My 2001 Mercury 15 hp four stroke bigfoot would not start today. I have put about 40 hours on it this year and it was running like a top. Came out of winter storage and it fired right up with no smoke. Today I tried to start it for sockeye fishing and it would not go. It tries and if you apply throttle advance it will try to idle but it never idles properly and it sounds out of time. I tried adjusting the idle mix screw on the lake without success. I did find a cracked fuel line and changed that on the water. I have since come home and checked a few things out. New plugs this spring. Both have a good clean spark. Opened up the carb. Squeeky clean inside. float bowl clean and float is working properly. I played with the idle mix screw some more. no luck. It sounds like it's out of time and will not pick up a proper idle no matter what is set it at. I am off to read my manual now and see what else I can check. The Mercury parts guy says shear pin. The timing belt is not broken and looks in ok shape to me. Boat ran great tow days ago now refuses to idle.
Ok I went a little further. I thought I should check the fuel supply so I disconnected the fuel line at the carb and ran the starter. It shot pulses of fuel out onto the floor. I used the primer bulb to fill a 1/2 gallon jar to check for separation/ water or cloudy fuel. It looks crystal clear after the bubbles rose out. My 125 hp also shares this fuel supply they also share a 10 micron spin on filter (new 40 hours ago) The 125 is not having any issue with fuel.
The engine oil is at the full mark. Clean and smells right.
I pulled the flywheel and the key is not damaged. The flywheel looks good and so do the charging coils underneath.
I'm going back out to try a new set of spark plugs just in case but I am pretty sure that is not it because I saw nice clean white sparks when I checked.
I can't do compression because I don't have a tester with the right thread.
I tried fresh plugs and it was better but still not good. It runs rough and sounds wrong like its out of time or something. I went back through old posts looking for the idle mix screw setting and found advice to check the fuel pump. I am on my way out now to check the fuel pump diaphram. Other than that I am fresh out of ideas.
It was running great when I shut it down on monday then wednesday it won't run.
Fuel pump was very dirty on the outside and the four bolts were full of salt residue. Diaphrams look soft and no cracks. When I manually pump the mechanical actuator it pumps fuel. I'm pretty sure that is not the issue. I'm stumped for sure. tomorrow I will take the carb down and soak it then re-assemble. Maybe I missed something there? Seems doubtful as I have extensive previous experience with cleaning carbs and putting them back to factory settings. I am not an expert in tuning or setting them up but I have always been able to clean and put them back as found with good results. The float and needle were closing and opening with manual activation.
When I had it apart i rolled the idle mix screw around a bit because I wasn't 100% sure the tip was straight. I'll look a little closer at that tomorrow too.
Anybody else got any ideas? I could by a compression gauge with the right threads. There could be an issue there.
Okay I went back through the carb today and found a little sediment. I cleaned it all throroughly, replaced all fuel lines back to the shared filter. The small inline filter in the cowl was breaking down. the glue was letting go and there were some small pieces of glue floating when I back flushed it. I put on a new one and fired it up. No luck.
I checked spark again and both are cycliders are sparking. But when I pull the plug boot off ofthe on the top cyl nothing changed. Dead hole. When I pull the bottom cyl it dies instantly. It is only running on the bottom cyl.
Checked timing of camshaft belt. Correct. Opened Valve cover. Issue found Broken valve spring on upper cyl. I pulled the power head after trying to remove head in place and finding out that was not possible. I am starting a new thread now that the issue has been identified.
