40HP Mercury knocking

[mattfish11]

I have a 40 HP 2003 oil injected 2 stroke... It starts right up every time, runs strong but has a knock to it...

Is there anyway I can tell if the oil is being injected correctly? The oil tank is up in the motor, but I can't tell if it's actually injecting...

Any other ideas I can check out before taking it to a marine mechanic to look at?

Thanks,
Matty Kling

 

[striper2101]

Re: 40HP Mercury knocking

I would like to see an answer for this as well. My dad has a 2002 50hp 2 stroke that knocks when you slow down.

 

[River - Runner]

Re: 40HP Mercury knocking

I'll get the knock also, running a 2004 40 hp. 3 cylinder 2 stroke, gas oil mix, manual rope start, tiller, short shaft (40M). It will sometimes happen when the motor isn't under a load or when slowing down. I said sometimes and it depends on curtain conditions or throttling.

My mechanic said it's a harmless exhaust noise.

It's a very strong motor for my light aluminum boat and capable of pushing it 50 mph. The boat and motor weigh the same, so sometimes cruising at a slow speed with little motor load, it'll do it. I've found prop pitch makes a difference for me. Running a lower pitch prop I'll get the knock more often than a higher pitch prop, as less load is on the motor with a lower pitch prop. I ran 7 different Mercury props with 6 different pitches and between aluminum and stainless steel, just because I could, not because of the knock.
This is something different, you can also get a clutch dog clatter in the lower unit from a heavy stainless prop when slowing down fast. This is but my experience of running it 5 years now. It is a great motor and for me a go fast motor.

 

[striper2101]

Re: 40HP Mercury knocking

Thanks for the info River Runner. My dad just got the boat and motor and we don't know what sounds are normal yet.

 

[mattfish11]

Re: 40HP Mercury knocking

Does anybody have any suggestions to test if the oil injector is functioning properly?

 

[River - Runner]

Re: 40HP Mercury knocking

To check if and how well it's functioning I'd premix oil in the gas, then check the oil reservoir level after running. The motor would be doubled oiled for a time, but that's OK.

 

[mattfish11]

Re: 40HP Mercury knocking

To check if and how well it's functioning I'd premix oil in the gas, then check the oil reservoir level after running. The motor would be doubled oiled for a time, but that's OK.

Over-oiled isn't a big deal? What's a good ratio for this kind of motor - any idea?

Thanks!

 

[River - Runner]

Re: 40HP Mercury knocking

Short term, over oiling is not harmful and double oiling is also recommend for a new motor break in period. 50 to 1 is the proper gas to oil mix for a premix motor. You doing a 50 to 1 gas and oil premix and with your motor functioning properly it would be operating on about a 25 to 1 ratio. There is no problem and no harm to the motor, but it'll smoke a little more.

 

[River - Runner]

Re: 40HP Mercury knocking

I should add... I'd then keep track of the gas consumption with the oil reservoir consumption to know if it's proper. Your motor should average a 50 to 1 ratio.