7 Reasons Your Outboard Will Not Throttle Up and How To Fix Them

Why is my outboard bogging down?

Why does my boat hesitate when I accelerate?

Why won't my boat motor go full throttle?

These are common questions for those who love to boat, only to suffer from poor performance from their outboard motor when out on the water. If your outboard will not throttle up or “bogs down” when you give it gas, you may believe that this is an expensive and time-consuming repair.

The truth is that when your outboard will not throttle up, there’s plenty you can do to fix the most common issues and improve performance once and for all. Let’s take a look at seven reasons why your outboard won’t throttle up and how to fix them:

• Tangled Propeller
• Poor Gas Quality
• Clogged Fuel Filter
• Fuel Hose Replacement
• Faulty Impeller
• Bad Spark Plugs
• Malfunctioning Fuel Pump

1. Tangled Propeller

If your outboard motor will not throttle up, the issue may be as simple as a tangle. Debris such as fishing line, excessive seaweed, beer can rings, or nearly any other material can wrap around the propeller and its spindle. This increases the friction and causes the bog, preventing it from rotating to propel your boat through the water. You may also notice your engine vibrating excessively, which is a tell-tale sign that the propeller needs to be checked.

How to Fix It:

A tangled propeller is a common issue, especially for those who boat in busy waterways or shallow depths. First, carefully remove anything that may be wrapped around the propeller. Once removed, be sure to look for any signs of damage. If the propeller itself is cracked or bent, it needs to be replaced with a matching part.

Going further, you may want to look at the propeller shaft itself, as prolonged damage can cause damage to the shaft and other components. Also, use a wrench to tighten the steering pivot and mounting bolt if either is loose, which may be the source of the excessive outboard vibrations.

2. Poor Gas Quality

The fuel used for your outboard motor can be one of the primary reasons why your outboard motor is bogging down. And this is most likely caused by the ethanol content of the gas. Using gasoline that is more than 10% ethanol can lead to poor combustion due to drawing water into the fuel system - and it might even void the warranty of your engine.

Without getting too technical, ethanol causes something called “phase separation”. Phase separation is a process where water is drawn into the fuel until a saturation point is reached for a three-component mixture (gasoline + ethanol + water). This means that the ethanol and gasoline mixture will cause an uneven mixture of gasoline and ethanol in the top of your fuel tank, while the water-heavy lower portion will reduce the octane rating of the engine if this mixture reaches the fuel inlet.

How to Fix It:

If you suspect that you have bad gas, completely remove it from your fuel tanks, fuel lines, primer bulb, and from the rest of the outboard motor. Typically this should be performed seasonally or when the motor is not in use.

Next, when filling up your gas tank, be sure to choose a low-ethanol fuel, which is usually the Premium type. Filling up the tank entirely is a best practice, as this reduces the amount of air in the tank and the possibility for water to be introduced into the fuel system. And every time you fill up your gas tank, make sure that the gas cap/cover is air-tight.

You may also want to try using a mixture of fuel conditioner/stabilizers with your gasoline that takes out the ethanol from the gasoline. Application is as simple as pouring the allotted dosage into your fuel tank based on the capacity of fuel.

3. Clogged Fuel Filter

Another common reason why your engine is bogging down when you apply throttle is a clogged fuel filter. The fuel filter’s primary purpose is to strain out any debris and dirt that may reach the engine, but the fuel filter itself can become blocked if there is enough debris. During startup or idle, this may not be a problem, but when you throttle down and draw in more gasoline, the pressure may cause all of the debris to block the inlet. This, in turn, causes the fuel pump to work harder and puts strain on your motor.

How to Fix It:

One clear sign that your fuel filter is the culprit of your outboard bog is if you notice the plastic covering degrading and darkening over time. Over time, the fuel filter becomes brittle and may even leak gasoline. Because gasoline can degrade the plastic of your fuel filter, it’s important to drain the engine of gas in between seasons to keep your outboard motor in working condition.

Start by inspecting the fuel filter. Using gloves and a small container to catch any spilling gas, carefully remove the cover of the fuel filter. Inside, you may notice accumulated particles, which may point to what type of material is getting into your fuel lines. Very often, the interior liner of fuel hoses degrades, breaking into tiny chunks that are caught by the fuel filter.

4. Fuel Hose Replacement

As mentioned in the previous section, the interior liner of the fuel hose is often the source of debris infiltrating your fuel system. This debris bogs down outboard motors when applying throttle, causing an uneven performance.

