Classic & Vintage - Suntour Cyclone RD, can't adjust it from 7 spd to 6 spd?

I have a Suntour Cyclone MKII RD, which supposedly is able to handle up to 32T on my Vitus. It was working well with a no-name 7 speed freewheel with (I think) 28T as the biggest, and a KMC Z-chain, and Campy friction shifter.

As an experiment I tried a 6 speed Shimano "corn-cob", which only goes up to 20T. Strangely, I can't get it to shift onto the 20T, and I'm out of adjustment! This does not make sense to me since I know it can handle the range. What am I missing?

Off the cuff, did you check your B screw as well as limiter(s)?

Is this on the same wheel or a different one with a different hub? I wonder if there is a difference in the placement of the freewheel in relation to the dropout that may be the issue.

Is this on the same wheel or a different one with a different hub? I wonder if there is a difference in the placement of the freewheel in relation to the dropout that may be the issue.

+1, that's about all that makes sense...the other end I could see there being differences...but not on the hub side.

OP are there any ring spacers on the hub?

Same wheel, didn't change spacing. The 6 speed is a narrower stack. The Cyclone has only 3 screws, L, H, and the chain wrap, which I dialed all the way in. The chain just doesn't make enough of an angle going in to pick up the sprocket, just chatters.

Sorry but it's not clear that you did try the limit screw? As mentioned, the freewheel body may have a different relative position. If your freewheel is a Suntour Ultra, they have a history of being fussy with some chains.

Sounds like chain is too flexible for the increased chain gap caused by the smaller large cog. B-screw should be dialed out, as opposed to in, for this problem.

Sounds like chain is too flexible for the increased chain gap caused by the smaller large cog. B-screw should be dialed out, as opposed to in, for this problem.

That's interesting...so taking a link (or 2) out of the chain might add enough tension to enable the shift.

Yeah, that was going to be my next suggestion - taking a link or two out of the chain. if you've got it on a stand, you could manually push back on the derailleur to mimic adding tension / adjusting the B-limit while turning the cranks to see if that solves the problem, or slide the wheel forward in the dropouts to do roughly the same.

Thanks,I'll try the B-limit the other way (vertical drop-out on this bike, no adjustment). I already have the L limit fully out. The purpose of the experiment is to see if I can eliminate auto-shifting I get under certain conditions. The narrower, 6 speed looks like it will have a straighter chain line, which should help. I also have a wider Nashbar 6 speed freewheel to try.