Road Cycling - Front derailer adjustment

I have an older set of 8 sped 105 STI shifters that I fitted to my older road bike. The rear derailer is new Ultegra with a new Mavic Open pro wheel, 8 speed casette. Everything out back works fine, but although the older 600 front derailer works well most of the time, it could be better. Here's what happens; with the rear at about 5th gear or higher and the front on the large chainring, the chain rubs against the cage. I like the fact that when this happens on the small chairing I always have the ability to do a partial shift and the cage will move to the right and out of the way of the chain. But when I try this on the large ring, I'd have to hold it there in order to stop the rubbing, it won't stay in that position. Is this the way this shifter is supposed to work, or is there something out of adjustment?
Thanks,
Rich

Try tightening the cable. Use the twist knobs. If you are looking down(from seat) on the FD tightening the cable will move it to the right. The opposite if you loosen.

I don't think 105 8-speed STI is compatible with Ultegra 8-speed STI for the front derailer. Years ago I tried to get an Ultegra 8-speed STI lever to work with a 105 8-speed front derailer but could not get it to work correctly because the derailer seemed to require more cable travel than the lever provided. Or maybe it was the opposite - the lever might have provided more cable travel than the derailer required. Anyway, I bit the bullet and bought an Ultegra 8-speed front derailer and everything has worked great ever since.

I'm pretty sure that 105 and ultegra will be compatible, but I won't put money on it. If you can't simply adjust the barrel to tighten the cable, you may need to turn the limit screw on the derailer.

On the top of the derailer you should see two screws, it limits the range of how far left the derailer can move and the other how far right. From the front or rear of the bike, you should be able to see the end of the screws "bottom out" as you shift the derailer to the left and right extreams. Find the screw the limits the far right and loosen it a quarter turn. If it's still rubbing, try another quarter turn.

cheers

The limit screws are set to allow the full travel w/o going too far to throw the chain (although this does sometimes happen). The cage travels far enough. but the STI shifter just won't hold it there.
As far as 8 speed, I didn't think the design of the front derailer was dependant on the number of gears on the rear. Is this so?
One thing I did notice, is that when I purchased the new rear wheel, I could see that the smallest cog is closer to the frame lug than the old setup. There is room to space the hub more to the left and this would cause the chain to move away from the outboard edge of the FD cage (where it rubs), but someone had advised me not to make any changes to the hub/axle position and since it was new, I left it alone.
Any other ideas appreciated,
Rich

Just because its new does not mean it is spaced correctly. In fact, one of my LBSs would always respace rear hubs to get the correct chainline. Think about it, if you have a hub that cant take 8 or 9 gears, the chainline will be off for one of the two. It may not be much, but it will be off, in most cases, it will work fine, but it is still off. This is something pretty simple to do. I assume you tighten the cable (not just the adjusting barrel). You could also rotate the entire front derailer ever so slightly, it sounds like (looking from top down) you should rotate it counter-clockwise (be sure to loosen your cable first). Or is there room to put on a shorter bottom bracket spindel? I would have your LBS check the chainline.

fogrider, I had rotated the FD to compensate, and now I will have the LBS check the chainline. That advice makes sense. Maybe the hub should be moved further to the left.
Thanks,
Rich