Volvo Penta 572 feels like it's cavitating...can't figure it out.

[Smad23]

I have a 1992 Volvo Penta 572A with a Volvo Penta DP outdrive in a 25' deck boat. Bought the boat and knew it was in need of maintenance, so I changed the lower unit gear lube, oil, water fuel separator, sparks plugs and thermostat for good measure. Put the boat in the water for the first time to find a few hiccups. 1. You have to pump the throttle over and over to get it to fire. Once it does, it would die on idle. 2. After running it around the lake for a while it started to make a loud popping noise from the engine when under acceleration and while in motion. 3. She takes on a ton of water, but the hull is sound. Turns out there is a huge rip in the driveshaft bellows. So, in an effort to remedy the engine issues, I siphoned all of the gas out since the boat had been sitting and filled it back up and added a can of seafoam. Ran the boat around the lake and it ran great, at first, then after a while it was doing a popping/cavitation thing under acceleration and on plane. It almost felt like it was either missing out or something was going on with the lower unit and it wouldn't hook up. Last week I decided to replace the spark plug wires, cap and rotor since they looked pretty bad. I took it out on the lake today and the good news is that it idles now and doesn't die. The bad news is it is still giving the feelings of cavitation or missing out (almost the feeling of being on a jet ski and it sucking air in the intake grate, the lunging forward once it hooks up).

I'm getting the bellows fixed next week, but I cannot figure out why the boat feels like it's cavitating or missing out when under power. Please help me figure this out!!!!!

 

[bigbrownbuku]

Its probably your shift cone is glazed or you have too much lost motion in your upper vertical shaft assembly causing the cone to not engage properly. Too much of this and it will just slip all the time until it's fed rpm and bangs in. They are the simplest upper to overhaul and setup. Not really a back yard garage job unless you have done on e before. Upper has to be removed to change bellows so it's the best time to do it.
On the water through the bellows topic. This is trouble. Your turtleneck bearings will have water damage, find out if you have a single bearing shaft then it's possible to repair without pulling the engine.

 

[Smad23]

Thanks for the advice bigbrownku. I am having the bellows replaced and will have them check this out.

 

[Smad23]

Since last post: Bellows replaced (exhaust and driveshaft), gimble bearing and driveline ujoint look good. Full lower unit service done. Running sporatic at times, replaced coil and seemed to help idling issue and backfiring issue. Took for a ride (2 adults) and it ran great with none of the cavitation feel at all. Tonight we had 10 people in it and the cavitation feeling was back. I'm confident it's not the engine this time. I had the adults move from the front of the boat to the rear and the cavitation went away. Even with weight distributed evenly from front to back it still does it. I have tried the lower unit all the way down, trimmed up a little, etc. no change. I thought the Duo Prop lower units were supposed to eliminate cavitation. Any thoughts on how I can overcome this?

 

[Smad23]

Also, regarding my shift cone, my mechanic said that I would see rpm fluctuation if it were to slip, but I don't have any of that.

 

[dypcdiver]

Just a shot in the dark but, have you checked that the 2 props are a match? If they are from different series it could cause the cavitation you mention.