Just bought '83 air cooled Vanagon L

Where is throttle, which way do you turn it, does engine have to warm or not? '83 air
cooled Vanagon L. Just had tune up, & probably lots of stuff that didn't need to be done.
Ran great (900+miles) until we filled up, just under half tank, with gas. Then just seemed
to loose all power. Couldn't go over 49-50 mph even with truck draft. Got home, barely,
changed oil filter, still not running right. Oh yeah, it backfired after fill up going slightly
uphill.