Engine knocks, less power

[momcat]

60 hp , 1995 Mercury 2-stroke. suddenly starts knocking while cruising on lake. Power loss, not total. Knocking reminds me of a stuck valve on a v-8. Anyone have this happen? What am I looking at for repair?

 

[IamCanadian]

Re: Engine knocks, less power

Pull the plugs and inspect, a comp test would be a good idea at this time too. So many things can knock and tick, hard to say where to start.

 

[momcat]

Re: Engine knocks, less power

Pull the plugs and inspect, a comp test would be a good idea at this time too. So many things can knock and tick, hard to say where to start.

It tested at 118 on the top cylinder, 120 on each of the other two.

 

[momcat]

Re: Engine knocks, less power

It tested 118 on the top cyl, 120 on the other two.

 

[momcat]

Re: Engine knocks, less power

I cranked it yesterday, it didnt knock. I am going to try to put it in a barrel of water and run it to see if it does it again.

 

[Texasmark]

Re: Engine knocks, less power

I cranked it yesterday, it didnt knock. I am going to try to put it in a barrel of water and run it to see if it does it again.

Ensure that the water remains above the water pump or it will run hot. Additionally, exhaust makes water hot so watch that....I used to keep a hose running in the barrel when testing and if you have the prop engaged and the water whirling around I noticed reduced tell tale indicating to me that the water pickup was not very efficient...with water whirling.

Mark

 

[CharlieB]

Re: Engine knocks, less power

Pull the spark plugs, turn the motor over by hand, look at the piston domes and see what color each of them are.

All should be an equal blackness indicating a factory rich fuel mixture.

If one carb is getting dirty, restricting fuel flow to that cyl then that piston may be lighter in color. A hotter running piston will swell and reduce piston clearance and loose power and can cause a knocking noise without heating the motor enough to set off the overtemp alarm.

Inspect those piston domes to get a better idea if this is part of your problem. If so stop running the motor until you clean the carbs.

 

[James R]

Re: Engine knocks, less power

Could be an ignition problem causing the knock. If one cylinder is not firing correctly the out of balance will produce a knock. Don't continue to run like that as permanent damage will result. Triggers and switch boxes get to fail when warm. Most often the switch box.