Expert: Jeff Grigg - 2/22/2008

Question
QUESTION: I bought a 16’ center console Key Largo with 50hp 2-stroke Yamaha. I purchased it Jan 2007. It seemed to run OK for a few trips then it began to skip and sputter. I took it to a mechanic and he took noticed the bottom coil ignition system was very loose, so he tightened it and it began to run fine again. After a few trips it again began to skip and sputter. Everything was tight this time. I changed the spark plugs and added Sea Foam to the fuel tank. It didn’t help. It idles fine and can run at ½ to ¾ all day. When I try to run full throttle “5000 rpm” it powers down to 2500 rpm. I hold the throttle down and every now and then it will regain power for a moment or two then powers down again with occasional sputtering. I poured the water/fuel separator into glass jar and there was a lot of water. I drained the tank using the fuel line and hand primer. I drained the carbs and sprayed carb cleaner into them. I added a water dispersing treatment to the tank and tried it again. It ran fine for awhile then same issue resurfaced. I drained and flushed carbs and connected fuel line to external tank. Still no fix, took it to the mechanic. He replaced the fuel cap assembly because he thought water may be getting in that way then he completely cleaned the tank and took the carbs off to clean them thoroughly. It ran great for an hour or so then the problem came back. I continued to run the boat all day until I ran a full tank of gas. I replaced the fuel /water separator 3 times. Every time I checked the fuel/water separator for water, but it was clean as a whistle. Please Help!!!

Cansas Sadler


ANSWER: You may have more than one problem, you could have even scored a piston and cylinder running it with water in the fuel.
Do a compression and leak down test, do a total output test of the ignition, making sure it fires on all cylinders, and check or replace the spark plugs.
You will also want to have the linkages and synchronization checked and the cooling system checked that it isn't overheating. Post back with what you find.
Jeff

---------- FOLLOW-UP ----------

QUESTION: Compression test and output test checked out fine. Have new plugs. It doesnt run hot. The fuel system is clear. Still have the same symptoms. I am lost on what to do next!

Answer
If the fuel system is OK, no water in the carbs etc. Next look at the ignition system. You could have a faulty power pack or coil. The loose coil could damage the power pack.
Run the engine with a timing light to the #1 cylinder. While a helper runs the boat watch the flashes of the timing light. You can do this with the engine cover on. When you reach the full throttle and the engine starts to lose RPM see if the timing light starts to fire erratically. If not repeat the same test on the other cylinders. If only one cylinder stops firing the problem is most likely in that coil. If it does it on all the cylinders then it is either the stator or power pack.
Make sure your Guardian system isn't activating from running hot or lack of oil. Check the warning horn that it is functioning.
If you find an ignition problem have it diagnosed before you start changing parts.
If the ignition is good the whole time, look towards the fuel pump and do an output test. You can pick up a fuel pump tester for about $15 at an auto supply store.
Post back with what you find.
Jeff