Preventive Maintenance is an important part of keeping your jet rig in peak operation all season long. Many hear this and draw the conclusion that jets are high maintenance. This is simply NOT the case. Truth is, prop or jet engines commissioned for use on shallow rivers will require more attention than many other boating applications. The main difference is that a jet unit requires greasing the main bearing zerk after daily use, this task only takes seconds to perform. The other attention required comes from normal use, a craft operated in shallow water tends to bump and bounce objects more so than in deep water applications. The lower unit or jet drive takes the majority of this punishment and this is the focus of The 30 minute PM. 

The impacts to the jet drive can begin to loosen the hardware that keep both the impeller and the lower unit attached to the outboard. Just as it is a good practice to check your trailer’s wheel lugs, doing a periodic check on the jet drive is a smart move. For those who have brand new rigs, do not assume you’re in the clear. Several years ago, I uncrated a new outboard jet engine, rigged it to a transom and ran the craft for a few days. When loading the rig on the trailer, I noticed a faint rattle sound when the engine idled down. I pulled the intake off and found that my impeller was not tight on the drive shaft. When I removed the impeller, I also discovered that one of  the 4 bolts that attach the jet drive to the motor were loose. This engine had less than 4 hours of run time on it.

Getting Started:

First step is to remove the intake foot from the pump housing. Keep in mind there are two styles of intakes, those with a flanged liner and those with the liner attached to the intake. Remove the nuts off the rim of the intake. If the intake has not been removed for a while, it may be necessary to knock the foot lightly with a rubber mallet to jar it loose.

Open up the wing tabs of the retaining washer (above A) and use a large adjustable wrench to remove the brass nut (above B). A solid bump or two on the wrench handle is often required to loosen the nut. With the brass nut removed, slide the retaining and 8-shim washers off the shaft and carefully remove the impeller. Be sure not to lose the plastic sleeve and impeller key.

NOTE: 8-shim washers are required on all but two of the Smallest Jet Kits. Regardless of impeller wear, each shim must remain on the drive shaft when the impeller is placed back on the shaft. For those who have the older OMC jet kits (Evinrude & Johnson), the Series H has 6-Shims & H9 has 9-shims.

At this point, with a wrench and long extension, check the four bolts that hold the jet drive to the outboard housing (below C). For completion sake, check the 4 bolts that hold bearing carrier to the shaft as well (below D).

The next step is to inspect the impeller for nicks and for cavitation burns. Depending on the year and make of outboard, the stock impeller may be made of Aluminum, Zinc or Stainless Steel. Companies such as Mercury, have ordered their jet units from Specialty Mfg without the impeller for the last few years. When the engines are assembled at Mercury’s distribution center, they use a 3-Blade stainless impeller on the 60/40Jet and a 4-Blade stainless impeller on both the 90/65Jet and the 115/80Jet for their engines. The Stainless Steel impellers are far less prone to wear than the softer impeller material. While there is a much debated payload advantage in Stainless Steel impellers for large size pumps, the main advantage is longevity and less maintenance. I honestly feel it is a bonus all around.

When the impeller fins are filed sharp, slip the shear key, plastic sleeve and impeller back on the drive shaft. Add all 8 shim washers and the retaining washer and tighten the brass nut. Bumping the handle of the wrench will help tighten the impeller on the shaft. Check that the impeller is tight by pushing upwards on it, also make sure that it does not rotate or slip on the shaft. If the tabs of the retaining washer do not line up with the nut, remove the nut and flip the retaining washer. The tabs on the retainer are designed with an offset for this purpose. If the shear key or the retaining washer is worn, replacing them is very inexpensive. Marine shops that deal with outboard jets should carry these common parts. If they do not have them in stock, either look for a better shop or order a few spares to keep on hand.

  Do Not fold the retainer tabs over the brass nut until the impeller clearance is checked. (Next Step)

Note: There are two different types of impeller shear keys. (below right) They are NOT universal and must be replaced with the correct key shape.

Before folding the retainer tabs over the nut, check the clearance between the impeller and the liner. Too little clearance will cause liner wear, too much clearance will reduce the volume of water into the jet pump. Use a 1/32” feeler gauge to check each fin. The 1/32” gauge should be a snug fit. For the flanged intake liners, slide the liner in place and check the clearance (‘A’ below). For intakes with the attached liner, (‘B’ below) the feeler gauge has to be slipped through the intake grates to check liner clearance.

Adjusting the Impeller Clearance:

If the clearance between the impeller and liner is greater than 1/32 inch, move a shim-washer above the impeller on the drive shaft. The impeller and liner are cone shaped. Moving shim-washers from the bottom of the drive shaft to the top, drops the impeller further into the liner, closing the clearance gap.

Once proper clearance is attained, fold the retaining washer tabs over the brass nut. Visually inspect the grill bars, after a few encounters with an unforgiving rock or ledge, these grill bars tend to lose their uniform shape. If left unchecked, several problems can arise. First, because the bars are bent, they can allow larger debris to enter the intake. When this occurs, the impeller, pump housing and exhaust nozzle are subject to severe damage. Another concern is that mashed or bent grill bars can impede the flow of water and reduce thrust. If you have significant time on the jet unit, and the bars are in rough shape, consider replacing them. A few years back, Specialty Mfg started using stainless steel bars. These are stronger and retain the shape better than those previously used.

The grill bars are pinned in place by two long rods that pass through each end of the grill bars. To remove the grill bars, start with a long punch or a steel rod similar in diameter to the intake rod. Start from the side closest to those that need attention. If you are removing them all, it does not matter which end you start from.  As the rod passes through each bar, remove and reshape or replace the damaged grill bars.  New grill bars can be purchased for a few dollars each, notice that one end ('A'  above) has an oblong hole. Pin the end with the circle hole first (‘B’ above), the end with the oblong hole allows the second rod to pass easier through each grill bar on the way back through.

Install the liner and intake to the pump and the 30 Minute PM is complete. Keeping an eye on the basic maintenance will provide early detection and often prevent regular wear from becoming a more serious problem. It will also ensure your outboard jet will perform at peak performance all year long.

Note: While this feature has been updated by the author in February 2004, segments of Outboard Jet Maintenance was published by Pennsylvania Angler & Boater magazine in May/June of 2001.