Samuel, assuming all other items are up to snuff...tune up, valve adj., no vaccuum leaks etc. One of the problems of that system are dirty ground connections. All White wire grounds, pull off and on, 5-10 times to clean them. Another item is the MPS or Manifold Pressure Sensor, it is also on the '68 and later VW type 3's. It is about 2-1/2" in dia. and mounted on the Pass. fender well. One Vaccuum hose and an electrical connector. These have a S.S. bellows (a Barometer)inside. This unit fills up with oil that comes through the hose from the intake manifold. 4 screws hold it together, take it apart carefully, the bellows will fall out into your hand, so do this rt. on the table top. Scratch an alignment index mark on the two halves for reassembly later.
First, chk. to see if the bellows is cracked from flexing for 30 yrs., if it is it's toast. Find a used replacement. It should be springy between your thumb and forefinger indicating that it's sealed. If it compresses readily or goes flat, when squeezed, then it is bad. DO THIS GENTLY. If it's OK, then clean out any oil inside. (the oil slows or prevents the "armature" from moving back and forth). One last thing, while you have this in two pieces, look at the end piece with the screw in the center. It is usually covered over with epoxy. Take a propane torch and gently heat this epoxy and scrape it out. Don't turn the screw. I put a small center punch mark on the screw and on the body for ref. later.
I have found over the yrs. that just doing the above, usually helps alot. The pupose of the screw is for Fuel enrichment adjustment, rich/lean. I have had quite a few VW customers cars (at 3500 ft. elevation)that ran too rich or too lean and having this adustment made it possible to get them running well again. Hope this helps, Tom