My 1967-made Marlin, SN AC30XXX, is a great little rifle.  I enjoy it.  But the action seems stiff.  This became readily apparent to me when I bought my Henry .357 Yellow Boy recently.  That action is buttery smooth.

I bought this gun used about 20 years ago.  It's clean and lubed on the inside.

Yes, it's a different caliber and design, I get that.  I have concluded that a lever action doesn't have to be stiff.  

I shoot everything from .22 CBs to .22 HV ammo through it.  Everything feeds.  I had to replace the firing "pin" several years ago due to a tendency to lock up.  The new pin cured that.

Is there anything I can do to smooth up the action.

My 1950 Mountie is stiffer than my Win 9422.

With the gun taken apart (side plate removed from receiver), how stiff is the lever to move? First is to determine if it’s the lever pivot pin too tight. Then check how polished the left side of the lever is (above the pivot screw), and determine if it could be dragging inside the receiver. Also check the polish where the lever engages the bolt.

Then remove the bolt, and check the finish of the parts beneath it. Just ahead of the ejector is a very shallow threaded screw in the receiver that could be rubbing against the bolt. If it has backed out, care must be taken to tighten this screw, as it doesn’t take much to strip it out. And replacement parts are hard to get.

One item to try, if you are comfortable with doing it, is to degrease the interior of the receiver, put a very thin layer of Prussian blue, or a sharpie pen, on surfaces that contact each other. Reassemble the rifle and cycle the lever a couple of times. Dissemble and check where the ink has been scraped away. This could indicate where parts need polishing, but experience is needed to decipher between the normal contact points vs. friction rubbing.

The model 39s I have are not considered ‘stiff’, but are positive and close fitting on the parts.

Thank you, sir.

A few years ago I rebuilt a 1950 Marlin 39-A that a previous family had attempted the usual trigger job on and ruined the hammer and trigger.
I gave them an estimate of the cost to restore it and instead he sold it to me for $50.
I almost broke my arm grabbing for my wallet.

I replaced all the dinked up screws, bought a new trigger, but couldn't find any hammers but the newer type with the grooves for the cocking attachment for use with scopes.

It was hard and stiff to cock.  I compared the original hammer to the new hammer and the front face of the original was smoothly curved, the new one had flats with sharp edges.
I rounded off the sharp edges and it was smooth operating.

Last year I lucked into an eBay sale of a pre-war checkered 39-A hammer and after cleaning up the checkering, it was a straight drop in.
Again, this pre-war hammer has a smoothly rounded front face.

Here's a picture of the older hammers.  Note the smooth curved front face that contacts the bolt as it's cocked..........

https://www.gunpartscorp.com/products/411500b

Here's the new style.  Note the sharp corners on the face.

https://www.ebay.com/itm/386637889034?itmmeta=01HSCF1N5E5VZW86BJPE24AXE6&hash=item5a0569fe0a:g:LJ0AAOSw9qVlpJoF&itmprp=enc%3AAQAJAAAAwPC3Tr37Mx5o0WZz3KSCFMjiase08M08%2FTvibcMdd5kgRIkDIYJvtiU6rqF94zRJxPD1v1TCEr9OBwMklK%2BSvca90g6H8OOfbDkbCBvfTyxQU1t6SGvvhVlwHOn2Gio5%2FPtaqiCaQOm0zSGwyOQbhvKdkuH0ieDasgk0PbNXvBydM27I%2FISMNOsv21yn18JexMRjPmp1GC5SHbHGW2qvE6JzQdx%2BP%2FnDEl8igAT6uFpPbtB8mUMIygKpm%2FDtB28Q2A%3D%3D%7Ctkp%3ABk9SR_DSho_LYw

After doing everything suggested in the other posts, try stoning these sharp corners to a more rounded shape like the original.
Also, try backing off the disassembly thumb screw a slight amount.  Sometimes tightening the screw too tight can cause binding.

NOTE:  What appears to be a silver screw on the front of the ejector is actually a slotted rivet used to lock down the ejector for cleaning the bore from the rear.
To clean, push down on the ejector and rotate the silver slotted rivet 1/4 a turn.  This holds the ejector down so the rod will pass.
Don't forget, after cleaning rotate the rivet back so the cut lines up with the ejector and it pops back up.

Actually never gave a thought that was the purpose of the referenced screw. Taking a closer look at my 39s, the 1954 A model has a steel retainer plate, polished, and appears that it will perform as stated/designed. The 1972 M model has a brass retainer that it is mounted on, but has been deformed and will not turn or engage the ejector . It got deformed sometime in the past before I got it. And have seen other rifles that had that screw/rivet deformed or missing.

I'd imagine it has never been completely disassembled and cleaned.  Start there.

How many rounds do you think have been fired through it? I'd take it out and put a couple hundred rounds through it. Then look for abnormal wear, try and see where it might be hanging up. It won't hurt to cycle the gun a few hundred times ( don't dry fire it)

Lube the shit out of it and cycle it a bunch. Rimfire guns shouldn't be dry fired. I'm not sure about the 39 but to be safe I wouldn't .

I also have a Henry 22 (H001) it's probably my most accurate 22 rifle. It doesn't compare in quality to my Winchester 94/22 or any of the Marlin 39s. These older guns are built way better and that's why they demand such a high price.

Sorry I didn't read this before. Thank you for this info...

Compared to the 9422 and the BL-22 the Model 39 has a long lever throw.

It’s not as light and smooth as the 9422 (Winchester really got that right) but it’s smoother and lighter than the short lever throw of the BL-22.

It should not be “stiff”.

I’ve got three 39s from 1965 to 1999ish vintage. None are what I’d call stiff.
Henry’s, well one benefit of pot metal construction seems to be that they wear in and slick up nicely. Whether that’s a feature or a bug is the owners decision.

Mine is not.

1974 date, IIRC.

My first one was, but my brother's was not(Christmas presents).  I bought one about 5 years ago (used) and it is smooth.

Butter.

Late 70's model here,  it's a tight lockup but nothing I would call stiff.

This.

Oil it, shoot it, clean it, oil it some more.  Repeat until it's smoothe.

Many years ago I inherited a 39A that has always had smooth, but positive cocking.  A few years ago I bought a used Mountie and was very surprised and disappointed at how difficult the cocking effort was.  Upon inspection and comparison with 39A parts, I determined that the firing pin was laterally warped. A light grinding and smoothing of the "high" region at the center of the firing pin fixed the issue.
Best of luck.
MHO, YMMV, etc.  Be well.

Since the 39 is a take down, you can have the bolt out in seconds.  Polish it.  Shoot it a lot.