SEARCH THE BOAT GALLEY 
How much do you really know about carburetors? If you’re like me, not much beyond the fact that if a gas engine isn’t running right, it’s probably the carb’s fault. While some gas engines – inboards, outboards, generators being the most common on cruising boats – have direct fuel injection, most have a carburetor.
Alejandro Uria is the co-founder of MarineCarburetors.com and I asked him if he could shed a little light on the most frequent carb problems. Here’s what he put together.
A carburetor is an important component of the correct functioning of your boat’s gas engine, whether inboard or outboard. It mixes the right amount of fuel and air for combustion, with the mix being sucked into the engine by a vacuum. As the vacuum pulls the air down through the carburetor, the carburetor directs the fuel from the fuel bowl to mix with this incoming air, forming a combustion mixture that enables the engine to run.
As the engine gains speed while working, the fuel flows into the venturi through metering jets. The air and fuel mix and flow down into the cylinders via the intake manifold and burn to produce power. At idle, the fuel enters the carburetor through a small idle port location right above the throttle plate. The process is quite basic. However, a carburetor relies on other add-on devices for idle control, emissions, and starting.
Typical Problems with a Marine Carburetor
A well-maintained carburetor will work effortlessly. The engine will start swiftly, accelerate, and idle smoothly. A clean working carburetor ensures a healthy fuel economy and emissions for years. However, a lousy carburetor may cause a multitude of problems with your engine, such as rough idle, stalling, flooding, poor fuel economy, and hard starting.
I have spent over 20 years rebuilding carburetors for many different types of applications. You don’t always need to replace one and that’s why I put together this guide to help you identify the symptoms of a carburetor having problems, and learn some possible ways you can work on damage control.
Hard-Hot
Starting
Hard hot starting is a problem that usually makes us think that the carburetor needs to be replaced, however, the real issue is that too much heat starts accumulating in the carburetor, fuel pump, lines, and its vicinity. It makes the fuel boil, creating a vapor lock. This condition makes it harder for a hot engine to start.
This usually doesn’t require rebuilding or replacing the carburetor; there is a more straightforward solution. I recommend rerouting the fuel lines away from exhaust pipes, manifolds, and other sources of heat. If that is not possible, then other options are to either wrap insulation around the fuel lines or insulate them by fabricating a heat shield.
Other components may cause your carburetor to face hard-hot starting problems, such as a faulty ignition module, improper battery connections, and a bad starter. Check them out before digging into your carburetor.
Rough Idle
As carburetors age, the material around the throttle shaft located in the base of the carburetor wears out from constant movement. It causes an increase in the tolerance between the base and the t-shaft. This increase allows air to leak, making the engine run rough at idle speed. It also makes the necessary adjustments very difficult or impossible to perform.
This issue can be solved by inserting new brass bushings around the throttle shaft. These bushings restore the tightness of the throttle shaft and eliminate the vacuum/air leaks. This fix allows adjusting the carburetor properly and makes the engine run nice and smooth.
Hard Cold Starting Issues
The problem starts when a choke fails to close; this causes a rich fuel mixture while the engine is cold. Again you do not have to worry about rebuilding or replace your carburetor altogether.
The choke housing contains a heat-sensing bi-metal coiled spring, which expands when hot and contracts when it cools down. This spring controls the opening and closing of the choke plate located on top of the carburetor.
You can find the spring housed in a black plastic choke by the side or down on your intake manifold. The black plastic cover contains an electric heating mechanism to heat the spring. Alternatively, heat from the engine turns the spring choke on your intake manifold.
A burned-out heating coil will cause the spring mechanism to malfunction. The coil can also stop functioning If it is not receiving voltage or if the heat riser is loose or rusty. Either one can cause the choke to stay open for too long, making the engine run rich and too fast, or completely close it.
Simple cleaning of the choke linkage and mechanism with a little bit of adjustment will fix this issue. In the case of a defective choke, you can get a choke repair kit or a new bi-metal spring to eliminate this problem.
Stumble Under Load
Sometimes your engine can hesitate or misfire when it is under load. A faulty power valve in your carburetor causes this issue. A carburetor allows intake vacuum to direct the fuel into metering circuits. The throttle opens wider and the intake vacuum drops as the engine load increases. It reduces the fuel flow causing the fuel mixture to go lean.
There is a vacuum-sensing mechanism within the power valve of the carburetor, which opens to increase fuel intake when vacuum drops. If this mechanism fails or the power valve is clogged, you will have to replace it. You can usually get a new power valve with a carburetor rebuild kit.
It is advisable to check other components that may cause misfiring or stumbling of the engine. These may include a weak ignition coil, a cracked distributor cap, or faulty spark plug wiring.
Flooding
This issue may or may not be the carburetor’s fault. Dirt can sometimes enter the needle valve, preventing it from closing properly. As the valve fails to shut, fuel overflows. The fuel either spills out of the bowl’s vent or down the carburetor’s throat. It will make all the spark plugs wet, preventing the flooded engine from starting at all. It is a safety hazard and may cause a fire as fuel overflows out of the carburetor on to a hot engine.
