Finally got my first VHF/UHF mobile rig installed in the truck - Yaesu FTM-400XDR with a Diamond NR770HB dual band antenna on a NMO mount. Testing with the antenna in final position showed good 2m SWR but 70cm is reading 2.1:1. Do I need to adjust the whip sections on this antenna model? The manual mentions adjustments but I want to make sure I'm doing it right. Also wondering if my coax routing could be affecting the readings - I have about 15 feet of LMR-240 running from the roof mount down to the radio.

Start with getting the 2-meter band perfect first, then work on 70cm - they can interact with each other. That's a quality antenna so it should tune up nicely. Check that your NMO mount has a solid ground connection to the roof too.

Keep that coax run as short as practical and make sure all connections are tight. Your LMR-240 is good cable choice. Also do your final tuning with doors and tailgate closed - changes the ground plane.

I had similar issues with my dual band Diamond antenna. The key is patience - alternate between bands and make small adjustments. Don't try to rush it with big changes. Are you using an SWR meter or antenna analyzer?