Knockin' at idle...

[55'Fleetwin7.5]

Hello again...

My motor makes a knocking noise at low, low speed... Is this normal? When the Rpm's rise it goes away. I have been running the wrong fuel/oil mix... 50/1 I am changing that today... Iam not sure if that might contribute to this phenomena...

Thanks, tc

 

[R.Johnson]

Re: Knockin' at idle...

If you are talking about a 1955, 7.5, that engine has bushed connecting rod's, and crankshaft. They don't like 50-1. As for the knock, that can be a hard call without hearing it myself. The engine has a die cast aluminum hood with snaps front, and back. These can sometimes set up a lot of resonance. Check the rubber bumpers between the hood halves, 2 in front, 2 in back. They are like a little rubber mushroom, with a 1/4" stem. Make sure the flywheel is not loose.

 

[F_R]

Re: Knockin' at idle...

Get prepared for a connecting rod to come poking through the crankcase.

 

[55'Fleetwin7.5]

Re: Knockin' at idle...

Anything I can do to stop this from being inevitable?...

 

[Kevin W]

Re: Knockin' at idle...

Knocks are usually a bad sign.
its possible it could be a carbon buildup. (if your lucky)
pull the head anc check for carbon.
you can also inspect the cylinders while you have the head off.
if its not carbon...its not a good thing.
most likely it is the connecting rod bearings or main bearings or both.

 

[55'Fleetwin7.5]

Re: Knockin' at idle...

Knocks are usually a bad sign.
its possible it could be a carbon buildup. (if your lucky)
pull the head anc check for carbon.
you can also inspect the cylinders while you have the head off.
if its not carbon...its not a good thing.
most likely it is the connecting rod bearings or main bearings or both.

Sounds fatal.... Is this fixable? Parts available? Or will the cost for the parts make it a bad investment...

 

[55'Fleetwin7.5]

Re: Knockin' at idle...

Also.... It only knocks "a bit" at the lowest rpms... almost stall... If I give it even a bit of throttle "still below the start point and near the shift point" the sound goes away...

 

[R.Johnson]

Re: Knockin' at idle...

Check the flywheel nut, take the hood off, see how it sounds. Don't jump to the worst case scenario here. If the rod is loose, it will rattle up to the point of harmonic imbalance. It does not sound like it is doing that here. Again! I can't here that sound

 

[ColdPat]

Re: Knockin' at idle...

Had this happen to a 25hp motor Johnson. I thought it was knocking but figured out that I was running to lean(air not enough fuel). Try while idling adjusting the low speed idle. You might have to pull off the top cover to really turn it further.

Worth a shot.

 

[55'Fleetwin7.5]

Re: Knockin' at idle...

Thanks guys.... I will check the lbs. on the flywheel nut tonight and try to adjust the low speed idle as well... Are you suggesting turning a bit counter clockwise for that? I think, if I am hearing you right that you want it loosened a little or turned left... correct?

 

[Kevin W]

Re: Knockin' at idle...

I would definately make sure you are using 24:1 mixture first and see how it sounds then.
do a decarb with seafoam and that may clear it up some.
It may be getting a but tired but may be usable for quite a while with the correct mixture.
it is also possibe the driveshaft has a little rattle at slow speed and we are scaring you with the worst case ideas.