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I have made sure the FD (Front Derailleur) is at the right height and angled parralel to chain.
Now, I have shifted the gears rear and front outward most cog (8 and 3 respectively) and gone to adjust the H limit screw to set 1mm gap however the FD is not moving when screwing in or out.
What could be causing this? I redid the tension at the pinch bolt but has had no baring on this.
The front derailleur (FD) H limit screw simply limits the travel of the FD to the right. It does not adjust the position of the FD, just the point where it will stop moving regardless of the pull on the shift cable.
If you are trying to "set" the High limit on the FD, you will need to pull the cable more to move the FD more to the right. On some derailleurs, you may have enough leverage to manipulate the lever arm [at the pinch bolt] to move the FD to the point where it will contact the H Limit screw to see where it is set (to achieve the 1mm gap you mentioned). If the gap between the outside edge of the chain and the outer part of the FD cage is > 1mm, you turn the H Limit screw clockwise to reduce the gap. If the gap is < 1mm, you turn the H Limit screw counter-clockwise to increase the gap.
Sometimes a person can help manipulate the FD to the right while a second person is free to make the adjustment. However, this is usually a one-person job.
Park Tools has an excellent video on FD adjustment. It covers the H limit screw adjustment in detail starting at about 6:00 in to this VIDEO. Prior to that, it details the FD alignment and L Limit screw adjustment, so it is all good content, and well worth checking out.
Note: The H and L (high and low) limit screws on front and rear derailleur (RD) are just travel limiters. They do not adjust chain line positions while riding per se. However a case can be made that the settings made on the L Limit screw on the FD and the H Limit screw on the RD are used as index points. For the FD with a slackened shift cable, the FD is usually on the the L limit screw setting. For the RD with a slackened cable, the RD is on the H limit screw setting.
