jump way up like 1000rpm very fast. Definitely not normal.

I turned around and slowly drove back. Before I entered my cove, i tested getting up on plane and it was normal again. I drove to back to the marina.

I got feedback on a message boards and calling a local prop shop that it was likely a spun hub. I just pulled the prop and I don’t have a rubber or cushion hub.

I have this plastic and metal system. I don’t see how it could have spun. I assume the plastic squarish piece would sheer off. But it looks fine.

So is there anything that looks out of the ordinary here?

If it’s not the prop, is there anything else? The prop place said the tach wouldn’t jump around like that but I’m not sure what else it could be.

I’m also attaching pics of the back of the tach. No corrosion or anything.

Thanks in advance for any help!

 

[dingbat]

From the looks of it, (brass gulling) the hub wasn't tight and the insert is spinning on the thrust washer.

Far end of the plastic sleeve looks like it's been hot at one time or another as well

 

[aggiedave98]

From the looks of it, (brass gulling) the hub wasn't tight and the insert is spinning on the thrust washer.

Far end of the plastic sleeve looks like it's been hot at one time or another as well

Thanks. I noticed the nut did come off very easily. It didn't seem like the 70-80 ft lbs that it's supposed to be tightened.

So what would you do?

 

[kenny nunez]

The rear of the gear case where the propeller fits in that makes the labyrinth seal looks like some of the rear edge is worn away. If that is the case then too much exhaust is going over the propeller than through it causing cavitation.

 

[aggiedave98]

The rear of the gear case where the propeller fits in that makes the labyrinth seal looks like some of the rear edge is worn away. If that is the case then too much exhaust is going over the propeller than through it causing cavitation.

So what do I do in that case?? I read up on cavitation last night and that sounded like it could be the behavior I'm seeing.

 

[kenny nunez]

So what do I do in that case?? I read up on cavitation last night and that sounded like it could be the behavior I'm seeing.

You will need a new lower gear case or try removing the exhaust bellows to cause the exhaust to blow out before the propeller.
Try to compare your gear case to another one where the propeller fits in the gear case just to be sure.

 

[aggiedave98]

You will need a new lower gear case or try removing the exhaust bellows to cause the exhaust to blow out before the propeller.
Try to compare your gear case to another one where the propeller fits in the gear case just to be sure.

Ok, thanks. I'll start with trying to compare to another one.