Over time, fuel hoses break down due:

• Heat
• UV rays from the sun
• Bad gas (high ethanol)
• Fuel left in the lines during the off-season

How to Fix It:

A complete replacement is needed to address this problem. More of the interior liner will degrade over time, causing repeated issues.

Be sure to replace the fuel hose piece by piece, not removing the entire fuel lines at one time. The reason for this is that it is easy to forget a part (i.e. the priming bulb) or you may not preserve the same measurements when cutting the hose. Also, be sure to adequately secure any hose clamps to prevent air, water, or debris from entering the engine.

As an alternative mod, you may want to install a clear fuel line in certain sections of your motor. The purpose of this modification is to give you a clear window on whether air, water, or debris is present in your fuel. When installed, you can observe the presence of contaminants from foggy lines, air bubbles, and discoloration.

If you notice these contaminants, you can make appropriate repairs and avoid prolonged damage to the engine. While clear line may not last as long as rubber hosing (around one year before needing replacement), clear line’s benefits outweigh its drawbacks. Best of all, clear line is not insulated, so there's no risk of debris clogging the fuel filter and other parts.

5. Faulty Impeller

For most 2-stroke and 4-stroke outboard boat motors, the cooling system uses water that is circulated through the engine to regulate temperature and prevent overheating. Under normal operation, the water pump draws water into the intake grates on the lower unit, pulling water through the cavity with a rubber impeller that is keyed to the drive shaft on top of the lower unit.

When the impeller is worn, clogged, or misshapen, the ability to cool the engine is drastically reduced. This in turn leads to the engine overheating, triggering mechanisms that prevent the engine from functioning at high temperatures. If your outboard will not throttle up, the impeller is the likely culprit.

How to Fix It:

First, look at the telltale hole, which emits a small, visible stream of water from the outboard motor. This helps indicate that cooling water is flowing. Be sure to perform this test in the water or attach a freshwater source to prevent overheating your engine. Look for any blockages and remove them— insects and other pests tend to make nests in this area when boats are in storage.

Next, take a look at your impeller. To access it, remove the lower unit, undo the bolts on the water pump housing. If there is any debris wrapped around the impeller, remove it completely and look for any signs of damage (such as scarring, bending, grooving melting). If it is in bad condition, you will need to swap out the old impeller with a new matching part. Luckily, this is an easy replacement requiring only a few tools.

6. Bad Spark Plugs

Your outboard engine’s ignition system is responsible for igniting the gas, firing up the engine, and keeping it running. If your outboard will not throttle up or your outboard motor dies at idle frequently, chances are you may have problems with your spark plugs.

How to Fix It:

Spark plugs are an easy fix, requiring only a spark plug socket and wrench to remove them from their housing. There’s an easy method to checking and replacing spark plugs in your outboard motor:

• Look for signs of corrosion and traces of salt on the spark plugs and ignition coil wires. These signs are often caused by evaporated saltwater.

• If the spark plugs are in good condition, you will need to check the ignition spark with a spark gap tester.
• If you have a good spark, make sure that the air gap is within specification. Adjust the air gap if needed.

If you don’t have a good spark, the spark plug caps or ignition coil leads could be causing the issue. Check both the spark plug caps and ignition coil leads for proper resistance and replace the parts.

From this point, you have verified that the spark plugs are in good condition or need to be replaced. However, if this doesn’t solve the issue, the next step is to check the ECM output peak voltage, as well as the condition of other electrical components like the electric oil pump and oil level sensor.

7. Malfunctioning Fuel Pump

The fuel pump is responsible for bringing gas to your engine from your fuel tank. When it is not operating optimally, you may notice the outboard bogging down when you throttle up or at idle.

How to Fix It:

Perform an outboard fuel pump test by removing the spark plug wire boot from the spark plug. Next, remove the spark plug with a plug socket and wrench. Attach a compression gauge into the spark plug socket. If you have an electric motor, you may need another person to crank the engine over while you twist your throttle position open. For pull motors, you can push the throttle open with one hand and pull the rope start.

If you notice a rating from the compression gauge at less or more than what is rated for your outboard, it is time to replace the fuel pump.

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Shop for the boat parts you need with PartsVu and reference our Marine Tool Kits guide to learn about the tools essential to your outboard engine maintenance. If the troubleshooting guide above provided no results, consult a mechanic. They may find the issue, or you may need to replace your outboard altogether.

This content was designed to serve as a general guide. Persons who are not familiar with marine engine repair and who have not been trained in the recommended servicing or installation procedures should have the work performed by an authorized marine mechanic. Improper installation or servicing of parts can result in engine or boat damage, personal injury to the installer, or harm to persons operating the product.