Jet Outboard Care: Grease, Intake Grate, Impeller Clearance and Shallow-River Habits

A jet outboard does not follow the prop-motor rule of setting the anti-ventilation plate relative to the hull bottom. Its height is set so the front edge of the intake sits at a particular spot relative to the bottom of the hull, which your jet maker specifies. Yamaha sells its jets by the transom they fit (20-inch or 25-inch), not as short or long shafts.

You don't need to measure anything if the boat runs well. Signs the height may be wrong:

• Too high: the pump sucks air, rpm jumps and the boat loses grip, especially in turns or rough water.
• Too low: lots of spray around the motor, extra drag and a pump that grabs more gravel.

Changing height means re-drilling or a jackplate change. Let a dealer who rigs jets do it, with the jet maker's figures.

The pump needs smooth, solid water flowing into the intake. Anything mounted on the hull bottom ahead of the pump (a transducer, a keel guard, a damaged strake, a big dent) can trip that flow and feed air into the pump. Look along the bottom ahead of the motor for dents, bent strakes and add-ons. Jets work best on flat or shallow-V hulls; a deep V can make them suck air in turns. Honda Marine's field-service manager put it plainly in Boating magazine: a jet works on a flat bottom or a slight V, but not on a true deep-V. If you are thinking of converting a deep-V boat to a jet, talk to a dealer who rigs jets first.

With the engine off, key out and lanyard pulled, tilt the motor up and look at the grate. It should have:

• all bars straight, with none bent, cracked or missing
• every mounting bolt present and tight
• no weeds, fishing line, bark or rocks jammed between the bars

Outlaw Eagle, a river jet boat builder, lists a clean, undamaged grate with tight hardware as a basic check. A bent grate lets rocks through to the impeller. Never run with the grate removed.

The driveshaft runs in a bearing inside the pump, with a grease fitting on the outside of the pump housing. Jet pumps need grease far more often than a prop outboard's grease points, because there is no gearcase full of oil around that bearing. Published advice varies: a Honda Marine field-service manager told Boating magazine (2018) that the bearing should be lubed after every use, and Outlaw Eagle suggests every 10 to 20 hours, more often in harsh conditions. Intervals differ by brand and model, so the jet section of your manual is the one to follow; if you are unsure, greasing after each day on the water is the safe habit.

• Outlaw Eagle adds a simple check: after a run, feel the pump housing. If it is noticeably warmer than usual, a seal or bearing may be starting to fail, so have it looked at.
• Wipe the fitting clean, then pump in the grease type your manual names, the number of strokes it says.
• On many units, old grease pushes out through a vent hose or opening. Look at it: normal grease looks like the new grease. Milky, gray or watery grease means water is getting past the seals. Note it and grease again sooner; if it stays milky, have the seals checked.
• Outlaw Eagle warns that both too little and too much grease cause trouble, so count your strokes rather than pumping until something gives.

Most gravel damage happens at low speed, not on plane. At idle and while coming off plane, the pump pulls water (and gravel) up from the bottom.

• Get on plane in water deep enough for the motor, and come off plane only where it is deep enough to sit.
• Don't idle, power-load onto a trailer or try to "push off" a gravel bar with the jet. Shut down, tilt up and walk or pole the boat off.
• Running over sand or gravel at low speed is what dulls impellers. Plan your stops and launches in deeper water.

A sudden loss of push, rpm that climbs with no added speed, or a new vibration usually means something is stuck on the grate or in the impeller.

1. Throttle to idle, shift to neutral, shut the engine off, take out the key and pull the lanyard.
2. Tilt the motor up.
3. Clear the grate with a stick. Look up past it with a flashlight for anything wedged in the impeller.
4. A rock jammed between the impeller blades usually needs the grate removed (engine still off) and a tool to pry it out. Don't force it if it won't come; a stuck rock can chip the impeller or score the liner.

The PWC intake grate guide has more on clearing debris safely.

On many jet outboards the cooling water pickup is in or near the pump, so the same debris that clogs the grate can starve the engine of water. Check the telltale (pee stream) every time you start and after you've run through weeds or gravel. If it is weak or stops, shut down and clear the intake; if the overheat warning comes on, see the engine overheating guide.

The impeller works best with a very small gap between its blade tips and the liner around it. Gravel and sand round off the blade edges and wear the liner, and the gap grows. Thrust drops a little at a time until the boat struggles to plane with a load.

What you can safely look at (engine off, key out, lanyard pulled, grate off):

• Blade edges: sharp and even is good. Rounded, chipped or bent leading edges mean wear.
• The liner: look for deep scoring or gouges.
• The gap: many jet pumps let you lower the impeller deeper into the tapered liner by moving thin shims from one side of the impeller to the other. Your service manual gives the target gap and how to measure it with long feeler gauges.

Re-shimming and touching up dull blades are jobs many river boaters learn to do, but only with the service manual's procedure and specs. A badly gouged impeller or liner is replaced, not adjusted.

Two very different problems feel similar from the helm. Check the tachometer:

• Rpm is normal or higher than usual, but the boat is slower: the pump is slipping. Look for a clogged or bent grate, weeds on the impeller, a dull impeller with too much clearance, hull damage ahead of the intake, or a motor mounted too high (steps 1 to 8).
• Rpm is lower than normal at full throttle: the engine is not making its power. Work through the loss of power guide (fuel, filters, plugs, warnings).

Write down your normal top rpm and speed with a typical load when the pump is fresh. That baseline makes the next diagnosis much easier.

Look at the nozzle for cracks or rock damage and make sure the reverse gate (the scoop that swings down over the nozzle) moves freely through forward, neutral and reverse with the shift lever. Lubricate the linkage pivots where your manual shows. A sticking gate can leave you without neutral or reverse at the ramp. The shift cable adjustment guide covers basic cable checks.

A jet outboard cannot use ordinary prop-style flush muffs, because there is no gearcase water inlet to cover. Your manual describes the flushing attachment for your model and whether the engine runs while you flush. Built-in flush ports are an engine-off method unless your manual says otherwise. After salt or brackish water, Outlaw Eagle also suggests running in fresh water for a few minutes when you can. Mercury builds its jets with anodes and corrosion coatings; check and replace the anodes as described in the anodes guide.

Follow the winterize outboard guide for the powerhead (fuel stabilizer, fogging, oil change on a four-stroke). For the pump:

• Grease the bearing before storage, so fresh grease, not water, sits in it all winter.
• Tilt the motor down so the pump drains, and make sure the grate and nozzle are clear.
• Look over the impeller and liner while the boat is out, and order shims or parts before spring.