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1989 yamaha 40elf squeak noise/idle issue

schmidty25

New member
First off Hi, I'm new to the forums here and just purchased my first boat. It's a 16' Carolina skiff with the above engine on it. I rebuilt the carbs, New thermostat, new diaphrams and gaskets on the side of the bottom carb and also the pump, changed waterpump, and lower unit oil.


The engine runs good when above half throttle but down low at idle it makes a chirp noise and will rapidly shut off. I've raised the idle and also adjusted the position of the black box on the side off the engine. This helped out but the engine still makes this noise and will shut off during idle. I've also checked all electrical wires found one that was badly frayed and repaired it. Also there was a inline fuse that was blown, fixed that. It seems when I take the plug off the bottom cylinder and let the top two run It wont squeak and idles fine but will occasionally run rough. I am wondering if it is the bottom piston making the noise, but why wouldn't it make it all the time like when given full throttle? anyway here are the videos if anyone has any input I would appreciate it before I start tearing this thing apart.


 
So just to update. I have brand new fuel, checked compression on all three cylinders 110 in each, spark on all three checked fuel lines they are all good most of them are all new. I was told might be a lean sneeze/ vacuum leak.
 
Seems like the reed valve is cracked. Check it using a hose, put the hose to the valve area and the other end to your ear like an stethoscope, listen if the noise is coming from there.
 
I stumbled upon this thread as I’m having the same exact issue with my Yamaha outboard. Did you ever resolve the issue ? Was it the reed valves ?
 
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