1975 Mercruiser 140 ticking/knocking sound under load

[brian8227]

Hello all this is my first post here. First I would like to let any other newbies know where I found a manual for my boat. Boatinfo - boat brochures and manuals for boats and marine engines! It takes a while to download but I was able to find a service manual there for my boat and many others it is a great resource for anyone looking.

I have been working on a 1975 mercruiser 140 there is a ticking or rattle sound but I can not pin point its location. It only happens while in the water under a load. It is louder when it is under a greater load pulling a tube or skier. It does not make this noise at all while at home with the muffs on at idle or any rpm up to 3000 which is the highest I tried at home. Does anyone have any ideas what this could be?

 

[Bondo]

Re: 1975 Mercruiser 140 ticking/knocking sound under load

Hello all this is my first post here. First I would like to let any other newbies know where I found a manual for my boat. Boatinfo - boat brochures and manuals for boats and marine engines! It takes a while to download but I was able to find a service manual there for my boat and many others it is a great resource for anyone looking.

I have been working on a 1975 mercruiser 140 there is a ticking or rattle sound but I can not pin point its location. It only happens while in the water under a load. It is louder when it is under a greater load pulling a tube or skier. It does not make this noise at all while at home with the muffs on at idle or any rpm up to 3000 which is the highest I tried at home. Does anyone have any ideas what this could be?

Ayuh,.... My guess is it's detonation rattle, caused by a lean condition,...

Are the fuel filters clean,..?? How 'bout the Carb,..??

 

[brian8227]

Re: 1975 Mercruiser 140 ticking/knocking sound under load

Ayuh,.... My guess is it's detonation rattle, caused by a lean condition,...

Are the fuel filters clean,..?? How 'bout the Carb,..??

Thank you for replying.

The Carb was just rebuilt. I have heard this in the past as well before we had the carb rebuilt.
The fuel filter looks clean but I plan on replacing it before I take the boat out again. It only makes this noise in the water under a load and it is about an hour drive to the nearest lake.


Like I said the noise I have heard in the past just not as bad. But it did have one other issue I was going to post separately I was cruising at about 3000 RPM and the RPMs would drop to 2800 with out touching the throttle for about 2 or 3 min then it would go back up to 3000 again with out touching anything. This was the first time it has ever done that. I was going to post this issue separately, I thought it may be a separate fuel issue.

 

[Bondo]

Re: 1975 Mercruiser 140 ticking/knocking sound under load

Thank you for replying.

The Carb was just rebuilt. I have heard this in the past as well before we had the carb rebuilt.
The fuel filter looks clean but I plan on replacing it before I take the boat out again. It only makes this noise in the water under a load and it is about an hour drive to the nearest lake.


Like I said the noise I have heard in the past just not as bad. But it did have one other issue I was going to post separately I was cruising at about 3000 RPM and the RPMs would drop to 2800 with out touching the throttle for about 2 or 3 min then it would go back up to 3000 again with out touching anything. This was the first time it has ever done that. I was going to post this issue separately, I thought it may be a separate fuel issue.

Ayuh,.... It sounds like a Fuel delivery issue,...
Everything from the tank's vent to the carb is suspect,...

'n,... Carb rebuildin' is 'bout a 50/50 shot, even in a well equiped shop,...

 

[brian8227]

Re: 1975 Mercruiser 140 ticking/knocking sound under load

Ayuh,.... It sounds like a Fuel delivery issue,...
Everything from the tank's vent to the carb is suspect,...

'n,... Carb rebuildin' is 'bout a 50/50 shot, even in a well equiped shop,...

It runs really smooth when at idle or even at 3000 rpm while sitting in the drive way so I am not sure we have a carb issue unless it just needs to be adjusted richer which I will try when I get It back on the lake.

A fuel issue is what a figured for the drop in RPM issue. But do you think the pinging noise is also related to a fuel issue. I was fearing something in the sterndrive side of things considering I do not hear the noise on land.

 

[Bondo]

Re: 1975 Mercruiser 140 ticking/knocking sound under load

It runs really smooth when at idle or even at 3000 rpm while sitting in the drive way so I am not sure we have a carb issue unless it just needs to be adjusted richer which I will try when I get It back on the lake.

A fuel issue is what a figured for the drop in RPM issue. But do you think the pinging noise is also related to a fuel issue. I was fearing something in the sterndrive side of things considering I do not hear the noise on land.

Ayuh,..... Without a Load, the motor requires little to no fuel,...

Add the Load, 'n it gets Lean,....

When it goes Lean, Detonation sets in, 'n yer motor is consumin' itself,....
Every 1 of those Pings is another piece of the motor goin' out the exhaust pipe,....

I don't think it's an Adjustment issue, I'd sooner think the carb is restricted, Internally,...

 

[brian8227]

Re: 1975 Mercruiser 140 ticking/knocking sound under load

Ayuh,..... Without a Load, the motor requires little to no fuel,...

Add the Load, 'n it gets Lean,....

When it goes Lean, Detonation sets in, 'n yer motor is consumin' itself,....
Every 1 of those Pings is another piece of the motor goin' out the exhaust pipe,....

I don't think it's an Adjustment issue, I'd sooner think the carb is restricted, Internally,...

We were planning on replacing the fuel pump as it is the last piece of fuel system left, and it is the original from 1975. We are just having a hard time finding one.

Is there an easy way to confirm the carburetor has an issue with out pulling it apart?

 

[Bondo]

Re: 1975 Mercruiser 140 ticking/knocking sound under load

We were planning on replacing the fuel pump as it is the last piece of fuel system left, and it is the original from 1975. We are just having a hard time finding one.

Is there an easy way to confirm the carburetor has an issue with out pulling it apart?

Ayuh,... The Color of the spark plugs, tells ya What the carb is doin',....