1983 720 stutters in 1st, then runs great.

[MinuteMilitia]

First car, found my dream truck! Dont know much of anything but have been working on the truck to the best of my abilities.
Changed all the fluids, replaced spark plugs, the basics. 
Most of the problems it had are gone, but one issue remains: When shifting into first, it stutters/stumbles. Any other time it runs as good as any vehicle ive been in.

What is causing this? How can I diagnose? What advice can you give! Thanks a ton in advance.

[datzenmike]

What differentiates first gear from all the rest as far as engine running? That's the question. Some questions...

 

This is first gear with the clutch up engaged?

 

Does it idle ok in first holding the clutch down?

 

What happens if you hold a steady speed driving in first? or is it only while accelerating through first?

 

 

I'll wait for your answers to the above.

There are transmission switches. Neutral and top gear. Nothing special about first.