Blasting over the waves on a jet ski is high-tier motorsports fun. I say this having done it a few times off the shores of Lake Erie. However, if your motor will start on land but not on the water, you’ll need to figure out why.
If your jet ski starts on land but not in water, there are four potential causes: an off-set reverse gate, impeller intake or exit nozzle debris, insufficient cylinder compression, or improper carburetor pop-off pressure.
In this article, we’ll examine each reason and provide a solution. Fingers crossed, you’ll soon be back on the water with full power.
Why Your Jet Ski Runs on Land but Not in Water
If you own a jet ski that runs fine on land but poorly in water, remember that water demands more power for a motor to work. Said motor may fire right up without resistance, but when systems are below nominal, any extra stress placed upon them hurts.
So let’s look at some problems your PWC may have in water.
Improperly Set Reverse Gate
Most jet skis come with an option for reverse; however, the technique does not use a transmission. A lever on the jet ski lowers a curved “gate” over the thrust nozzle, redirecting the flow of water under the ski, and making it move backward.
At full throttle, the reverse gates should be wide open. While in neutral, they come down to cover about 80% of the thrust nozzles (or nozzle). To see a reverse gate in action, have a look at this video:
Debris in Impeller Intake
On the trailer, an impeller will spin to life at higher revolutions per minute (RPM) than it does in the water. This is because the craft is designed to run against resistance from the water.
It only takes a tiny bit of foreign material to affect how things go on the waves. Already the motor is doing more work. With the added stress of, say, a weed or a bit of moss or some dirt, your jet ski might not run properly.
Additionally, with less water running through the system due to blockage, a collapsed exhaust hose could occur. This is known as cavitation—the induction of air into pressurized fluid. In this case, the jet ski will likely stall out altogether.
Low Compression in One or More Cylinders
To run strong, your jet ski motor pushes a mixture of air and fuel into a cylinder. This is known as compression. Performance is reduced if compression is low in one or more cylinders.
A 130-150 PSI (896.32 – 1.03 KPa) compression is good for jet skis, with no more than a 5%-10% difference between cylinders. If you suspect low compression in your PWC, you will need a compression tester.
Fixing this issue can be quite a chore. Damaged valve seals or worn piston rings are common perpetrators. Or, if you’re lucky, the ski may just need a new battery or new starter.
Improper Carburetor Needle/Spring Pop-off Pressure
If your jet ski uses a carburetor (or carburetors), it may be suffering from incorrect pop-off pressure of the jet needle. A jet needle rests inside a hollow cylinder in the carb, regulating the flow of fuel through that cylinder. Pop-off pressure is the term used for how much pressure must be applied to the needle before fuel can pass.
If the carb needle becomes jammed or is demanding too much pressure to induce pop-off, the ski may still run just fine on land, but have trouble with the extra effort required for it to run in the water. See tip number 4 below for a way to fix this.
Starting Your Jet Ski on Water–4 Ways to Make it Happen
Depending on what’s preventing your ski from starting in the water, the solutions will vary. Let’s go through the above issues one at a time and get them resolved.
1. Adjust the Reverse Gate
If your jet ski sputters on the water or creeps forward too fast in neutral, adjusting the reverse gate can make things great again. Here’s how to do it.
Go to the thrust nozzle in back of the ski. The reverse gate looks like a big hood that hovers over the nozzle. In the back of the gate (usually) you’ll find an adjusting nut. The nut will be attached to a rod, which will detach from the gate when loosened. Turn the adjustment cylinder on the end of the rod clockwise to raise the gate, or counter-clockwise to lower it.
If you are not comfortable with adjusting the reverse gate yourself, have it looked at by a professional.
2. Clean the Impeller Intake
As mentioned, gunk in the ski’s intake is never a good thing. To clean it, you’ll need to get up close and personal with some delicate parts.
You can clean the jet pump/impeller of a jet ski with your hands or even a long screwdriver. However, if the gunk is deep or really lodged tight, you may need to dismantle the impeller. Either way, it’s a simple matter of getting that unwanted stuff out of there.
3. Test the Cylinder Compression
Too much or too little compression in a motor’s cylinders will result in improper air/fuel mixture. Normally, compression that is too low is what jet ski owners worry about. Remembering that the motor should run between 130-150 PSI (see above), it’s time for a compression test.
To test cylinder compression:
- Remove the spark plugs from all cylinders.
- Take off the fuel pump and fuel injector fuses.
- Attach the compression tester to one cylinder.
- Open the throttle all the way and crank the engine.
- Give the engine four or five cranks and check the needle.
If the results kick back low, start your deductions on the easy end. Make sure the ski’s battery and starter are good. Next, check the fuel line for aging and damage. Should all of these prove good, it may be time to have your PWC’s piston rings and head gasket checked at a reputable jet ski dealer.
4. Adjust the Carburetor Needle
A carburetor’s jet needle has grooves cut around the top for the attachment of a tiny clip. Depending on where you choose to set this clip, the needle will rest higher or lower in the carb, allowing for rich or lean running of the jet ski motor.
Jet needles are attached to a spring that must be of the correct size and tension in order for the needle to allow the proper amount of fuel into the carb. Correct settings for both spring and needle should be available in your jet ski manual. In the meantime, here is an instructional video on how to test and set carburetor pop-off pressure:
Jet Ski Starts on Land but Not in Water – Final Thoughts
With the above information, you should be able to pinpoint why your jet ski runs on land but not on the water. The process involves a lot of trial and error, but as you tick off the boxes, you’ll be closer and closer to a solution.