Suzuki Intruder won’t start up properly

My Suzuki Intruder 1500 has a starting problem, sometimes it starts fine, but about 50% of the time it will turn over really slowly and not start. I have adjusted the decompressor cables, fitted a new battery and a new decompressor solenoid. The battery voltage is still well up (12.4v) even when it won't spin over, what else can I do?
Ian Anderson, email

The 1500's starting system is peculiar. When you push the starter button the battery sounds flat as it's trying to shift two massive 750cc pistons while the decompressor solenoid pulls open on both cylinders. I'd get the compression checked on those cylinders. If there's too much because the valves aren't adjusted properly for example, then the solenoid's 0.5 second lift isn't long enough.

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