outboard Jet Maintenance

For those of you who run a outboard jet on your boat, there is some required maintenance to keep it functioning properly, & to assure that you can remove, adjust or replace needed parts if required. I am are not covering inboard jets in this article. I will not really go into the a lot of details here of history, how it works or installation as it is available on the manufacturers website listed below.

The bulk of all the current outboard jets are made by Specialty Mfg Co.. The jet units you find on the factory fitted motors, no matter what name is on the motor is made by this company. Below is a working view of the unit (taken off this company's website). There is at least one other newcomer, Durajet, that is using a design very similar to a airplane jet engine in the location of the prop, sucking the water in the front & out the rear.

This Specialty Mfg company was started by Dick Stallman after he experimented with an idea in 1960. The refined product you see today is the result of his efforts. Dick was good friends with the Wooldridge family boat company from the inception & both being far sighted, did lots to get us where we are today. Stallman's oldest jet motor is proudly displayed in the showroom of Wooldridge Boats in Seattle, WA. today.

The exploded view of the design shown below was taken from Specialty Mfg Co. website.

On The Water Repairs : The one thing I will say is that it is very beneficial to have a motor that has power tilt. Jets don't like weeds. Depending on the quantity of floating weeds, running thru them at speed may work of you have a Vee bottom, but outboard jets are usually on flat or a slight Vee boat & more often than not, you will clog the intake grill if there are many of them. The intake will plug up & the motor will rev up but the boat will not go anywhere, or if it does it will be a a drastically reduced speed.

Many times if you just shut off the engine & let it set for a minute, start it back up & see if you regain power. If the stop & go did not clear the obstruction, shut it down again, tilt it up, and reach back and clear the intake. Be sure you are wearing you PDF. It is usually not that hard to clean out the intake grill on an outboard jet. You may want to keep a long handled screwdriver or something similar with a lanyard tied to it in case you drop it while you are hanging over the stern.

Ingestion of rocks is another matter. The smaller ones will go thru the impeller & out the rear with no problem other than possibly chewing on the impeller's leading edge. Larger rocks however approaching a quarter size coin may well get wedged between the grill bars. If that happens, a special pry bar with a cross pin needs to be fabricated so that you have leverage on the grill bars against the wedged rock. Remember you are on the water, hanging over the stern trying to dislodge these rocks, so you will need to drop anchor or get to a sheltered location.

If you are running in shallow mud infested water, watch the motor's overboard tell-tail indicator, (pee hole). This water indicator HAS to be functioning otherwise you WILL get the motor hot enough due to no water cooling it & burn the motor up, usually by seizing the pistons in the block. If it just stops peeing & you caught it before the engine faltered, you might try shutting it down, wait a few minutes, & try again. This may take 6-8 tries, but it usually works. If not then you need to row, as the motor CAN NOT be run without water circulating thru the water passages. You might try using a wire poked up inside the pee hole to see if just that indicator is plugged as water may be in the engine but not coming out a plugged pee hole. Start it up again & watch this pee hole for water being squirted out in a steady stream. If so then you are on your way.

Off The Water Repairs : For those of you who only run in clean freshwater, the following paragraph does not apply. To flush the motor, there is a 3/8" stainless bolt head on the LH side just below the juncture of the motor's midsection & where it bolts to the jet lower unit. This is a flushing fitting hole. Mercury builds a flushing adapter that uses a 3/8" course bolt thread on one end & a female garden hose end on the other. You can remove the 3/8" plug bolt, screw the garden hose into the adapter & then into the motor, then turn on the water. It does not need to be full force of the standard house water pressure, but about 1/2 force. Start the motor up & let it run for long enough to get the motor warm enough to open the thermostat allowing water to flush thru the power-head. Shut off the motor, then the water & replace the plug bolt.

Read your manual. EVERY TIME you run the boat & come back in for the day, freshwater or especially in saltwater, use the recommended low pressure grease gun & lube the impeller shaft's bearings that are located under the water pump & above the impeller. This is accomplished by removing the rubber hose/grease nozzle that is snapped onto the Zerk fitting close by. Do not use a regular hand pump grease gun , but the tube type gun that has the Lubriplate tube attached for less pressure. Pump a couple of pumps, or until the Lubriplate grease starts to come out of the nozzle. Stop, wipe off the excess & replace the nozzle. This nozzle is merely a cap covering the Zerk fitting. On the opposite side of the unit as the grease fitting is a small hole in the base that is also a relief hole for the excess grease to be pushed out. It is imperative that you grease the unit each time it is used to be sure that IF any water has gotten inside, that the new grease will force the water out & not allow this water to stay trapped inside giving the chance of rust to start in the bearings when the boat & motor may set idle for a while.

