Fuel tank cap key question

Found a barn bike I couldn't pass over. 1982 GPZ1100 complete and all original. A lot of work will be needed though but I'll do this after my current KZ750 project.

First question regarding the GPZ: it comes with two keys; one that operates the ignition and one that works on both helmet locks.
Which one is supposed to open the fuel tank cap?
Both keys are OEM but only one has a stamped code and there's no code on any lock. I tried both keys (almost to the point of breakage) but nothing worked.

Because the bike has been is storage for something like 10 years, the fuel tank cap lock mechanism is maybe stuck. I sprayed some "Jig-a-loo" but will wait tomorrow to test it again.
However, if the mechanism remains impossible to operate, is there a way to remove the cap anyway? I'm thinking a trying to push out the pivot pin but I ignore if such a thing is possible, Anyone knows what I can try?
Thanks!

Not positive on this but I think any double-sided key should work on the lock. I'd apply your lubricant of choice liberally and try to work it back and forth gently - you should be able to get it free eventually.

As far as pushing out the pivot pin, that may work if you can access it, but it seems a bit more risky if you're trying to save the paint.

If the lock doesn't open after a couple days, you might try "Metal Rescue" or a similar product, poured into the upside-down tank through the holes of the fuel sender or petcock (not sure of term for fuel injected bikes). 

It may be lockpicking lawyer time.
I've had to pick a Honda fuel cap before.

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Not positive on this but I think any double-sided key should work on the lock.
 

This is not true, unfortunately. I have some KZ tanks (440/550) that will not cooperate with any of the keys I have, even though the locks appear to be clean/not rusted.

As far as pushing out the pivot pin, that may work if you can access it, but it seems a bit more risky if you're trying to save the paint.

This seems to be the recommended option - carefully, of course.

I have a few of the standard 80s fuel caps & the keys are interchangeable.

I would make sure the fuel cap doesn't have a replaceable locking cylinder, found on some models.  On these I'm pretty sure you need the correct key.   Partzilla doesn't show that type on your GPz, but the diagrams are for USA (N. American) models.  So your motorcycle could have the type:  either stock or someone could have swapped it out.  The ones I've seen are more robust around the lock cylinder and have kind of a ring around the cylinder.

Kawasaki fuel cap w replaceable keyed locking cylinder.
 

 

TIP:  The original fuel tank cap is black, not chrome.  What's on your bike now?

The key code is definitely stamped on the backside of the seat lock.  You may need a magnifier to see it, but it's there.

If you can get Kroil penetrating oil, douse it well and wait.  It will likely get it working again.

Just wondering.  Is Kroil going to attack the fuel tank paint?

I've never had Kroil attack painted items.

Here's where the seat lock, key code is stamped.  It will begin with Z.

 

Sorry for late answer; last days were quite busy.
Below is a picture I just took of my GPZ1100. The cap seem to be the same exact one than on my KZ750L4. And both are black chrome.

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TIP:  The original fuel tank cap is black, not chrome.  What's on your bike now?

The key code is definitely stamped on the backside of the seat lock.  You may need a magnifier to see it, but it's there.

If you can get Kroil penetrating oil, douse it well and wait.  It will likely get it working again.

The GPZ came with two keys; one for ignition and one for seat locks. I assume the number stamped behind the seat lock will be related to the key needed for the said seat lock.
However, I checked on my KZ750 that also has two keys; one for seat locks and one that works in igntion AND fuel tank cap. Based on this, I believe that, for the GPZ1100, it is the ignition key that should open the fuel cap. So, far, I tried both keys. The key turn a little but not enough to unlock the cap. I could force a little bit more but I'm afraid to break something. All the time, I'm strongly pushing on the cap itself while turning the key but no luck.

That being said, I believe the fuel tank cap barrel key is not stuck and it is possibly the spring loaded locking piece (shown at upper portion of Martin's second picture) that could be stuck.
I did not work on the GPZ so far and I could have a bad surprise (something like a brownish liquid filling the tank) when I will open it. When I'll give a look at the GPZ, I'll first try to push out the pivot pin of the cap. If it doesn't work, I'll remove the tank and try to work from underside.

If your cap is like the one pictured below, you can see the 'gaps' in the corners of the cap on the hinge end.  I know on KZ1300 models, it's possible to get in that gap with a thin punch and punch the hinge pin out to remove the cap.  Once the cap is off, you can take it apart to find out what's stuck.

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Kawasaki fuel cap w replaceable keyed locking cylinder.

 

It's likely that the latch (circled in red) is stuck.  You can remove the fuel sender and get a good idea of the tank's condition.

I would drain it (if necessary), invert it and then pour several gallons of boiling hot water into the tank (through the fuel sender opening).
Then put the fuel sender back in.  Let it sit for 5-10 minutes, put the tank upright and try to unlock it. 

I've used boiling hot water to free up totally stuck and heavily varnished carburetors.  I think it would work in this situation too.