Nelson Muffler Loud

[Bar Nuthin 2,012]

I never thought much about it until I added a couple of tractors to my collection. My original C-120 still has the stock Nelson Muffler (on a K301) - but it's twice as loud as my other two tractors.

C-141  K321 w/stock Nelson muffler

'73 No Name  K321 with aftermarket muffler (surprisingly, the quietest of all 3)

 

 

I asked ChatGTP about it and it provided some solutions and a drawing.  I really want to keep the Nelson muffler on my C-120. Curious if anyone has successfully operated on one with some success.

 

Here's what AI suggests

 

 

Repair options (from least invasive to most involved)

1. Internal baffle stabilization (best first step)

If the muffler rattles when shaken, this is likely your fix.

Method

• Drill a small access hole (⅜"–½") on the underside

• Use a stiff wire or rod to confirm loose internals

• Tack-weld or plug-weld the loose baffle through the hole

• Weld the access hole shut

Result

• Often restores 70–90% of original quietness

• Keeps original Nelson muffler intact

• Invisible once done

This is the most common successful repair.

---

2. Internal repack (moderate skill, very effective)

Original Nelsons were not fiberglass-packed like modern mufflers, but adding discreet packing helps.

Method

• Cut a small rectangular access window on the underside

• Add stainless steel wool or basalt exhaust packing

• Re-secure internal plates if possible

• Weld panel back in place and grind smooth

Result

• Significantly deeper, softer exhaust note

• Slightly quieter than factory

• Still visually original

⚠️ Do not use household steel wool—it burns quickly.

---

3. Hidden secondary diffuser (period-safe trick)

This avoids opening the muffler at all.

Method

• Insert a short perforated tube or cone into the outlet

• Secure with a hidden tack weld inside the outlet

• Paint flat black

Result

• Reduces sharp crack without choking the engine

• Often used by restorers who want no shell modification

---

4. Full internal rebuild (maximum effort)

Only worth it if originality matters greatly.

Method

• Split the muffler along a seam

• Recreate baffles from stainless sheet

• Weld back together and refinish

Result

• As quiet as new

• Labor-intensive but museum-grade

---

What not to do

• ❌ Don’t drill random holes (makes it louder)

• ❌ Don’t stuff fiberglass loosely (blows out fast)

• ❌ Don’t neck down the outlet (hurts K-301 breathing)

• ❌ Don’t assume “they were always loud” — they weren’t

A healthy Nelson on a K-301 should be deep and mellow, not sharp.

Edited yesterday at 07:02 PM by Bar Nuthin

[squonk 45,043]

Check your timing. 

[Bar Nuthin 2,012]

11 minutes ago, squonk said:

Check your timing. 

Interesting, as I do have a frequent backfire (through the exhaust) when I shut off the engine. Related possibly 

Edited yesterday at 07:47 PM by Bar Nuthin

[Handy Don 15,527]

7 minutes ago, Bar Nuthin said:

backfire (through the exhaust)

Backfire is unburned fuel entering the exhaust system and burning/expanding really fast (kinda explosive). NOT good for mufflers. 

 

Interestingly, the 1990 small engine pollution abatement rules caused many engine makers to put solenoid valves on their carburetors to instantly shut off fuel when the engine is switched off. Absent one of these, as long as an engine was spinning it’d draw fuel even with no spark. That fuel would go out the exhaust as a pollutant. To lessen “spinning with no spark” is one reason for bringing an older engine to idle before shutting it down.

[kpinnc 16,461]

I have a K301 that runs great but is stupid loud even with a new muffler. Much louder than a 14 or even FILs 16hp. Valve adjustment made no difference. 

 

Figured I'd have to go into it to figure it out...

Edited yesterday at 08:17 PM by kpinnc

[squonk 45,043]

I've had a couple of tractors that were stupid loud. Timing was advanced on both. Engine was laboring a little . Setting the timing helped some

[ebinmaine 76,356]

3 hours ago, Handy Don said:

. To lessen “spinning with no spark” is one reason for bringing an older engine to idle before shutting it down.

