Diagnosis
The bar has worn unevenly, a common issue called "rail height discrepancy." The guide bar's rails—the two edges the chain runs in—are not the same height. The side that's shorter (more worn) allows the chain to sit lower and feel loose, while the taller side pinches the chain, making it feel tight. This uneven wear is often caused by not rotating the bar regularly or by a chain that's been sharpened unevenly.
How to Fix It
First, remove the bar and chain. Clean all sawdust and debris from the bar groove, oil holes, and mounting area. To fix the uneven rails, you need to "dress" the bar. Use a flat mill file or a dedicated guide bar dressing tool. Lay the file flat across both rails and file the higher rail down until it is perfectly even with the lower one. Ensure the groove is clean and the bar's oil holes are clear. Reinstall the bar, making sure the tensioning pin fits correctly into its hole on the bar, and adjust the chain tension evenly.
If That Didn't Work
If the problem persists after dressing the bar, inspect other components. Check the drive sprocket (clutch drum) for excessive side-to-side play, which can misalign the chain. Also, ensure the bar mounting studs and the adjuster pin are not bent or damaged. Finally, the chain itself may be damaged or have unevenly worn drive links; try a brand new, properly sharpened chain to rule this out.
Heads Up
A severely worn or improperly dressed bar will rapidly ruin new chains and is unsafe, as it can cause the chain to derail or break. To prevent this, rotate the bar 180 degrees every time you sharpen the chain to distribute wear evenly, and always keep the bar groove clean and lubricated.