Ariens S-16H engine trouble

Looking for some second opinions before I start the process of elimination. Have an Ariens S-16H with the ole Kohler single cylinder 16hp cast iron block boat anchor engine. It acts like it pings or predetonates really horribly. It does have an aftermarket carb. What it does is this: starts fine, runs fairly smooth for a Kohlooser, but as soon as it comes under any kinda load it starts knocking around like a suburban solicitor and pings itself until it dies. Now that said, sitting there at full throttle in Park, if I rev the throttle butterfly on the carb up by hand it doesn't have any issues. But any kinda rapid acceleration with the throttle lever or when the engine goes under any load, starts banging and knocking around like crazy. I checked the points gap and it was at 0.020. I know for all the things I hate about the Kohlers those cast iron engines will run even if they are dead and dont know it, could it just have that much blowby that it leaks past and causes insane back pressure? Clogged crankcase vent? Bad carburetur? Is my points timing wrong? Could it be the spark plug? Is there something I'm not thinking of? Any help is appreciated, its one of those, I'd love it to run but don't want to devote a bunch of time and money to making it run right.

Is it the OHV 16 HP? If so probably a valve seat

Original engine was a K341S so no OHV.

That model was made from 1976 to 1978, over 45 years ago.
Like someone said earlier, the engine and transmission are virtually bulletproof.
The points setting is tricky. The .020 is measured when the points are open widest - most of the revolution.
Another reason a Kohler engine will bounce around is failed/broken/missing isolation mounts.
Make sure the engine is bolted to the mounts and the mounts are bolted or riveted to the frame no matter what kind of mounts you have.
All the other possibilities previously mentioned are legitimate as well.

The K341 was known to vibrate a lot under the best of conditions.

Have you tried changing the fuel?

My old Kohlers in Cub Cadets are about as tough an old engine as you can find. One thing ive seen is head bolts come loose from time to time. When that happens they run really hot and will display detonation or preignition. Otherwise I'd recommend replacing it with a Predator and send me that boat anchor! I can always find a use for an old Kohler!

I'll check the head bolts. I had never heard of that before, thanks for pointing that out.

Not a fan of the Predators either they're fine but my personal favorite has always been the ole Briggs and Strattons. Easiest to work on, least finnicky, and longest lasting, at least from the ones I have messed with. I like Kawasaki's too. And I have heard a lot of great things about the new Briggs and Stratton Vanguard engines.

Yeah it has fresh fuel and a relatively new inline fuel filter.

That is how I set the points, .020 at their widest points

Its not so much that it is loose or rattly than that it is preigniting and basically causing the whole front of the machine to bounce and torque around

Thanks! I didn't have the manual when I got it, I didn't realize it would be a good one with lots of information. Sometimes they are, sometimes they are useless.

The reason the Vanguard is so good is because Briggs didnt build it.

Don't know what we have against these engines.. "Kalooser?"not by a long shot. That series of engine was among the best ever created.
My first alarm bell goes off when you say "Chinese carb". Now you wanna talk JUNK, you can start with the Chinese carbs.theyre cheap FOR A REASON. Not to mention that engine is over 40 years old. It's bound to need some maintenance. I have had a few of those same tractors (currently have 4) that tractor has been made virtually unchanged since 1969-70 until the early 1990s. First from Sperry rand then aroens took them over about 1973, only changing the colors, and becoming the "GT" series thru the 80s and lastly as the Grand sierra. By this time they had a 22hp v twin. Over the years they had anywhere from a 10hp to that 22hp.
First few years they had a Vickers hydro, then going to a15 series sunstrand until the end.

Also check to see if varmints have taken up residence in the shroud and clogged the fins, those have to be clear so air can cool the cylinder. Often you can take the tins around the carb off, and get in there with an air hose. To take the main shroud off the engine will have to come out.

Just came to.mind now, but if mice may have nested in the muffler, if it's even possible on that tractor, ( I'm thinking of the old Cub Cadets with that wide open muffler), that would cause a huge back pressure condition. Which would likely result in detonation.

This engine is directly bolted to the tractor frame on these tractor. The only "mounts" are 4, 3/8" bolts.