The fuel bowl within the carburetor has a float inside; this may also cause flooding if the bowl has a leak or the float is set too high. All you need to do is to replace the float.
Speak to Experts
Need help with a carb? Fixing or maintaining a carburetor is not necessarily hard; however, it’s not a quick DIY project either. Consult with an expert. It’s always easier when you know a bit about the likely source of the trouble from the information above. I’ve spent over 20 years working on marine carburetors and am happy to tell you if you need to replace your carb or not, and if so, help you find the right one. Visit our shop MarineCaburetors.com for a big selection of rebuilt carbs for multiple marine applications: Mercruiser, Volvo-Penta, OMC, Crusader, and more.
Warning: These products can expose you to chemicals, including lead, which is known to the State of California to cause cancer or birth defects or other reproductive harm. For more information, go to California Prop 65
About the Author
Alejandro (Alex) Uria is co-founder and developer of the online store Marine Carburetors. After a 10-year career in web development and media helping online magazines grow their audience and sales, he decided to turn his greatest hobby — sailing — into a business. Since 2009, he has used his experience in the online marketing industry to support his love for the water and help others in the boating industry.
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Dan Sullivan says
Most of the problems with carburetors for sailors involve their dinghy outboards. While I have no problem with most of the authors points, the one problem that consistently curses outboard performance is gumming up of fuel in the carb bowl and microbial growth, both of which happen in the long inactive season when so many of our boats are in storage.
The longer an engine sits in storage with old fuel in the carb, the more certain this problem will occur. Carb cleaner spray will not fix it. You have to remove the carb bowl and clean all the passages and jets with some type of probe. i have used strands from regular electrical wire. Strip the insulation off any multi strand wire and twist 2 or 3 strands together. They must maintain a very narrow profile, not much more than that of 3 strands of human hair. Use them as a pipe cleaner to route out all passages and scrape all growth out of the bowl. You’ll be amazed how much stronger the idle is, and you don’t need an overhaul kit. Just be careful pulling the bowl off the body. If the gasket tears, put it back together, gasket shreds intact with a smearing of vaseline. As long as the mating surfaces pretty much match each other they will seal perfectly. Performed this on a 6hp Mercury that wouldn’t start or run and it has been stellar in performance for 3 years ever since.
Lydell says
If a lot of gas is coming out the carburetor what could it be from?why?
Dave Skolnick (S/V Auspicious) says
Building on Dan’s point – the biggest cause of the biggest cause in the US is ethanol in gasoline. Ethanol sucks water right out of the air which leads to corrosion of carburetor parts. Ethanol also causes phase separation – water and ethanol combine in the bottom of your fuel tank and the octane rating decreases, sometimes to the point at which fuel won’t combust in the engine. Ethanol is itself very corrosive and even “ethanol resistant” fuel line starts to deteriorate leading to tiny near microscopic balls of rubber and silicone clogging jets.
Finding non-ethanol marine/off-road gasoline is becoming increasingly difficult in the US. It’s hard to describe the situation without dipping into politics and lobbying so I’ll leave that to readers to pursue on their own.
The best you can do is install a good-sized fuel filter and water separator in your dinghy fuel system (most people mount them on the inside of the transom) and turn over fuel as quickly as possible.
Aurelio Del Rio says
Hi, and thanks for your help and knowledge.
I got a problem on my 200hp 2 stroke outboard mercury. On idle speed work fine but when i rise the accelerator or rpm engine go down and stop.
That don’t happen if i don’t engage.
Thanks .
larry pierre says
having an issue with my Yamaha f100, when i accelerate it shuts down i had the carburetor cleaned and still, it works fine in gear but cant go any further….
Ernie says
I’ve found that the single best thing I can do with my Yamaha 9.9s is to unplug the fuel line and run the engine dry anytime I will leave it more than 3 or 4 days. In a week, I accumulate growth and some form of sediment. I have 10 micron filters in the fuel line and this still happens.
I use an ultrasonic cleaner and Simple Green to clean them, using 10 and 30 lb fishing line instead of wires.
Brandon says
I have a 115hp 1982 evinrude starts right up
After running and stop
To fish will
Not start as soon as I get home starts on muffs everytime no problem
Carolyn Shearlock says
Sounds like it’s not starting when hot. That’s beyond the scope of this article, but an outboard mechanic should be able to help you.
Anthony B Kabbeko says
My Johnson 175 will idle fine throttle slow fine medium stalls out but if I punch it to full throttle it’ll go in the full throttle fine so anywhere in between she’ll stall out
Douglas says
I purchased a 1992, 40 hp Yamaha. It ran a little rough so I replaced all the fuel line and fittings. Also installed a 10 micron water separator. Now it idles and runs great. How ever the carburetors are leaking fuel. I’m thinking I have a couple of floats stuck open. This is after I ran a fairly strong mix of sea foam through it as well. Any ideas what else I should look for when I open them up? Besides floats and needle valves.