I have an older abu garcia ambassador  5000 that works great .. but I have to adjust spool tension every couple cast to avoid backlash... any ideas why tension won't hold ?

There should be an o-ring under both tension caps.  If they aren't there, the knobs will move too easy.  

This real only has 1 tension cap 

Tension cap had no o ring,  put 1 in ... should it have shims behind o ring?

Look at the reel foot and cross reference the schematic, it will indicate the number of shims that go inside of the spool tension cap.  Most will use 2, but some will use more.

You can usually feel the number of shims needed by counting threads remaining to bottom-out.  

E-replacement parts or Dad's Old Tackle (also sells on ebay) will have both shims and the O-ring you need.  

Rubber has a finite life, especially under constant compression - replacing the O-ring at least every decade is a good idea.  

 

Found the shims behind o ring ,, they had divots  .. cleaned and reversed then ... seems to work fine now ..  Thanks 1 and all for advice and info 

My dad used to crank his down so hard he's turn them into donuts, I had to make him new shims from phosphor bronze sheet I keep around for fabricating antique reel springs.  

I have a nice hand press-brake cutter for making spring strip - cut in the other direction, and you have a shim.