Many of the common issues with older Marlin Rifles have had "Fixes" that are fairly well known for decades.
Most all of these came from Cowboy Action Shooters and were listed in the Marlin Section at Paco Kelly's Leverguns.com website. (www.leverguns.com) I have been using many of those mods for years on my own guns and the good thing is that you can go as deep as you feel comfortable or all the way to "Full Race." Anything you do to the gun improves the way it runs.
I have added a few tweaks of my own and have also conversed with Eric Lundgren at Ruger/ Marlin and they were all incorporated in the new guns coming off the line.
The biggest one was "Chamfering the Chamber Mouth" This has been discussed many times here and there is a "Sticky" at the top of this page on the subject.
Next: You need a .01-.015 Radius on the Point of the Snail Cam on the Lever. This has been sharp until the Ruger take over. This will eliminate the "Marlin Jam" which is caused by that point digging into the bottom of the lifter over time. However this will take literally hundreds, if not thousands of rounds to show itself.
The fix for this has been known about for 30 years but nobody did anything about it until Ruger. I did it in 2005 when I got mine.
Next: To smooth out the Lever travel back and forth, the top edge of the hammer needs to be radiused so that the cam on the bottom of the bolt that pushes the hammer back sees the same area on the hammer when it goes forward. As the hammer is pushed back it must go past the Sear. When the bolt moves forward it has to push the hammer down past the sear again, but since the bolt is moving forward the hammer face hits the ramped portion of the cam. By putting a radius on that portion of the hammer it smooths out the closing stroke a lot.
Next: The groove on the side of the bolt that the Ejector rides in, is notoriously rough or more properly wavy from the Saw Cutter that is used to cut the slot. This could have been by an out of round cutter blade or just a high tooth. Take a Flat File and use the edge to smooth out the bottom of that slot. This has a surprisingly large effect on how the bolt moves back and forth.
Next: Reduced Power Hammer and Trigger Springs. Brownell's sells really neato Trigger Spring Kits for about $10-12. They work great and reduce the amount of effort to open and close the bolt.
If you do these simple things your gun will run smooth as silk and feed every kind of boolit known to man. And there are several other things that can be done to improve the gun even more, but some of them are not for the inexperienced owner, as they require skills that are not commonly known about... I draw the line at messing with Trigger Engagement and Stoning on Sears etc. as the gains to be had are miniscule and the possibility of screwing it up or making the gun unsafe are real and easy to drift into. I avoid them simply because they don't do me any good. I can get a more noticeable gain in function by changing the Hammer Spring, as opposed to doing a "Trigger Job." The Spring costs $10 and takes 10 minutes to accomplish, and anyone can do it. The trigger job not so much.
Continue on.
Randy