Jet Outboard Impeller Shimming and Sharpening: Win Back Lost Thrust

Worn impellers and big gaps usually show up as:

• Rpm normal or higher than usual, but the boat is slower than it used to be.
• A slow, mushy hole shot, or trouble planing with your normal load.
• The motor "revs up and slips" when you punch the throttle from a standstill, then hooks up.
• Cavitation-like surging, especially when loaded.

Compare with your logged baseline of wide-open rpm and GPS speed with a typical load. A slow drop over a season of gravel running points to wear. A sudden drop after one trip points to debris or damage.

Before you take anything apart, check the things in the jet outboard care guide:

• Weeds, bark, line or a rock caught on the grate or in the impeller.
• A bent grate bar or damage to the hull bottom ahead of the intake.
• A reverse gate that isn't locking fully in forward.
• A heavier load than usual, or a dirty, waterlogged boat.
• High elevation, which reduces engine power.

If the tach shows the engine itself is down on rpm at full throttle, the problem is more likely the engine (see the loss of power guide), not the pump.

Follow the safety box above every time, even for "just a quick look": neutral, key out, lanyard pulled, battery disconnected, plug wires off if your manual says so. Tilt the motor up and lock or block it. Put a tag on the key switch or helm so nobody tries to start it while you work.

Depending on your pump, you may be able to look and measure through the grate, or you may need to remove the intake grate or the whole intake housing. Follow the manual's order for loosening bolts. Put each bolt in a labeled cup. Clean the threads and note any that are seized or corroded so you can replace them.

With a flashlight, look at:

• Blade leading edges: the edges that bite into the incoming water. Sharp and even is good. Rounded, nicked or rolled edges mean wear.
• Chips, cracks or bent blades: these are a replacement, not a touch-up.
• The liner: light scratches and gravel marks are normal. Deep gouges, or wear that is heavier on one side, are worth showing to a pro.
• Cavitation burns: frosty, pitted patches on the blades. They often go with dull leading edges or running the motor trimmed out too far.

Slide a long feeler gauge between a blade tip and the liner. Find the thickest blade that slides through with a light drag. Turn the impeller slowly by hand (the motor must be disabled) and check every blade, at a few spots around the liner. Write down the largest and smallest readings.

Compare your readings with the range in your service manual:

• Within range: the gap is not your problem. Leave the shims alone.
• Larger than the range: move shims (step 8).
• Very uneven around the liner: the liner or intake may be off center, or the impeller is running off center. SBT, a personal watercraft pump parts supplier, notes that a gap that is tight on one side and loose on the other points to a bent shaft or worn bearing, which new shims or a new liner won't fix. Ask a pro before you shim.

SBT also says a liner with a glazed, polished or discolored band where the blade tips run has been slipping and heating, and is a sign it's near the end of its life even if the gap still measures close to spec.

Bend back the tabs of the nut retainer or pull the cotter pin, then remove the nut. Slide the shims and the impeller off the driveshaft, noting the key that locks the impeller to the shaft. Lay every part out in the exact order it came off, and count the shims above and below the impeller. A photo helps.

On most pumps you move one shim at a time from the stack on top of the impeller to the stack behind it. That pushes the impeller deeper into the tapered liner. Your manual confirms the direction for your pump. Then:

• Reinstall the impeller and key, the remaining shims, and the nut.
• Remeasure the gap.
• Repeat, one shim at a time, until the gap is within range.

Don't go tighter than the manual's minimum. A blade that rubs the liner will scrape both parts and can lock up the pump.

If every shim is already behind the impeller and the gap is still too large, the impeller, liner or both are worn out and need replacing.

We could not find a jet maker's sharpening procedure published on the web; where one exists, it is in the jet section of the service manual for that pump. So the rule is simple: if your service manual describes sharpening, follow it exactly; if it doesn't, have a jet dealer do it. Don't copy a method from a forum or from another brand's pump, because the blade shape and the edge profile differ from pump to pump. If your manual does cover it, the points to watch are:

• File only the side and angle the manual shows, and don't change the blade's working face.
• Aim for the edge the manual describes. It is not always a razor edge.
• Keep the blades even so the impeller stays balanced.
• Stop if a blade is chipped, cracked or bent. That impeller is a replacement.

A filed impeller has less metal on it, so it can run out of shim adjustment sooner. Check the gap again after any sharpening (steps 6 and 8).

Torque the impeller nut to your manual's spec. Fit the nut retainer and bend its tabs, or fit a new cotter pin, as the manual describes. A loose impeller can rattle at idle and damage the shaft. Many owners keep a spare nut retainer aboard, because a broken one costs far more than the part.

Turn the impeller by hand through several full turns. It should spin freely with no scraping or tight spots. Recheck the gap on every blade one last time.

Refit the intake housing and grate with anti-seize on the bolts if your manual calls for it, and torque them to spec. Grease the pump bearing (see the jet outboard care guide). Reconnect the battery and plug wires, and put the lanyard back. In a safe spot, run the boat with your normal load and compare rpm, hole shot and top speed with your baseline.

An impeller can settle slightly once the pump is loaded. After the first outing, make the motor safe again and recheck the gap. Then log the date, the shim count above and below, and your new rpm and speed in BoatMend.