When you shift into any gear on the Honda 3810 riding mower, it takes about 15-20 seconds before the unit starts to drive. Once it's moving, it's fine, doesn't appear to slip or anything. Any suggestions would be great.

Find the tensioner bolt on the swing arm attached to the spring, tighten that bolt about 4 turns. Does it get better?

You know I am not talking about the mower deck, correct? I.e. not the tensioner at the front of the tractor, but the clutch for the transmission.

When you push down on the cltuch you operate a spring loaded tensioner arm that gives slack to the driove belt. Letting the cltch out allows the spring loaded arm to reapply tension to the belt, you need more tension.

I think I have figured it out, although, this unit does not use a belt, it's direct drive (via drive shaft). BTW, what is the typical donation people leave, haven't been on this site for many many years. I did post another article, the unit sometimes will just not start, other times, it fires up right away. Appears to have spark, gas to the carb, checked all connections. If you have seen this unit before, you'll know about all the electronics and relays etc. I am sure there is a "magic" part which will / would resolve the issue. Any ideas?

If you check on the Honda 3810 there is a device on the shift lever called a damper. It does just that, it softens the shift of the user to keep the transmission from jerking (like a Craftsman, etc.). The dampre can be adjusted and it should bring the mower into gear within 2 seconds. Look on on of the web sites that sells Honda(like germanbliss) and you will see the damper and the adjustment is on the bottom.