I have an '87 pickup, carbureted 2WD. I'm having a problem getting it to smog. When the tech disconnects and plugs the vacuum lines to check the timing the engine stalls. The CO is way high. I took it to a friend at a community college and everything worked fine. For a week I'd intermittantly pull the vacuum lines and it wouldn't stall. Today I took it back for the retest and same problem. I'm at my wits end. It seems to be an intermittant problem but what? Could it be one of the vacuum switches?