 

 

Idle for a minimum 10 seconds. 

Turn off engine. 

IMMEDIATELY  bring throttle right to FULL.  

 

No more backfires.  

 

@wallfish I believe,  taught me that gem originally.  

I do not recall WHY it works. Maybe @Handy Don can elaborate.  

 

 

 

[Handy Don 15,527]

1 hour ago, ebinmaine said:

Idle for a minimum 10 seconds. 

Turn off engine. 

IMMEDIATELY  bring throttle right to FULL.

I can only guess that the suddenly open throttle significantly leans out the fuel in the “last gasps” of the engine. No fuel; no bang. 

[kpinnc 16,461]

8 hours ago, Handy Don said:

To lessen “spinning with no spark” is one reason for bringing an older engine to idle before shutting it down.

 

Agreed. Carbon inside your muffler is glowing hot during operation, and will cause a backfire rather consistently. 

 

Idling down works very well to reduce this for sure. 

[oliver2-44 11,132]

Attempting to weld on an old muffler is challenging since everything is rusty and thin. Time for a new muffler. 

[Retired Wrencher 6,246]

I still like those old gravely mufflers style. I know they’re not like the originals, but it’s much quieter than the screw on type from Kohler. Those are really loud. And of course they’re a bit ugly. Definitely no style points there. I have two engines using those.

[Bar Nuthin 2,012]

2 hours ago, Retired Wrencher said:

I still like those old gravely mufflers

If you get a chance, post a picture of how you have them installed. Thanks!

[Kenneth R Cluley 654]

Definitely don't care for the "steel wool" suggestion in the drawing. That won't last long. We used to use fiberglass, hence the term "Glasspack Muffler" That didn't last very long either. Maybe stainless or copper gauze?

[ebinmaine 76,356]

26 minutes ago, Bar Nuthin said:

If you get a chance, post a picture of how you have them installed. Thanks!

 

 

You could do a quick search for a couple of mine.

My 1975 C160 Automatic  & The Ugly Bruce tractor  both have one of those Gravely mufflers. I leave them on the outside because I don't like having the heat next to the fuel pump and carburetor. But you could definitely use one in the stock location if you were handy with a welder.

 

[Bar Nuthin 2,012]

1 hour ago, Kenneth R Cluley said:

Definitely don't care for the "steel wool" suggestion in the drawing. That won't last long. We used to use fiberglass, hence the term "Glasspack Muffler" That didn't last very long either. Maybe stainless or copper gauze?

In the text, it cautions against regular steel wool and suggests stainless steel wool or basalt exhaust packing. Not sure what basalt exhaust packing is. Looks similar to insulation.

Basalt Silencer and Exhaust packing sound insulation from Merlin Motorsport

[Bar Nuthin 2,012]

1 hour ago, ebinmaine said:

The Ugly Bruce tractor

Looks like Ugly Bruce is a stack setup. I wouldn't be opposed to that!

I'm most likely going to take the following steps in order

1. Check/adjust timing
2. Slightly richen the low idle mixture

If no improvement... (both of these things I did when I got the tractor)

1. Pull the head for inspection and cleaning
2. Check valve adjustment

If still excessively loud...

1. Pull the muffler and decide if it can be reworked or gets replaced

[ebinmaine 76,356]

2 minutes ago, Bar Nuthin said:

Ugly Bruce is a stack setup

 

 

Cinnamon is a stack.  

 

The Ugly Bruce is a red tractor.  

[Bar Nuthin 2,012]

8 minutes ago, ebinmaine said:

Cinnamon is a stack.  

 

The Ugly Bruce is a red tractor.  

You must use a database to keep track of all your stuff 

[ebinmaine 76,356]

8 minutes ago, Bar Nuthin said:

You must use a database to keep track of all your stuff 

 

In fact, yes.  

 

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