Occasionally you may want to look up inside the grill grates & inspect the leading edges of the impeller blades. If they have been chewed up enough by sand or rocks, there will be a disturbance in the water flow, & it is beneficial to remove the impeller & dress, (file) the leading edge of the blades. While you are at it check the clearance of the impeller to the liner. The factory suggested clearance is 1/32". If you happen to run, or even start up in shallow water, the impeller will suck sand & small rocks up inside. These act like grit & wear the soft aluminum impeller & liner. The design is so that the impeller & liner are both made tapered, allowing the owner to adjust for wear, by shimming the impeller down, tightening the fit to the stationary liner. When there is enough wear on either or both, the clearance increases, which decreases the efficiency of the unit.

To repair this wear, pull the 5 or 6 nuts off holding the foot on, dropping the foot. Bend the impeller nut's lock washer flanges back allowing the nut to be loosened. When you pull the nut & lock washer off, catch the flat washers that are under the nut. Count them, REMEMBER THE NUMBER & place them separately in a secure place. Try to pull the impeller off the shaft. If it will not move, tap the impeller with a mallet in the opposite direction that it is normally rotating. What has happened is the impeller's key may have became wedged against the flat on the driveshaft. Tapping it backwards may loosen things enough to allow you to pull the impeller.

If that does not do it, you might have to resort to a Oxy-Acetylene torch & heat the impeller. In doing so you will ruin the nylon busing that is between the impeller & the drive shaft. But at this point, it is sacrificial & inexpensive. When purchasing a new bushing, you will need to know what size your jet unit is. On the RH side there will be (on the newer models anyway) a small aluminum plate held on by 2 nuts. Here will be the model in alphabetical letters & the serial number. The letter, or even 2 is what you need, like A, F, L or S etc. If your unit is an older on without the model number affixed, then you will need to measure the shaft size at the impeller.

If the above technique does not work to remove the impeller, you may have to use a small 3 jaw gear puller. Place one jaw claw on each impeller blade, tighten the center screw against the drive shaft's center hole & tighten until you get it to come up tight. If by tightening a bit more & the impeller does not move, heat it again & try to pull using the puller while it is warm. The white nylon bushing will begin to bubble & ooze out, but that is an indication you are softening it enough to possibly allow the pulling process to proceed.

OK, you now have the impeller off. There are more shims above the impeller, again COUNT THEM. Inspect the impeller, & file the leading edge according to the manual. If there is wear & the clearance has increased you will want to shim the impeller down decreasing the clearance. In reinstalling the impeller under this circumstance, you will need to take one of the washers from below the impeller & place it above the impeller. This pushes the impeller deeper into the tapered liner, decreasing the possible excess clearance.

You will not know just how many shims to move until you reattach the foot which contains the liner. Move one shim, bolt the foot back in place with about 1/2 of the nuts & inspect the clearance. If it is still loose, pull the foot & impeller off & move another shim from the bottom to the top. Reattach the foot & inspect again. If close, rotate the motor's flywheel to see if it is not bound up in any way. If everything looks OK, remove the foot & bend the lock washer's tabs into the impeller's nut flats. Reassemble the foot & you are back in business.

Yearly Repairs : It is recommended that every couple of years that you remove the unit from the motor, inspect, adjust the impeller if needed & if nothing else remove the bolts & reassemble them to assure that when you do need to disassemble this unit at the boat launch if the impeller needs shimmed or you get sticks imbedded in the impeller that you can get it off when needed without having to go to a marine repair shop for help.

This is even more important if you happen to fish in salt water or even bays, that depending on the tide & where you fish in the system, if there is some salinity in the water that you should do this repair every year. The combination of stainless steel bolts screwed into aluminum is not very conducive to easy removal unless it was assembled using some form of anti-seize on the threads. It may even be beneficial now to repaint the lower unit. The older units were painted red, newer ones are black & the ones supplied especially to the manufacturers will be the same color as the motors they are attached to.

The foot has slots in it to accommodate the grates. These grates, shown at the bottom of this article, are held in place by a 1/4" stainless steel rod going thru a hole on each end. The front hole in the grate is a round hole, while the rear is elongated. If this rod is not removed periodically, it too can become seized in the aluminum housing. To remove a stubborn rod, you need a short 1/4" punch with a shank about 1 1/2" long. This short punch will take a lot of pounding to the rod to start moving. Once it starts, you can then use a 12" piece of 1/4" drill rod to drive it out the rest of the way. When reassembly slightly grease this rod with boat trailer wheel bearing grease.

When assembled the grates are flush with the foot's front but protrude about 3/8" below the foot's rear body. This is probably designed to allow a stick or other debris to be carried down & away from the foot when the boat passes over it.

Remove all the bolts, wire brush off the debris, & reassemble using some kind of anti seize on the threads. You do not need a lot, but just enough to give some protection.

