Chain

[twangin4u]

Well an old stretched chain cause a hercky -jerky, or pulsing  momentum at low speeds?

[Country Joe]

Yep, it sure can.

[red]

Well an old stretched chain cause a hercky -jerky, or pulsing  momentum at low speeds?

Twangin,

A worn chain, which usually means worn sprockets as well, can cause this.  A loose chain may do the same thing.  Put the bike up on the centerstand, and select Neutral gear.  Check the chain tightness at the place where the chain is the tightest, when rotating the rear wheel by hand.  If the engine is simply running rough, try a can of SeaFoam or other cleaner in the gas tank, for a few tankfuls.  Then, if not improved, check plugs, et c.

[Schurkey]

I would have said that a loose chain makes the jerking worse, but the root cause is typically lean carburetion.

The engine surges because the fuel-curve isn't right, and the loose chain amplifies the surge into even worse speed fluctuations.

I'm dealing with this on my '86 FJ12.  A prior owner scrapped the OEM airbox in favor of those crappy pod filters, so there's major fueling problems.  The owner I bought it from apparently couldn't adjust the chain.  I put an entire turn on the adjuster nuts, and I don't think I've got the chain tight enough yet.

[balky1]

If the chain has developed a tight spot it will be like you describe and it is time to change it sooner rather then later.

[FJmonkey]

A worn chain can cause holes and crack in the case when it breaks. Check your chain.