Merc SeaPro 25 2 Stroke knock on down throttle

[SalmoTrutta]

Hi all, Brand new here, I recently bought a lightly used '10 (I think??) Mercury Sea Pro 25 2 stroke, manual start outboard, starts,runs,idles excellent but there is a intermittent knock when it revs down. Kind of sounds like knocking but not really sure and is not there all the time and sometimes its louder than others. I use good fuel pre-mixed exactly 50:1, checked fuel, spark, rod play, and everything else for which I had the resources. Motor looks literally brand new. I'm thinking maybe a timing issue and what I'm hearing is bad ignition?? I'm pretty mechanically inclined but not use to outboards so I'm kind of hesitant to mess with it.

Any input would be greatly appreciated

Thanks

ST

 

[windy5849]

Re: Merc SeaPro 25 2 Stroke knock on down throttle

Hello and welcome to iboats.Is the knocking sound heard when the boat is going at speed and then you back off on the throttle quickly to slow down?If this is when you hear it,then that is the clutch ratcheting in the lower unit.The water pressure when you slow down quickly is trying to spin the prop in reverse and making the clutch ratchet and would be normal operation.If you back off the throttle slowly and still have the noise,then my theory is wrong.Hope this helps.

 

[SalmoTrutta]

Re: Merc SeaPro 25 2 Stroke knock on down throttle

Thanks for the reply, I have it set up in my shop on a stand with the leg in a barrel with water just covering the intakes up to the plate. It makes the sound in neutral, forward and reverse, I removed the prop to be able to test it in gear. The noise seems to be coming from the powerhead.

 

[Mi duckdown]

Re: Merc SeaPro 25 2 Stroke knock on down throttle

Removing the prop is BAD..When testing. Need resistance.
Do a compressin check

 

[SalmoTrutta]

Re: Merc SeaPro 25 2 Stroke knock on down throttle

Thanks for the tip, prop back on..compression is 125 each side, hoping to try and get a video uploaded.

 

[SalmoTrutta]

Re: Merc SeaPro 25 2 Stroke knock on down throttle

doing some more poking around on the net and came across this video, same motor, same noise you can hear the "knock" right after her rev the engine and it idles down, is this normal?? the sound is identical. I'm use to two stroke engines (dirt bikes, snowmobiles) and they can be loud and "tingy" .. but I'm not use to outboards and this just does not sound right??? Could it be this is just how the motors are?? Anybody else with the same motor know this sound? around 2:20 to 2:30 when he revs you can clearly hear the noise.

http://www.youtube.com/watch?v=e0N2-ReYVoQ

 

[Handyman1]

Re: Merc SeaPro 25 2 Stroke knock on down throttle

Hi it almost sounds like a beck fire check the timing and the carb could be a little dirt in the carb. I worked on an atv with a back fire it was a dirty carb.