Knocking trouble

[AJC49]

Any opinions would be greatly appreciated as I am giving a refund to the guy who bought my boat with 2001 125hp merc from me and I need to know before putting up for sale again.I suppose it could be minor piston slap that I had never noticed but other than this knock it performs great throughout the whole powerband.Could it be bad gasoline,old 2 cycle in oil reservoire,etc.
Could anyone tell me if a marine shop has to take down an engine to diagnose what is causing a knock coming from the powerhead?Could they give me a good accurate ballpark estimate of a repair without tearing it apart very much?Again any and all thoughts are very much appreciated.Thanks

 

[corm]

Re: Knocking trouble

Hi,

A knock,clunk clink or a ping? hard to tell without being in the boat.
A good mechanic should be able to give you a educated guess using a stethoscope and a dial indicator gauge. Measure the piston travel. You can check the rod bearings with a screwdriver by bringing a piston up to TDC as you go past TDC the piston rings will usually have enough friction to hold the piston as the crank moves down towards BDC. Stop just past TDC or up to 90 degrees and push on the top of the piston through the sparkplug hole. If the piston moves without the crank moving (watch the flywheel for movement) and you hear a clunk type click that clyinder has a bad wrist pin or crank end bearing.
I always found it hard to hear piston slap. Unless it was real bad. One of my mechanics could always hear it, he use to make me mad. hehe
Anyway if they are experienced they should be able to give you a pretty good idea about what is going on. Good luck.

 

[AJC49]

Re: Knocking trouble

Thanks for the reply Corm.Have her at the shop now and my mechanic WILL get a printout of your response.It just may help him decide how much he has to tear it down(the motor and my bank account)for a good diagnosis.