Before making Jetting changes, the basics must be correct. Maintenance adjustments completed. Ignition system checked, valve adjustment, and compression test, all would be a good Idea to insure engine is correct. Fuel Level in the carbs must be checked, and set if necessary. It would be best to make sure the Carbs have not restrictions, or damaged parts, such as blocked jets, air bleeds, transition circuits. Torn or leaking slide diaphragms, or Air-cut diaphragms. Vacuum leaks between Carbs and engine must be eliminated, also Air leaks between Carbs and Air-Box.
So jetting for the A5 model is in the Supplement section of the FSM. You may have the Later model. But here this info is, and nowhere is a 138 main mentioned.
There are a couple of very cheap ways to make sure your Ignition is up to it. First Check both the Primary and Secondary resistance of both coils, and then the Boot that attaches to the plug, as it contains a resistor. If you have the Original plug wires, get new 7mm hypalon plug wire, available by the foot and 4' is plenty. You can replace it easily for a couple of bucks. Don't run a Spark plug that has an "R" in the part # as these are resistor plugs, you already have a resistor in your Boot that goes down on the plug. So D8ES, not DR8ES. This will give a Hotter Spark, simple stuff. Taking plug readings, after this work will show if it is rich.