Adjustment screws on front derailleur no longer screw in
 

Here's the problem. I bought a new Cannondale road bike and have been testing it today. I noticed that the gearing was not set properly as the chain came of the crank set two times. I fiddled around with the adjustment screws and now have the problem that I can't screw them fully back in again. Orignally they were more or less fully screwed in and were flush with the derailleur surface. When I was playing around with the screws to get the gearing right again, I suddenly noticed that I could no longer screw them back in again. They only go half way and then stop. Any idea what the problem could be?

Take it back. That is really about the only advantage of buying new, you can take it back. I only buy used, so I have to work through problems when they occur.

1/2 turn probably would have done the job.
When screwing in the L screw, make sure the you aren't on the small ring, so you aren't trying to use the screw like a jack.
Ditto on the H screw, make sure you aren't on the largest ring.

I'll guess here that it's just a maladjustment to your FD - which can be fixed in short order. Here's a link to help clarify what's going on there:

http://bicycletutor.com/adjust-front-derailer/

And another:

http://www.parktool.com/repair/readhowto.asp?id=75

I think that this may be the problem in that the poster is adjusting the screw with the wrong ring selected i.e. trying to turn the "H" screw inwards while on the big ring.

Update

I took the bike to my bike shop and told them that the gearing was not adjusted properly. I didn't tell the guy anything about the screws I could no longer screw in as I had the feeling I may have damaged the threads. Anyway, he took the bike away and when he came back again said that he had adjusted the gearing and that I should not touch the little screws anymore because they were set totally wrong. I checked the little screws and they were both more or less completely screwed in again – as they were when I received the bike.
So the gearing is working again now, but I still can’t explain the problem with the little screws, i.e. that that they are so difficult to turn - which they still are.
When I first noticed that the gearing wasn't adjusted properly, I got a LITTLE precision screwdriver and fiddled around with the adjustment screws. It was no problem using this little screwdriver for this task. And yes, I had the chain sitting on the correct cog and so on as I checked on the internet what I had to do. At the time, I seem to remember that when I turned the screw, I could see the front derailleur slightly move outwards or inwards.
For some reason I kept fiddling around with these screws and at some stage completely unscrewed them. Now the problem began. I could screw the screws back in again, but only about half way and then they simply wouldn't move anymore. I had to get a much bigger screwdriver and apply some pressure to screw them back in again. That's the reason I thought I may have damaged the threads, although it did seem rather unlikely as I had tried to re-align the screws over and over again before I finally decided to get the bigger screwdriver.
As I already said, the gears are now properly adjusted. Out of curiosity, however, I got my little precision screwdriver again and tried to turn the screws slightly to check whether now the guy had fixed the gearing, the screws could be turned with the same ease as they could be when I first got the bike. But they can’t. I’m sure I could adjust them with a bigger screwdriver, but as there’s not need, I haven’t.
If anybody has an idea what could have caused the problem, I would be grateful to hear from them.
Thanks, Zweibel

Its already been stated by a few and I will state it again. You have to adjust the limit screws when there is NO tension on them. ie, you adjust the (H) screw when the front chainring is on the small ring and you adjust the (L) screw when on the big ring so that you are not trying to adjust against the cable tension. Once you have made the adjustment you need to change the ring and see how it goes. Also its the cable tension that has more of an effect of the front derailer performance. Ideally you never need to use the limit position.

The front derailer requires subtle adjustment so turning the limit screw all the way in or out isn't the way to do it. See, http://www.parktool.com/ for instructions.

Anthony

It's a fact of life that there are a few people that should not, on any account, try to repair or adjust their bike. I fear from the above account that you are one of them. If your local bike shop owner has a daughter, then marry her as a matter of urgency.

"and when he came back again said that he had adjusted the gearing and that I should not touch the little screws anymore because they were set totally wrong."

"Out of curiosity, however, I got my little precision screwdriver again and tried to turn the screws slightly to check whether now the guy had fixed the gearing"

You simply don't learn, do you?
The other posters TRIED to explain and you refuse to listen and/or are incapable of understanding!

+1 Take the advice of the mechanic.

If you want to learn about wrenching on a bike, I would suggest picking up a junker at a garage sale or thrift store, tearing it down and reassembling. You can quickly learn all about it and not risk messing up a nice new bike.

And if you continue to fiddle with your new bike, the mechanic at the shop will probably get less patient quickly.