Stalls at idle

Discussion in 'Troubleshooting & Diagnosis' started by frozencaveman, Jul 23, 2017.

  1. frozencaveman

    frozencaveman Member

    May 24, 2010
    Davis, CA
    Hey all,

    I've been fighting a stall condition on my 67 250 w/PG and A.I.R. system for quite some time. The motor has 120k original miles on it, head and transmission 40k miles after rebuilds. Pump is going out on the tranny, probably owing to my lack of experience mating the tranny to the engine. I've had the car for 18 years and frequently drive it to work.

    Symptoms: Within the last few thousand miles, I'd drive it a mile or two, no problems at the first stop sign, stalls out at the 2nd. Usually I can fire it back up fairly quickly. Generally, after it was completely warmed up, no more problems. I would change my route to work to accomodate the problem. This had been going on for a couple years until it got really bad last year and I couldn't even put the car in gear without it stalling. I swapped out the Carter YF (great little carb), which was starting to have a throttle leak, and put in a new (from Classic Industries) Rochester BV. Symptoms remained the same. I finally figured out I had 1 bad spark plug wire, and repairing that, I was able to drive again with the occasional stall.

    Recently, I drove 25 miles on the freeway, exited, stalled turning into a driveway. Managed to get home by turning in the idle screw.

    I can fire it up with minimal effort (1-2 cranks), and under load, the car seems to drive pretty well. The distributor control valve was leaking, so I've replaced that. Replaced the plug wires. Replaced all the narrow vacuum lines in the engine compartment. None of this seemed to make a difference. PCV valve still functioning. Reset the timing to about 13-14 degrees (vacuum line plugged) and idle at around 750 RPM. Car runs now, but still idles a little rough. Sprayed carb cleaner at various locations to check for vacuum leaks and didn't find anything. No loose manifold bolts to the head or the carb. A shop told me I do have a slight leak on the manifold, probably because when I rebuild the head at 80k miles, I didn't have the intake/exhaust manifolds machined.

    I wanted to see if anyone had input on the vacuum line routing, just to make sure my setup is OK. I have line running from the vacuum modulator on the tranny to a T that splits to the base of the carb and to the distributor advance. At idle, I read at least 15 Hg with a vacuum gauge. As idle increases, that goes up to about 17-18 Hg. Should I have a reading on there at idle? I haven't taken a reading off the intake manifold where the PCV hose attaches. I also haven't checked hose at the vacuum modulator yet to make sure it's intact, but given I have that reading of 15 Hg on that line, I'm thinking it's intact. I don't see any significant amount of fluid inside that line to suspect that there's a leak in the modulator.

    I don't know how the A.I.R. system works, so maybe there's something catastrophically bad there, but I don't know how to go about troubleshooting it. I replaced the one vacuum line that goes to some sort of contraption (diverter valve?) attached to the intake manifold, and sprayed carb cleaner around it to see if there was a leak where it fits into the manifold, but the engine speed didn't seem to change.

    I'm pretty close to wits end here. Everything except the carb is stock. I don't know if the BV is different enough from the YF to cause this since I still have the A.I.R. system intact. I don't know if a failing automatic transmission can cause these symptoms.

    Thanks for your thoughts...


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