Due to a lack of a FW/FG hub, I dished a rim and moved spacers to use a road hub to mount a SS freewheel.
I did the appropriate chain line measurements to ensure good chain line.

When the crank is rotated backward, periodically there is a jump in the chain. Upon inspection of engagement between the chain and the sprocket, the alignment looks good, the ramp on the tooth looks fine and the chain does not have any resistance to rotating on all the pins, but the chain tends to ride up the tooth and then pop down.

1/8" chain, sprocket and ring.

What did I miss? I spaced the freewheel further towards the DO but didn't notice any significant difference. Used digital calipers to measure from the centerline of the bike to the center of the teeth, they measure within 2-3 mm between the ring and the sprocket.

Usure, but probably something to do with the sprocket. Just for the fun of it, I would check it out by grabbing a steel sprocket of the same or similar size, from the middle of a cheap-o 7-9sp cassette (LBS might have a suitable loaner doner), and see if it does the same thing. If it don't, alignment is not the bigger issue; if it does the same thing, it's probably not the sprocket...

<blind guess> tight chain link? want to buy a 3/32" thick freewheel? still keep the 4/32 chain?

So you didn't get a 3/32 chain did you, and thought otherwise?

Why do you have ramps on a single speed? Maybe the wrong sprocket?

As far as riding high on the teeth, that sounds like the chain has stretched. Have you checked for chain wear?

Or, perhaps the sprocket is worn from a previously stretched chain.

As fietsbob mentioned, it could also be the wrong chain.

Thanks, I will check. The freewheel is new, the chain came with the bike as did the ring. No tight chain link. My understanding is the chain is new. I have not checked for wear/stretch. I know the bike was sold with 1/8 sizing and the ring is original. Because of that, I ordered a freewheel to match. Will take pics.

should have done that first , Instead of posting,

Ageed, please forgive me!

The chain was purchased in 2008 and checked two months ago and found to be good (not verified by me). I did check to see if the tooth was engaging the chain early, or on the upper part of the tooth, but it doesn't. It touches near the root for the links that do not have the problem. The tooth does not even touch the side plates, internal or external before engagement, so it is not "riding" on top of the teeth at all. If that were the case, the chain line would be the first suspect variable.

The jumping is after the point of the tooth is past the top of the chain. I will take a video and post it.

i would mark the link that jumps. if it's always the same link. i would assume it's probably a bad link. if it's the same tooth all the time it's probably the tooth. once that is done, i would have eliminated about half the possibilities.

if it only happens when the bike is on the bike stand or upside down, i may be able to forget about it... ...or maybe not.

In addition to checking for stretch, try re-lubing the chain. I had your experience (chain runs fine forward but rides up on cog teeth when reversed). After oiling, the problem went away.

Another thought: is your freewheel really freewheeling? With the chain off, does it spin smoothly in reverse when turned with the fingertips?

[MENTION=406276]habilis[/MENTION] - yes it is good. Surprisingly so. I didnt' think Shimano had freewheel hubs with as many detents in it. It is pretty quiet.

[MENTION=44555]Huey[/MENTION]oolihan - good idea. I had thought of that but didn't go that far. It is not freewheel, definitely the chain but I don't know if it is always in the same location. Another thought is that the side plate may have a deformation with a sharp edge that catches. I need to look more closely after marking the chain. Stay tuned if you still have interest. I'll be back!