Interesting drop in revs- Mk 2 2.0L Escort

[Pazuzu]

Hi all.

While cruising out near Wiseman's Ferry the other day, I noticed that my tacho was reading about half of what it should, i.e. reading a little over 1,000 while I was sitting on 60 in 4th gear. The engine sounded right, like it was doing a bit over 2,000 r.p.m., but when I got it back to the garage it was idling at around 400, and would stall as soon as I took my foot off the gas (yes, it was in neutral!) I've set the idling speed at about 750- 800, so it seems to have halved somehow.
Has this kind of thing happened to anyone else? I've just rekitted the carbie, and -farted around with the vacuum hoses, but my mate reckons it has been doing it on and off the last few times he's taken it for a spin, before I fiddled around with these things.

Any thoughts as to what could be doing this?

[grey_esp]

Is it a monster or an after market tach with a switch on the back for 4, 6 or 8 cly? because the switch has probably vibrated into the 8 position.

[whooligan]

While idling, pump the brake peddle. If the revs drop when you apply the brake, its clearly a vacuum problem. If its stalling on idle, its probably crank case ventilation. ie PC valve get gunked up with crap. Just wash it out with WD40 and re-check all your hoses on the inlet manifold for air leaks.