Swapped out a Shimano freewheel for a Suntour New Winner and just went for my first real ride. I shifted down to my largest cog on a climb and the derailleur kept skipping back down. Never did that on the old freewheel. Doesn't do it on the trainer. Do I chalk it up to a bad day or should I make some adjustments?

The bike has Suntour Cyclone M II drive train. Friction shifters. It gets into gear, just doesn't stay there.

Thanks .

The two freewheels could well have large cogs that are different distances from the dropout and derailleur, Did you check the low gear limit screw?

This sounds like a bent derailleur hanger; the extension of the dropout the derailleur bolts onto. (Easily bent any time a bike falls to the right.) Any bike shop can check the alignment of the hanger quickly with a special tool (and straighten it if bent and steel). You can make your own tool for very little money.

Did you take off the Shimano and put on the SunTour? THe SunTour should be less prone to accidental shifts than most Shimanos as the teeth have far fewer cutouts and relieving to promote easy shifting. If you went the other way, to Shimano you may find that derailleurs settings, both limit screws and the lever position are more critical if you don't want unplanned shifts. (I for one actually dislike Shimano FWs and cassettes for friction shifting.)

Ben

You should always check your derailleur stops when changing freewheels, especially with different brands.

It should be pretty easy to do, just put the bike up on the stand and make sure you can shift into all gears, don't over-shift into spokes, or off the high end. And, the chain goes straight into the derailleur.

If you have a B-Screw adjustment, you can make sure the derailleur is as close to the freewheel as practical without touching.

You should always count on making minor indexing and limit adjustments when changing freewheels/cassettes/wheels, even like for like but especially when changing brands or tooth counts.
Edit: You might need a longer chain if you changed to a freewheel with a larger big cog.

Thanks to all for the advice. A few bits I failed to mention before:
The chain was changed at the same time as the freewheel.
Hanger seems straight to my eye, never gave trouble with the Shimano freewheel.
Cogs are the same sizes.
Bike has no issue shifting to the biggest cog on a stand or on flat ground. The only difficulty was during a hard climb.

I have not bothered to check limits or B adjustment because of how well things felt on the stand. Today it felt like I really had to pull my shifter back hard to engage. I'll go back and check my limits further.

I frequently have to adjust the barrel adjuster a click or 2 from stand to street.

Last night I made an adjustment to the low limit and it seems to have brought the derailleur in a bit more without tearing into my spokes. I'll hopefully get outside on the bike today and check the response.

Thanks for the help everyone.