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Merc 150 fresh rebuild missing until 3000 rpm 150hp

Winddance

New member
I have about 6 hrs on a fresh rebuild of a 1987 merc 150. Seems to miss until revs are up over 3K. Works great on the top end. Trigger and stator outputs are within spec. Switchbox outputs were fluctuating on one of the banks so we swapped it out and although it stopped fluctuating it is not reading the same as the other bank. The new switchbox came off of an newer model would these have changed much? Is there another check on the switchboxes. Can a bad coil affect these readings even though it is downstream? Carbs were redone so we dont think that is the problem. Thanks
 
Re: Merc 150 fresh rebuild missing until 3000 rpm

Did you do the carb rebuild yourself? My first suspicion would be fuel delivery related. I'd suspect that your main jets are fine and your idle jets are off.
 
Re: Merc 150 fresh rebuild missing until 3000 rpm

No, rebuild done by someone else (marine mechanic who does weekend work). My first thought too but he insisted carbs were ok. I will ask him to have a second look at the carbs.
Thanks Jake.
 
Re: Merc 150 fresh rebuild missing until 3000 rpm

Yeah, I'd ask him to reconsider the carbs. It certainly could be something else, but it's also very possible that the idle jets were not properly set when the carb was cleaned. I suspect that when you break 3000 rpm your main jets are taking over.
 
Re: Merc 150 fresh rebuild missing until 3000 rpm

Use a timing light ( observe the flashing light )on each cylinder as you accelerate.---------------This is the simple and cheap way to find out if you have an ignition problem or a fuel problem!!!-----------Next you use a STRONG flashlight in the carburetor bores to look for a fuel problem , you look carefully at how the fuel is coming up the main jet / nozzle.-------------The poormans way to trouble shoot and it costs you nothing / teaches you a whole lot.
 
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