Slight knock in 1981 Chrysler 75 hp

[thenotsofreeboat]

Hey all,

I was doing some driveway boating a week ago and I have a very slight knock in the motor. It is pretty hard to hear but it is there. The history of the motor is a mystery, i got it for free i know it sat for quite a while and was only used 2-3 times a year by the people i got it from. The compression is good 150-150-145, and it starts and runs great usually first crank and away we go. This is the 5th time i have ran the motor and i don't think i have heard it before. I have been running the oil thicker 5 gal per pint and there is some seafoam in the gas is that causing the knocking i am hearing? Is this normal for a oversized weed whacker motor? Could it be spark plugs, they are the same ones that came with it when i got it. It is very faint but if it is a rod bearing or something like that i would like to fix it before it shoots a hole out the side of my block. Also how hard would that be to fix it looks pretty straight forward to get in there. Any help or comments are appreciated.

 

[Frank Acampora]

Re: Slight knock in 1981 Chrysler 75 hp

A slight knock in these outboard engines is like saying you are slightly pregnant or slightly dead. Investigate and correct it BEFORE running the engine on the water.

Bearings are roller and Usually don't knock --they tend to screetch--however, when they do knock, it is terminal in a very short period of time.

Most knocking (pinging, rattling) is pre-ignition due to a lean running condition and again leads to melted pistons in a very short period of time--especially at or near full throttle. Look up the thousands of threads here about setting carb low speed needles. Buy a Clymers manual.

The only thing extra oil will do is to foul the plugs faster.

 

[thenotsofreeboat]

Re: Slight knock in 1981 Chrysler 75 hp

I was worried about something being wrong with it, darn. I might have to cancel the rest of the boating season. I will check out the carb settings, hopefully that is all it needs. If not i have all winter and Iboats to get it figured out before hitting the water in 08.

Thanks again Frank

 

[Nate3172]

Re: Slight knock in 1981 Chrysler 75 hp

If the knock is at an idle and is faint, probably a good chance it is just a wrist pin that has a little bit too much play. Like Frank said, the rod and mains are needle type bearings and probably will not create that type of noise. Besides, rod knocks are usually when you de-celerate and the load is taken off the bearing and rod itself. Knocking at an idle is usually a main bearing or wrist pin. My first guess would be a wrist pin. Does it speed up with the engine or does it go away at any point?

 

[thenotsofreeboat]

Re: Slight knock in 1981 Chrysler 75 hp

When i up the throttle i can not hear the noise, that may be from the sound of the motor drowning out the sound (it is really faint) or it may disappear. I am not to sure. I am going to check it all out here in a week or so when i get time to do anything but work.

Questions a wrist pin is the pin that connects the piston to the rod right?

Thanks

 

[Nate3172]

Re: Slight knock in 1981 Chrysler 75 hp

Correct. The wrist pin is the pin that connects the piston to the rod. As a possibility, (I say possibility because I am not familiar with your particular engine and do not know how it is configured), if it has a fuel recirculation system than debris from the engine can accumulate there. Now, if it does have a fuel recirculation that is similiar to the later model force engines than it would have some screens to filter the puddled fuel before it is re-introduced into the intake. If these screens exist on your engine, than you can determine if a bearing is going bad simply by looking at the amount of metal debris that is there. It can also give you an idea of which section of the crank case has the problem. It will be normal to see some metal debris on these screens but if there is alot than if I were you I would have the engine gone through asap. If there is just a little bit of metal debris at the screens than I wouldn't get too worried about the slight knock you are hearing. You can investigate it more in the off season. Unless it gets worse of course.