244 won't rev when in gear

[Johnno]

A few months ago my '81 244 developed a problem - it simply won't rev when in gear; works fine in Park or Neutral, idles fine and revs up n down - but as soon as it's in gear I have no throttle response, I get nothing but idle.

If I take it on the road, I have to wait till the car gets some road speed, once it has crept  up to 10kph or so, then the throttle will respond and I can rev it and get going - but it takes a while before it gets any road speed up, so it's not good to go into traffic like that. As soon as it's back to idle, I have wait for road speed to pick up again before I get any throttle response.

I've replaced the air mass meter with a spare from another '81 244 (complete with fuel distributor, lines and injectors), and have installed all brand new vacuum lines from/to it, the throttle assembly, flame trap, idle mixture bypass etc; but I still have the same problem as above 

Before I get it towed to a mechanic, does anyone have any ideas what would be causing this behaviour ? Fuel filter, vacuum accumulator, brake booster whatever

[Johnno]

After a great deal of time eliminating one cause after another, it seems the vacuum advance unit on the distributor, although functional, wasn't being activated properly - probably a perforated, or perished, diaphragm inside.

As I didn't have another distributor to grab the advance unit from, I decided to test it by manually advancing the distributor a little. That fixed it; the motor runs fine, still starts ok, and most importantly revs in gear - it's now drivable again; just need to fit another vacuum advance unit to fix it properly.

Such an easy fix, but took forever to figure out