Troubleshooting a Rough-Running Outboard

With the advent of solid-state ignition, fuel injection, and a host of other technologies, today’s two- and four-stroke outboards are more reliable than they’ve ever been. They’re so dependable, in fact, that we often take their reliability for granted. Still, there are others of us out there that have older, more temperamental outboards and, believe it or not, even relatively new outboards can act up at times. It’s related to Murphy’s Law of Boats.

So, what should you do when your outboard engine is puttering instead of purring? Luckily there are some common reasons outboard engines act up, and most of the solutions don’t involve getting an expensive mechanic involved. Some problems can even be solved with the twist of only two fingers. Read on to find out about some steps to take when your outboard’s got an attitude problem.

Check the Tank Vent and Hoses

If we had a dollar for every time our boats started sputtering and running rough because we forgot to open the fuel vent on our portable fuel tanks, well, we’d have quite a few bucks rolling around. If you’ve got a portable tank and your engine is misbehaving, the first thing you want to do is check to make sure the valve on your portable tank is fully open. If it isn’t, it will cause your engine to become fuel starved, run rough, and eventually stall. Simply twist it open and monitor your engine for improvement.

If your outboard motor isn’t running at its best, check the fuel vent on your portable tank to make sure it’s open. Photo courtesy of Attwood.

If your boat doesn’t have a portable tank, but instead an integral tank with a fuel vent that’s plumbed to the outside of the boat, inspect that vent. Wasps, spiders, and other critters are well-known for clogging these fittings up, and that can significantly affect engine performance. If you do find a clogged vent, simply clean it out with a pair of tweezers. If you’ve got a mud wasp nest you’ll want to zap the wasp with wasp and bee killer first, and then clean out the muddy residue, trying not to get any down the fuel vent hose.

Also, check all your fuel hoses from the tank to the engine. A kinked or pinched hose can cause significant performance problems in an outboard engine, especially at higher RPMs, while pinholes in fuel lines can introduce air to the fuel, as well as create a safety problem.

External fuel vents can become clogged with the nest-making materials of spiders, wasps, and other insects. Be sure to check this fitting if you’re having outboard engine problems. Photo courtesy of Sea Dog.
External fuel vents can become clogged with the nest-making materials of spiders, wasps, and other insects. Be sure to check this fitting if you’re having outboard engine problems. Photo courtesy of Sea Dog.
Fuel fittings, such as this one, can corrode and clog up, causing significant performance issues in outboard engines. Photo courtesy of Yamaha.

Check the Fuel Connector

Another trouble spot in the fuel delivery department—if you’ve got a portable tank setup—are the snap-in connectors that attach the fuel line to your outboard. A haven for corrosion, the ports inside these connectors often clog up with nasty bits of corroded, gas-soaked metal powder that can impede fuel flow. The ball bearings and check valves inside some of these connectors can also fail.

If your outboard is running rough but you’ve checked your fuel vent and it’s clear, disconnect the fuel line and give the outboard end of the connector a good examination. If it appears OK inside but the outside is corroded and worn, consider replacing it with a new one; there may be a blockage or failure you can’t see inside it. Once you’ve replaced or unblocked the connector, reinstall it and check the engine for any improvement in the way it runs.

Check the Fuel Filter/Water Separator

Though many smaller boats using portable tanks are not equipped with these fuel filter/water separator devices, most boats with outboards that draw fuel from an internal fuel tank are. The purpose of these filters is not only to filter particulate matter out of fuel before it reaches your outboard, but also to separate any water that may be present in your fuel. These filters sometimes become clogged or filled with water, causing an engine to run poorly.

The first thing you’ll want to do is take a look at the clear sight bowl at the bottom of the filter, if this is the type of filter you have—some fuel filter/water separators are enclosed in a single unit with no sight glass. Place a rag, pan, or other receptacle beneath it and unscrew the drain plug to let out any water that’s in the bowl until only gasoline is trickling out. The presence of water is a tipoff that you may have a bad load of fuel in your boat. You may want to have it professionally filtered, as the presence of too much water can cause engine problems.

If you can’t remember the last time you changed your fuel filter/water separator, you ought to go ahead and swap the element, taking note of any water in the sight bowl. Excessive amounts of water here can mean you have a tank of bad fuel. Photo courtesy of Suzuki.

Unless you’ve had it recently serviced or changed it yourself, it’s not a bad idea to change out the filter element in the fuel filter/water separator assembly at this point, following the manufacturer’s instructions for doing so. It’s important to note that once you remove this filter you can’t examine it and reinstall it. Luckily most are a relatively inexpensive purchase.