One thread that does not need a anti seize is the 5 or 6 nuts that hold the foot to the housing as they are stainless nuts attached to stainless studs. These nuts should be stainless steel with the aircraft type nylon lock imbedded in the nut. If not, replace them with this type. It is a good idea to check these nuts occasionally as I have had one marine mechanic tell me that if the foot comes loose during the motor running, that bad & expensive things happen.

When it is apart, do not worry if the drive shaft wobbles a little. When it is installed in the motor, the upper end is aligned into the crankshaft's spines & things tighten up as designed.

When reassembling the driveshaft & bearing housing into the main body, there are 4 bolts holding it in, but the bearing housing needs to be installed to align the housing grease holes with the grease holes in the main body. There will be 3 small 1/8" holes that in the bearing housing have a slight counterbore around them that has a Neoprene O-Ring around the hole to act as as a seal. These O-Rings are 5/16" inner hole diameter & 1/16" thick. In the main body housing are mating holes. Be sure that you have these holes align otherwise the grease will not be able to get into the bearings.

Other adjustments : The shift linkage is critical in that the cable's end needs to be set so that when the motor is in full forward that the clamshell be clamped down tight against the bottom of the unit's exhaust tube. If not, in forward movement thru the water, the clamshell will be pushed rearward & you will not get maximum speed as you will think you are in full forward as indicated by the throttle, but the clamshell will be pushed back enough as to have you in say 80 % forward but at the same time 20% reverse. Not good. (Take my word for this as I being a newby a few years ago to jets, been there-done that).

This above situation is usually seen on the old style that did not have the side locking cam plate. Also since this impeller is always spinning when the motor is running, it is hard to find a neutral position so the boat does not want to move at least some. Do not think that you can adjust the neutral position on the shifting control box to match the motor's idle. IMPOSSIBLE because of the previous paragraph.

One thing that can be done as an improvement however, is that on most throttle/shift controls that are a single handle unit, the motor can not be shifted into forward at anything other than idle speed. There is usually a lockout pin in these control boxes. Removal of it allows you to (especially with a cold motor) to shift into forward to get away from shore at a slightly faster throttle speed thereby eliminating the usual scenario of the motor dying unit it warms up.

Flushing conversions : Many of the older units did not have the flushing plug. This can be installed on most of the older units by removing the unit, inspecting it & verifying just where to drill & tap the hole. If in doubt contact the manufacturer.

With the water pump removed, showing the flush plug & grease fitting on a newer unit	Waiting to tear down an early unit for a photo to show the difference

Improvements : There are a few things that can be added to improve performance.

Shown below are two simple but beneficial add ons. The intake side skirts & a aluminum bottom extension. The 3/16" or 1/4" aluminum skirts are very beneficial if the boat's bottom is rather flat & the water has just enough small chop to allow the motor's intake to intermittently be airborne. This allows repeated cavitation which means you either stay with it & allow the motor to over-rev, or slow down & pound thru the chop. The side skirts provide just enough of a funnel to keep air from being sucked in on the sides. They are installed in the same holes that the grate retainer pins are in. The 1/4" stainless steel pins are shortened enough so that the holes are tapped to 5/16" course threads & the bolts can be also used to secure the skirts & keep the pins from moving out.

The bottom extension is made of 1/8" aluminum & provides an extension of the bottom that just lays on top of the front of the intake foot with some tension. This is attached to the bottom of the transom & eliminates the chance of water coming rearward under full power & then being forced on top of the foot's front, which can then be forced back against the front of the foot & forced upward possibly spraying into the boat at high speed.

You will notice in the RH photo below, the zinc anode bolted to the rear of the foot. Clean this with a wire wheel or sandblaster to renew the effectiveness of this corrosion inhibitor, & do not paint it.

Another improvement is to take the grates & grind a chisel edge on one side. This edge is then installed down giving less resistance as the grates pass thru the water. This edge does not need to be a sharp knife edge but merely a simple chisel shape. Some boaters even do the same on the top side thinking they are eliminating a turbulence, hence a better flow.

The newer grates have a slight bend on the front end. This is so that when they are assembled in the foot, that there is some side tension on them, eliminating a rattle.

You can also look at the aluminum impeller & if there are any roughness left from the sand casting process, using a air die grinder, remove them, again creating less resistance. Also look inside the water housing & remove any imperfections or protrusions at the same time. Remember these units are semi mass produced & anything you can do to improve a unrestricted water flow will improve your end performance.

I should not have to remind you that the motor has to be maintained in top condition otherwise all you have done to improve things on the bottom end will not really help. Then there is the bottom of the hull that needs to be maintained also.

Here is a slightly dinged impeller front edge	Here is a dressed up impeller blade front edge

Outboard Jet on a 70 hp Johnson & an old 16' Hewescraft sled showing the bottom extension & side skirts	Bottom rear view of anode, grates, impeller & skirts

Two modified grates on top & an unmodified on the bottom	Cleaned up impeller castings