Welcome to the forum.
Here's a helpful link to a good CIS trouble-shooting site:
911 CIS Primer - Index
A '75 has the hand throttle (and I'm assuming it's adjusted correctly) and the fuel pump will run as soon as the key is in the "on" position. Something to try next time it won't start when warm:
- Hand throttle all the way down.
- Key to "on"; listen for the fuel pump and let it just run for ~30 seconds. The point of this is to try to prime the fuel system if the accumulator or check valve are bad.
- Pull the hand throttle up ~1/4" and engage the starter.
If the thing starts then it's most likely the accumulator or check valve on the fuel pump. If it doesn't, then next step is an ignition check (presence of spark). If no spark, check points, coil, CDI, etc. If you've got spark then it's time for a full CIS pressure check.