Here is a photo of the slack in the chain. Also some photos of the machine.
I took video of the chain skipping when I back drive the wheel, but I can't get it to load. The system says the file extension (MP4) is not allowed. I will try to figure that out and post the video, and maybe a video of it running.

 

Here is a photo of the slack in the chain. Also some photos of the machine.
I took video of the chain skipping when I back drive the wheel, but I can't get it to load. The system says the file extension (MP4) is not allowed. I will try to figure that out and post the video, and maybe a video of it running.

I'm no chain expert but I have replaced quite a few over the years. To me the chain looks OK but the sprocket is worn, which would increase the pitch and explain the looseness.
However to be fair, it is difficult to know just by looking at the chain.

A chain gage would give you a definite answer.

 

even with the clutch disengaged, it wants to drive

That would mean the friction disc is still engaging the drive plate ... possible a return spring missing? etc,..

the bigger issue, when driving it skips and start stops.

I would surmise a damaged friction disc rubber ....

 

Thanks for the ideas, but this unit doesn't have a friction disc. It has the Tecumseh transmission.
To me it looks like the chain is too loose and is jumping the teeth, but I can't see any way to take up the slack.

 

To me it looks like the chain is too loose

It's not loose; it's either worn out (see "stretch" below), or simply the wrong pitch for the gears (it looks like the pitch is too short?).

Chain "stretch" is caused by lack of lubrication and then the chain’s pitch grows in length as it wears. The growth comes from the bushings wearing with the chain pins. Over time, the inner diameter of these bushings increases and the pins groove out. Measure the length of 10 links and then divide by 10 to get the average pitch. If it's 0.5% greater than the original pitch (or more), it's time for a new chain, and perhaps new sprockets.

These are the specs for your 50640 chain:

• Chain Pitch: 0.500"
• Roller Diameter: 0.306"
• Roller Width: 0.250"
• Pin Diameter: 0.141"
• Plate Thickness: 0.050"

Here is a replacement for your 50640 chain:

NEW - CRAFTSMAN Snow Blower Drive Chain Replaces 50640 21133 STD316412 S4162WL | eBay

Craftsman 1318010 Lawn Tractor - Lawn Tractor. Craftsman 1318020 Lawn Tractor - Lawn Tractor. Craftsman 1318050 Lawn Tractor - Lawn Tractor. Craftsman 1318051 Lawn Tractor - Lawn Tractor. Craftsman 1318070 Lawn Tractor - Lawn Tractor.

www.ebay.com

They can probably make a 50641 if you need that one.

 

Sorry, been gone for a while. Here's the most common reason for chains slipping. Unless you machine it and replace the sprocket, it's easier to replace it if you can find a parts machine. You may have to shim the sprocket if it's got slop in it, but that's not too difficult. If anyone wants the doc on the rebuild I did, send me an email to pch-hcbph@msn.com and I'll forward it to you.

 

Thanks for the responses. I have been out of town and buried in other projects also. I measured the chain, and it didn't seem to be overly stretched. However 10 feet of chain was only $30 from Grainger, so I went ahead and bought that. It just arrived yesterday, so hopefully will install that soon and give it a try.
HCBPH
I tried emailing you and it bounced as message blocked, mailbox unavailable.
If you don't mind emailing your rebuild writeup to me, it is my username on the forum @gmail.com.

 

Thanks again for all of the help.
I found the idler sprocket at Adam's small engines, so I went ahead and ordered that. I finally got a chance to install the new chains on the drive system and the new sprockets on Friday. The slack does not seem to be any different, and the chain still slips. It appears that it is slipping on the small idler sprocket and not on the larger axle sprocket like I thought it would. So I am looking for options.
I can see if I can get a new axle sprocket and weld it on the shaft. I am not an experienced or skilled welder, but I can probably get it to hold. My biggest concern is if I cause any warpage of either shaft or sprocket.
Another thought is to replace the bushings with bearings, and redrill the mounting holes to move the axle down a little to remove slack from the chain. I was looking at these bearings:

SBPFT204-12 Stamped Steel 3-Bolt Flange Bearing | 3/4'' Bearing

SBPFT204-12 stamped steel 3 bolt flange bearing has a 3/4' bearing ID and is made from high-quality steel. Our SBPFT204-12 bearing is made to perform! Shop SBPFT204-12 bearings now!

www.usarollerchain.com

Again thanks for all of the help. Sorry I have been slow to respond but am in the middle of a construction project right now that is taking all of my spare time. I am hoping to get this going before we get into the full snow season here in Colorado.

 

An easier way to have fixed it would have been to just install a tensioner .... done.