Some portable outboard fuel tanks have inline primary fuel filters installed in the fuel supply line, too. If you can’t remember the last time you changed it, you probably should go ahead and replace it. Last, always use fresh fuel. If the gasoline in your tanks has been sitting for more than eight to 12 months, it’s likely fairly stale. And, always add fuel stabilizers designed to combat problems associated with ethanol fuel at every fill up. No excuses.

Check/Replace Secondary Fuel Filters

Many outboards—especially newer models—have a slew of onboard fuel filters that can become clogged and detrimentally affect performance. On the simplest of outboard engines these are easily replaceable, often installed inline between the fuel inlet and the carburetor. Some smaller or older outboards may not even have one. Newer, fuel-injected outboards can have as many as 12 of these filters—two or three primaries and one or more for each fuel injector.

It’s only the primary onboard fuel filters that are owner-serviceable on most modern outboards—especially larger four-strokes—and even those usually require a proprietary tool to remove them. Other fuel filters, such as those aforementioned fuel injector units, and vapor separator tank filters, should be changed by the pros. Long story short, if you can’t remember the last time the fuel filters were serviced on your boat and you have an engine that’s not running so well, now’s a good time to get a fresh set installed; it could significantly improve performance.

Check/Replace Air Filter

Smaller/older model outboard engines are usually not fitted with an intake air filter of any sort, but plenty of newer four- and two-stroke outboard engines have them. If they get clogged with dust and dirt, this can affect engine performance significantly by starving the engine of air.

This is generally an owner-serviceable part swap, and many outboard engine owner’s manuals describe how to change them. Once you’ve located the filter, give it a thorough inspection. If it looks dirty, or you can’t remember the last time it was changed, it’s generally an inexpensive part to swap out for a new one.

Check Spark Plugs/Ignition

Fouled or damaged spark plugs can cause all sorts of problems in any engine, much less in an outboard. Their condition can also indicate lurking internal problems, too. If you’ve eliminated fuel problems and air problems, consider pulling each spark plug and giving it a thorough inspection.

Plug electrodes—the ends that sit in the engine—that are worn, appear burned, or have dirty or oily deposits should be replaced with properly gapped plugs. Hold on to your old plug(s) if they were fouled with oil or fuel gum and then have an outboard mechanic take a look to diagnose the possibility of more serious problems inside your engine.

A set of fresh spark plugs can often smooth a rough-running outboard engine, but make note of any severely fouled or damaged spark plugs. This can point to problems inside your engine. Photo courtesy of NGK.
A set of fresh spark plugs can often smooth a rough-running outboard engine, but make note of any severely fouled or damaged spark plugs. This can point to problems inside your engine. Photo courtesy of NGK.

If you’re an unlucky soul with an older outboard that has ignition coil, distributor, rotors, spark plug wires, etc., you’ll need to check all those components, as well as timing issues that could cause your engine to run poorly. The good news is that there are many tutorials online for diagnosing and repairing ignition system components in older outboards yourself, versus taking the engine into a shop for repairs.

Check/Clean Carburetor

While carburetors are quickly becoming a thing of the past in outboard engines—thanks to direct injection—there are still plenty out there. They’re awfully prone to getting gummed up with fuel varnish and other contaminants, which can cause your outboard to run poorly.

The first and easiest thing to do is to spray carburetor cleaner in your carburetor. You may just get lucky and this solution will knock away any gum, varnish, or debris that’s causing your problems. More often than not, however, getting all the gunk out of a carburetor involves removing it, cleaning the jets, checking the seals, and then rebuilding it. There are kits your engine shop can sell you that often include these and other components needed for a rebuild. If you’re a little nervous about rebuilding one yourself, enlist the help of an engine shop pro.

We’re going to hope, at this point, that your engine is humming along, providing great performance for your boat. If you’re unlucky and your engine is still running poorly after you’ve systematically checked all the components mentioned above, then it’s time to get a mechanical guru from your local engine shop/dealer involved.

An inefficient boat engine may well be a source of pollution. For more information check out our eco friendly boating guide. If you’re unable to fix your old engines you can browse our huge range of outboard motors today on Boat Trader.

Happy motoring.

Written by: Gary Reich

Gary Reich is a Chesapeake Bay-based freelance writer and photojournalist with over 25 years of experience in the marine industry. He is the former editor of PropTalk Magazine and was the managing editor of the Waterway Guide. His writing and photography have been published in PassageMaker Magazine, Soundings, Fly Fishing in Salt Waters, Yachting Magazine, and Lakeland Boating, among others.


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