[throughput]

Because outboard engines tend to sit over the winter time, holes in the intake manifold gasket can occur. These increase the air intake, causing a lean fuel mixture and vacuum pressure problems. The result is poor acceleration, rough idle and stalling. A manifold leak is easily detected by a hissing sound coming from the source of the leak. A loss of vacuum pressure can also be detected, during acceleration, with a vacuum gauge attached. Once the leak is found, replacing the gasket, rubber intake or manifold components responsible for the leak will fix the problem. here

[alkor]

Outboard engines are popular in small watercraft because of their light weight and wide range of size for both low and high power applications. Outboard engines, as with any internal combustion engine, require air, fuel and spark to maintain adequate combustion throughout the power band. Problems with the delivery of any of these combustion components can cause an engine to stall while under acceleration.

[bretthart32]

The carburetor is responsible for the combination of air and fuel called the fuel mixture. If the ratio of fuel to air becomes too high or too low, the engine will run poorly, especially while accelerating. This is because a rich fuel mixture is over-saturated with gas, causing ignition problems. Mixtures that have too much air and not enough fuel are lean and tend to reduce the amount of power available under load and can cause stalling. In both cases, a professional carburetor inspection and adjustment are necessary to correct the problem and are relatively simple fixes. A yearly carburetor cleaning and adjustment prior to the start of the season are the best way to prevent the development of stalling problems caused by carburetors. more

[Frost987]

Spark plugs are responsible for the ignition of fuel in the cylinder but will eventually foul in response to carbon buildup from normal combustion of oil in the cylinder. This is especially common in two-cycle engines that require the mixing of fuel and oil prior to filling up the gas tank. Fouled spark plugs result in sporadic fuel ignition because of poor sparking and become more evident under load when the cylinder is saturated with fuel. Spark plug fouling can be detected by removing the spark plugs and comparing them with a plug color chart. Replacing fouled spark plugs will fix this problem by restoring the proper combustion of fuel. here

[bravenec]

A clogged fuel line inhibits the flow of fuel from the boat's gas tank to the engine. Fuel filters help reduce this problem by filtering particles from the fuel before they can reach small, easy-to-clog lines. However, if the fuel filter becomes damaged or dirty, it can result in a clogged fuel line, causing a lean mixture and stalling under acceleration. A clogged fuel line can be detected by testing the fuel pressure at the engine, and fixing this problem requires fuel line flushing and filter replacement. here

[ansible adams]

If he ran it out of gas it's possible he sucked some deris from the bottonm of the tank into the system a good carb cleaning might be in order. As for the fuel pump I had a similar problem with a 70 horse and a fuel pump rebuild ($11 kit about 15 mins to install) made the thing run like new again [links]

[kiba]

Re: 40 HP Mercury stalls when given throttle #2

[jeffersons]

I had a similar problem with a 35hp. If you pump the primer bulb while running, does the engine continue to run? Make sure the whole fuel system checks out, Vent on fuel tank open, fuel line and primer bulb ok. Has the fuel pump ever been replaced? I went through everything on mine and it ran great but could never really pinpoint what it was.

[chrisadamshtm]

Yep. check the parts list and see if the rubber plug isn't the bane.<br /><br />-W

[Mackendra99]

It was new gas right from the pump. Never heard of seafoam, ill look into it. here

[Stephen Gornick]

The primer Bulb did not help, I checked the Built in tank vent and it was clear. Motor is about 1 1/2 years old, bought new, never been serviced.<br />have some new info. Was looking at the Carb and next to the mixture adjustment screw at the top of the Carb is another threaded hole with nothing in it. Looked around and did not find anything except a rubber plug laying at the bottom of the motor case. Is this right? A air leak could easely cause this lack of power I believe.

[stbri]

So I'm thinking that the tach temp light not coming on at start is an indicator of some malfunction and that this stall issue with the starboard engine may somehow be associated with it.

[Art Gallery]

I've tested the engines with 30 min and 1 hour runs in the bay near where I live (mostly between 4000 and 4500 rpm). The starboard engine did not stall after each run so I think I've fixed the problem. However I did notice that the port engine has a slightly stronger tell tale than the starboard engine.

[jaredreed496]

I'm planning on ordering the water pump repair kit on Monday for both engines. It is due.

[midnightmagic]

I am hours old to this forum and apologize in advance for my mechanical ignorance. My mechanical work is being done by a local Yamaha mechanic who has been very engaged and supportive. That said, I have a reoccurring and intermittent problem that appears related to fuel starvation. My motor runs without issue at higher (3500 to 4500) rpms. However with extended *****ing speed (800 to 1000 rpm) after about 1 hour the motor will begin to stall. Sometimes I can get to the ball and pump up the pressure and sometimes no matter how hard I pump it will stall out. At times it has emptied the fuel water separator of gas before stalling. If I let it sit for a few minutes I can get it started. It can be a bugger if the fuel water separator is empty but I have learned to shut down the motor if it starts to struggle for fuel so this doesn't happen.

[throughput]

I read that another member with similar problems reported: ". without the fuel cooler able to function, the bypass fuel is constantly being recycled thru the heat producing fuel pump until the boiling point at the fuel pump pickup (vacuum) is being reached, thus creating the vapor lock situation inside the pump itself." He reported "I took the elbow off the hose ( #17) and rerouted it thru the hole in the lower cowling to give me a straight shot thru the hose, and ran a piece of mechanic's wire thru the tube. I felt it when I hit the hose nipple, then a twist or two and it went into the nipple. I could feel some restriction there but whatever it was gave way."

[dragon]

I took it to my mechanic and he found a bad pulley that had caused the serpentine belt to get chewed a bit and this was replaced. He changed the bulb and fuel line and removed the anti-siphon valve. I am able to blow air through the gas tank vent and checked the vent screens. He tells me the VST is fine.