Bad Vibrations!

Over the last 20 years the one question I get asked the most is, "Is my jet supposed to be making that noise?" With some noises being normal and some not, we are going to dive into a project that will reveal these noise making gremlins and how to fix them.

I have the perfect candidate, a 1989 Carrera 20.5, otherwise known as the "Red Rooster." This boat was brought to BJ's Marine to hunt down a bad vibration in the jet, or at least that?s where the owner thought it was. I was excited to see how clean this boat was. It was obvious that this boat has spent most of its life in a garage and is well cared for.

Before I tore into the boat, I thought it would be a good idea to run the boat and get a feel for the problem. Upon firing the boat up, it sounded great, it idled smoothly and there was no sign of any vibration. That drastically changed as soon as I ran the motor up to 1500 RPM?s. It then became obvious why the boat was here.

My initial thought was that the vibration was not in the jet. I grew concerned that the problem was in the engine. I had a few questions, was the flexplate ever changed on the engine? Is the driveline or harmonic balancer comi ng apart? I had to call the owner to gather some info. He stated that the flexplate has never been changed (since he?s owned the boat), and that he just rebuilt the driveline when he freshened up the motor. He?s only owned the boat for a short period of time, and he doesn't recall the boat vibrating like this in the past. With that info, I decided to pull the jet and get some answers. This boat is equipped with a 3 point mounting system, so with the jet in the boat, you have very limited access to the driveline. For a thorough inspection, the jet has to come out.

My customer had decided that he wanted me to rebuild the jet regardless of the outcome on the vibration problem. The impeller has seen better days, you could visually see that the impeller blades were pretty dull and dinged up. That was enough info for him to green light the rebuild. We also decided that the pump should be re-powdercoated while it's apart. The jet was the only original part on the boat that showed its age. Now would be the time to clean it up and we can make the jet look better than new.

Once I removed the pump from the boat, I locked it down in the pump bench and began disassembly. It became obvious that the splines on the pump shaft were damage because my spline tool wouldn't fit. Once the pump was apart, I could take the pump shaft and fit it into the driveshaft on the engine. Those two components slid together with a significant amount of slop between the splines. After taking a close look at the splines on the pump shaft, I could tell this pump had suffered a main bearing explosion at some point in time. The stainless s haft had significant heat discoloration indicating something got very hot.


The pump shaft splines were badly worn. This would also explain why my customer was a stating that the boat was making noises while idling. When a jet drive is idling in the water, it's not uncommon to hear some grumblings thru the hull. This is called spline cha tter. There have been several companies manufacturing jet drivelines and pump shafts over the years, so it's not realistic to think they're always going to fit perfect. A slightly loose fit between these two components is invited. You don't want the driveline to be seated tight on the pump shaft. This can prevent your engine from experiencing normal crankshaft end play and potentially cause engine failure. On the other hand, when the splines are too loose, they can make some noise, which is no big deal. As soon as the boat accelerates the splines lock up and the noise goes away. In this case, the splines are too far gone and the pump shaft will be replaced. Pump shafts are made of stainless steel and the drivelines are made from hardened steel, so the p ump shaft will wear out first usually leaving the female splined yoke on the driveline in good shape.

I had a nice used pump shaft in stock that we?ll use on this project. I test fitted the new shaft into the driveline on the engine and it fits perfectly. Not to loose, not to tight. By installing this pump shaft we will eliminate the spline chatter noise that my customer was hearing.


With the jet drive completely apart, I was able to confirm my initial gut feeling, the vibration was not coming from the jet. The impeller was in tact, not missing any blades and the pump shaft although not re-usable, wasn't bent causing any vibrations. The pump was in fair condition and the main bearing was still in good working order. When a main thrust bearing goes bad in a jet, it will make significant noise, and it can allow the rotating assembly to shift. The thrust bearing is the only thing holding the shaft and impeller in place. When a bearing fails, the forward thrust of the jet allows the impeller to move forward. As this problem gets worse, the impeller can run into the suction housing causing major damage. In severe cases the pump shaft can actually bottom out in the driveline and the forward thrust of the jet?s rotating assembly can push your crankshaft thru your crank bearings. A failed eighty dollar bearing in your jet can potentially cost an awful lot of money. Proper maintenance should not be ignored.

In order to absolutely confirm my thoughts, I installed a break-in pump in the boat. A break-in pump is a jet drive that is just the main suction housing, a special shortened shaft, and a main bearing. Installing my break-in pump allows for me properly secure the engine in place and run the engine just like it would normally run. It?s accepts the driveline just like the normal pump does. Once I had the break-in pump bolted down, and the engine bellhousing re-secured to the bearing cap, I took the boat outside to run it. Just as I suspected, the problem is in the engine..

Tune in next week for Part 2!

Bob Jeanblanc
BJ's Marine Recycle Performance
1260 N Redgum St
Anaheim Ca 92806
714-630-3905
info@marineiw.com
www.marineiw.com