Strange knocking noise

[Mahoman]

So I’ve been using my cub to cultivate and hill 3/4 acre of potatoes and the most bazar thing happens. It’ll go great for more than half the field, but eventually it will start a horrible constant knocking noise, and if I throttle down too much it will die, but when I fire it back up it’s gone and runs perfectly. This is a brand new fresh rebuild with all new bearings and all machined and everything. I put assembly grease on every bearing. I was thinking governor because the part where the keyed shaft goes is worn and the throttle is very finicky, I have another governor I just need to install. But has anyone heard of this problem? It’s scaring me. The last thing I want is a hole in my block and having to dump another grand into this thing.

[Cubfriend]

Is the knock in time with the engine rpm’s or could it be something else like a loose rear wheel?

[ricky racer]

Did you gap the rings to the correct gap? Did you check bearing clearances with Plastigage? Did you replace the bushings and pins on the small end of the rods and pistons?

[Waif]

Heat thinning your oil with run time?
Is pressure staying up?
Is engine stalling from heat?
Clean fuel/no water? Good /decent octane? Vent working?
Knock can be bigger than a wrist pin slapping.
Knocking wants to hint of slop somewhere.... I'm not familiar with governor caused knock. Unless your engine is trying to backfire or timing is off from it.

[Stoffregen Motorsports]

Here's another thought. Maybe the cooling system is clogged or otherwise not up to capacity and it is causing the tractor to run hot. Hot engines can make all sorts of noises when starting to run hot. Heat affects carburetor air/fuel tuning.

Could also be that the timing is off, causing it to run hot, leading to the noises.

Old engines may have worn wrist pins and slightly collapsed piston skirts, both of which will make clunking noises, but are rarely fatal.

[Mahoman]

Thanks for all the replies but I just don’t see how any of those problems you guys might have come up with would go away with simply turning the engine off for a second.

[Waif]

Thanks for all the replies but I just don’t see how any of those problems you guys might have come up with would go away with simply turning the engine off for a second.

You're breaking the "rhythm" the knock had going. Starting over again until it gets paced to where it can knock again.

[Matt Kirsch]

First thing you gotta do is investigate where the knock is coming from. When it starts knocking, stop, get off, and use an oak dowel or a mechanic's stethoscope to listen around for the source of the knock.

Another thing that was asked about the knock but not answered is, does the knock vary with engine speed? Does it stop when you push the clutch? Does it vary with ground speed?

[Mahoman]

https://youtu.be/Lesn2prbsGc
Skip to 5:15 for the noise

[tst]

is the fan good, has oil in it, not all worn out

[Magnum]

Is the knock in time with the engine rpm’s or could it be something else like a loose rear wheel?

This happened to me. It was noisy , but very easy to hear where it was coming from.

[Clemsonfor]

Is the knock in time with the engine rpm’s or could it be something else like a loose rear wheel?

This happened to me. It was noisy , but very easy to hear where it was coming from.

So what caused your noise?

[Magnum]

that cub just had a restoration done to it , we did not tighten the inner part of the rim to the outer rim . So sometimes depending on the weight shifting from side to side it would make some noise . This sound would not be mistaken for engine noise .

[Shadow_storm56]

That is odd, mine sounds kinda like that under really heavy load like up hill cultivating in second gear but it's definitely not a knock in my case just normal engine noise and it dosen't stick around. Yours sounds like it loses alot of power all of a sudden too? Check your fuel system as I had the sudden power loss happen from a sticking needle and maybe jiggling the throttle knocks it loose? I keep listening to the video and it sounds alot like it just suddenly gets really loaded down hard.

[Glen]

Hi,
If the engine starts missing steadily when it is hot, and you just rebuilt it, if they put in new valves and valve guides, it may have 1 or more valves that stick open.
The holes in the guides need a certain clearance with the valve stems, it is in the Cub service manual.
When the engine warms up, the clearance gets smaller, the valve can stick in the guide if the clearance is too small.
If the clearance is too small, the guide needs reaming to enlarge the hole slightly.
The clearances should be checked when the valves are installed.
I think sometimes shops don't check them, I remember someone on here had a sticking valve after a rebuild in the past.

The gas cap seems to leak gas out onto the hood, that isn't very safe to use it that way.
If the surface where the gas cap gasket fits on the gas tank is good, and the gas cap gasket is old and hard, the gasket probably needs replacing.
TM Tractor has a new gasket for the original style gas caps, below is the listing.

http://www.tmtractor.com/new/fl/785fp.htm

They also have a free replacement gas cap, it looks different than the original. You can read about it.

http://www.tmtractor.com/new/fl/fuelcap.htm

They won't seal well if the surface on the gas tank is bent.

I sent you a PM.