Been working on a ‘75 dt400 I picked up. I have the correct key for the gas tank, and have been able to turn the key fully after a few days of wd40 & wiggling the key. However I can’t seem to get the cap to pop open or anything, but it kinda sounds like the latches might be moving.

Which position of the lock is unlock, and how should I be trying to force this thing open (without breaking my one key)? Cap won’t at all





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The lock keeps the cap from turning.  Cap twists CCW about 1/4 turn to remove, it doesn't "pop open".

When key is turned CCW it lines up with rivet as shown in your second picture.  Locking tabs are retracted in this position and cap can be removed and installed by turning whole cap.  (key can be removed and cap left unlocked). 

 

How I would proceed is: 

1. Turn key counterclockwise to line up with rivet and remove key. 

2. Twist cap counterclockwise approximately 1/4 turn and remove cap from tank.

If your locking cap has bad seals or something, it can be replaced with the early 1968-71 non-locking gas cap that should still be available from Yamaha

 

Might want to get a second key made also. Just in case!!

Here are photos of mine.

1) Key in locked position.

2) Key in unlocked position.

3) Inside of cap with locking tabs retracted (ie cap would be unlocked).

4) Inside of cap with locking tabs extended (ie cap would be locked).

Push cap down firmly whilst twisting anti clockwise.