Engine RPM drop

[Jon1229]

Hi all,

I have a 2001 mercury 150 carb saltwater series and a new issue just started that I haven’t had in the past. The motor runs fine and gets up on a plane no problem. Also, it can maintain 4500 rpm for as long as I want it to without any issues whatsoever. When I pull the throttle back to slow down, the boat hits 4200 and then the rpm immediately drops to around 3000. Any ideas what could cause this?

Thanks!

 

[Texasmark]

Don't have that engine but I bet the spark advance mechanism is binding, either in your engine linkage or something under the flywheel. On smaller Merc engines, timing is around 0 before or after TDC at idle,....matter of fact you set the idle 675 rpm by changing the timing, while at WOT it's at around 22 BTDC. Coming off WOT it is slow to retard while your fuel supply is being cut down...deliberate for fuel economy reasons in the cruise mode. At some point it picks up and starts to decrease also.

If asked and I was there with the engine off, I'd disconnect the remote and just using the pin on the input throttle linkage ( to which the remote connected) for something to grab, slide the throttle all the way to one end and back to the other watching the advance arm for erratic action. If that seemed ok, hook the remote cable back up and and have someone do it with the remote and watch for binding.

 

[Jon1229]

Thanks for the info. That probably is what is happening. Today I tried to replicate the issue and investigate it, the issue was completely gone. My guess it’ll be back, and then I’ll know where to start. Thanks!