1972 911T - Timing not advancing to 30-35 deg BTDC at 6000rpm

I've just starting tuning my 1972 911T with the original 2.4 MFI motor. The timing was out and it had old plugs, oil and filters. The engine was not pulling strongly and popped and spluttered at points throughout the rev range.

I've replaced them and set the timing to 5 deg ATDC at idle, but when revving up to 6000rpm it only gets to around 20 deg BTDC rather than the 30 - 35 BTDC that it should be getting.

A bit of reading around PP indicates this could be wear or dirt in the distributor or a problem with the vacuum / diaphragm. I'm not an experienced tuner, so would welcome any suggestions on how to start troubleshooting this please.

When timing to 6000rpm you have to remove the vacuum retard line. Also, at idle, with this vac removed the timing should be around 5 deg BTDC.