So I recently changed one of my bikes slightly...

I replaced the old wheel (6 speed freewheel) with a new wheel with 7 speed cassette system. Previously I had 14-30T freewheel and I opted for a 11-28T cassette as I needed bigger gears more than smaller ones...

The drivetrain uses Suntour Cyclone GT Derailleurs and shifters and I put on a new 7/8 speed chain as well.

I adjusted the upper/lower derailleur limits and while on the stand had no problems shifting through all the gears... On the road was another story.

While riding I had no issues whatsoever with my smaller gears - anywhere from 18T to the 28T run perfectly with no issues... (It's an 11,13,15,18,21,24,28 fyi)

However, when riding home on the downhill and wanting to increase my speed, I have real problems with the 11T and 13T (11 wants to skip onto the 13, 13 wants to skip onto the 15 or 11)... finding the perfect position even with my friction shifter is impossible, and depending on the load I'm putting on it, it'll skip. (No load = almost no skipping...)

In a nutshell, It skipped constantly on the 11T, occasionally on the 13T and would skip on the 15T while standing... regardless of where I tried to place the derailleur (I have down-tube friction shifters FTW)

I've checked that the lockring is properly tightened - that's not an issue... The R. Derailleur certainly doesn't LOOK like there's any issue with the hanger being bent... But I have yet to pull things apart and grab my alignment tools/gauges out from under my workbench wherever they are...

The other thing I thought of... is it possible the top derailleur pulley doesn't have enough lateral play/float for a 7 speed index-compatible cassette/chain?

I'll be digging into it tomorrow, but was looking for a bit of advice from y'all as well... Any thoughts?

"But I have yet to pull things apart and grab my alignment tools/gauges out from under my workbench wherever they are..."

That would be a good first step; since you have the tools you may as well use them. Did you size the new chain properly? Did you readjust the B-screw to accomodate your smaller big cog? Did you thread the chain through the rear derailleur correctly? (We've all done it wrong at least once.) Did you remove any un-needed cog spacers and are all of the required ones present? There must be something basic wrong, friction shifters should cover a multitude of sins.

While you describe it as an auto shift problem (skips to a different gear) I suspect that it might be the more classic skipping or slipping under load problem. This is especially likely if you replaced the cassette with a used one.

Otherwise, there's the possibility that you left a slightly stiff link which is making problems when it comes around under load. Check this by back pedaling while pushing the lower pulley forward to slacken the chain's lower loop. Watch for any link that doesn't straighten immediately as the chain spools off the lower pulley.

If it isn't the chain, then a bent or misaligned hanger is the next suspect. You don't have to take it apart yet. Shift to high and stand behind the bike and eyeball whether the RD cage is vertical. Also look to see if the chain is feeding from a slightly inboard angle and touching the next larger sprocket on the way to a small one. If it is, that's your problem, pull the RD off, straighten the hanger and you should be OK.

I agree with FB - sounds like a wear issue - "wants to skip onto..." does not make sense to me. Either it occasionally shifts to the other cog on it's own or it skips under pressure. Yes, align the hanger just to eliminate that as a cause, and because it's silly not to if you have the tool. But if you mounted a used cassette odds are high that is the cause of your problems.

As for the float issue, you don't need float when you are friction shifting - float is designed into index systems to allow some leeway in the index position.

I'd go further and say you don't want lateral float when friction shifting - it'll only make it harder to judge, and less accurate and responsive. I don't even like it on index systems, since I'm prepared to take as much care as necessary to keep it adjusted right.

Now there's a statement you don't see every day...

You aren't using an old "fat" 5 speed chain are you?

Check your rear wheel for proper alignment. I fixed a bike yesterday that wanted to skip around in the two small gears out back. As I looked closely, I saw the chain was running a different line than the gears on the wheel.

Let's see... 7-speed... 11-28T cassette... is this an Interactive Glide cassette? They had thicker teeth with ramps on both sides, so the chain will be tempted to "climb" either of the neighboring cogs if the chain isn't perfectly aligned.

So figured it out...

Once I got my alignment tools out (shoved so far under my workbench it took me 10 minutes of moving things to even GET to them, LOL) ... I did some investigating...

For some odd reason the rear spacing was off... the right-side rear triangle was nearly 5mm too-far right... This was, of course, causing my chainline to be way off... when in the smallest cogs of my cassette, the extreme chain angle was causing my chain to hit the pickup teeth on the next largest cog...

I never noticed it while it was 6 speed as it was all pre-uniglide or hyperglide anything, thus no pickup teeth to be concerned about.

Honestly I prefer them to stem-mount or brifters... I'm waiting for my bar-cons to arrive, I think I'll MUCH prefer those to the downtube... I ordered 5 pair of them, since I got a deal, LOL.