There definitely may be more than 1 thing wrong with the engine. But stalling typically means the engine is running out of fuel. And fuel is not getting from the tank to the inside of the engine. When the cap, rotor, plugs or plug wires go bad what will happen is those effected cylinders will either misfire or not fire. If the engine was running on say just 4 cylinders instead of all 6 because of a bad ignition component, it would still stay running as long as those firing cylinders were getting fuel. The engine might very well benefit from a tune up. But that by itself is not likely causing the stalling, The stalling is most likely because is the engine is simply running out of fuel. Here is what we would do in the shop.
1. Change all the fuel filters. If the filters are clogged then obviously fuel is not going to make it's way from the tank to the carb. Some models have filters inside the fuel pump as well as one in the carburetors fuel inlet.
2. As simple as this sounds you always start by running it off of a different fuel tank, like a small plastic portable tank. Half the time there isn't anything wrong the motor itself, but rather something wrong with the boats fuel system. This can be a pinched or collapsing fuel line, bad or old gas, or a bad anti siphon valve. Marine engines are very sensitive as to the quality of the fuel, and if any part of it is over 90 days, the fuel itself is always going to be suspect. This is a pass or fail test, and the engine is either going to run better or it is not. If it does run better, on this portable tank, first change the anti siphon valve and retest with the engine hooked back up to the boats fuel tank. If it goes back to running poor on the boats fuel tank, that tank will need to be pumped out. Bad anti siphon valves are a common problem, when they go back they create a blockage and restrict the amount of fuel that flows to the engine. If the engine does not get enough fuel it obviously will not make full power. Click this link, this explains more on the anti siphon valve and why they go bad. http://www.sterndrives.com/anti-syphon-valve.html. On this anti siphon valve there is an inlet side and an outlet side. The inlet side faces the fuel tank, the outlet side faces the fuel line. When the engine runs, the fuel pump creates suction. This suction pulls the check ball off of it's seat and fuel is allowed to flow around it. The problem with this is over time the spring in the valve will get weak or simply break. When that happens the check ball will be free to jump to the outlet side of the valve and create a blockage/restriction on the outlet side of the valve. If the spring is simply weak and not broken, this will more or less happen at random. Click this link for a picture, it will make more sense once you see it Http://imgur.com/a/tpq4H. The anti siphon valve itself is located at the fuel tank, where the fuel line connects. If you pull the fuel line off the tank you will see a hose barb, that hose barb is the anti siphon valve.
3. Check the fuel pressure. Fuel pressure needs to be at least 3 psi at all speeds. This needs to be checked with the boat in the water and the boat going at full speed. If fuel pressure is below 3 psi then the fuel pump is weak. You want to make sure you test this after #2. If there is a bad fuel line or bad anti siphon valve will cause fuel starvation and low fuel pressure.
4. Finally. If all the filters are changed, and if there was no change when running it on the portable tank, and if the fuel pressure was good, then the problem is going to be inside the carb itself. The carb's internal fuel jets are going to be semi-clogged and not allowing fuel to make it's way from the carb and into the cylinders. Does that all make sense. That is where you stand on your end and what the problem is. I think that should do it but I work for tips so I do want to make sure you are happy with my service before you go. If you had a further question on the issue by all means feel free to ask. If not, Just in case you do not understand the way the website works (and some folks do not). You do have to put forth a positive rating in order for it to credit me for helping you. When ratings are not done, the website simply keeps your deposit and they will not credit me. The ratings box is located at the top of the screen. To rate, you must select the star you wish and also confirm it. Please let me know if you run into any problems or errors when trying to do it. If you do have a problem, or if you can not see the ratings box which is at the top of the screen. Please reply back "I rate Jason's service _______" and fill in the blank.
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Thanks in advance, and good luck