Having an occasional problem with the chain skipping when I shift the front derailleur. Sometimes it is when shifting up to a higher gear and sometimes to a lower. Could the problem be not enough tension on the chain? The effect when pedalling is as though the crankset axle had broken, a little unnerving to say the least. The amount of lateral movement is adjusted pretty well but its as though the chain does not lock down in the cog teeth immediately as it should.

Does this happen when you are pedaling with full force? If so, try easing off a bit during a front shift and see if your chain settles faster.

As Jason said, you have to lighten the pedal load and allow the chain to properly engage the new chainring before applying full load again. This is more important when not using matched Hyperglide (gated shifting by any name) chainrings which are phased for smoother re-engagement during a shift.

When you shift the FD the chain may not meet the new sprocket perfectly in phase to drop in and engage. For example, the rollers may line up with the tops of the teeth rather than the space between them, that means that there needs to be some relative movement before the chain engages, causing the slippage you feel. In some cases with high loads the chain may skitter along on the tops of the teeth rather than dropping in. This was more of a problem in the rear before Hyperglide came out.

Actually, I think the problem seems to be more frequent when shifting without load. Should point out that this is an older vintage bike, a '78 Centurion Le Mans 10 speed. It happened both when climbing a hill and on a slight descent. In both cases I believe I had unloaded the pedals slightly prior to shifting. Think it might have something to do also with the speed of shifting. These are the old friction levers I am talking about. Happened also on my old Fuji S10S 12 speed. The first time I thought the crank shaft had failed. If I keep spinning the pedals eventually it drops in to the teeth.

You have to observe carefully what is going on. For example, if you've upgraded to a newer, narrower chain, it's possible that the chainring separation is slightly too wide, allowing the chain to lean on the inner face on the larger chainring with the plates lined up on the points of the inner ring. With correct separation the face of the outer ring pushes the chain over far enough to drop in.

Test for this problem as follows. Shift to the inner chainring and an outer rear sprocket. Backpedal slowly while lifting the chain to the top of the inner rings teeth and see if you can get it to stay up there trapped against the face of the outer ring.

That covers one possibility, phasing as i described earlier is another, and since this is non-indexed shifting, the positiveness of how you shift is a third factor.

Try to make note of when it happens, and see if you can figure out how to duplicate the problem at will. If you firgure out how to make it happen, that will be a guide to how to avoid or fix it.

it sounds like both FD travel limit screws are slightly tight. back off both 1/4 turn. check shifting on a stand before riding.

Its not a new chain. In fact it probably is the original. Appreciate the info from all. When I looked carefully at the forward derailleur limit screws I saw that they were not quite adjusted as well as the should be. Found it difficult to turn the little screws with my phillips head screwdriver until I added some penetrating oil and filed down the tip of the driver because it seemed not to fit the slot properly. Also they are so close to the down tube that its hard to use the screwdriver efficiently. After adjustment it seems to function pretty well but will have to examine it carefully and test (on the stand) for shifting front derailleur back and forth repeatedly and with varying degrees of speed.

Those chainrings don't have modern ramps and pins, so sometimes you'll get a lousy shift. This bike is from when it actually took skill to shift well repeatably, so don't fret the occasional bad shift. Of course, there could be other issues related to chain/ chainring wear, chainring spacing, etc.

Just a follow up: After a little careful observation I realized that when it happens the way to correct it is to ease up on my pedaling slightly just the reverse of what I was doing. That seems to give the chain the slack it needs to drop into the teeth correctly. Not really curing the problem but at least dealing with it when it happens.