Running rough

[rpmerf]

1970 Chevrolet C20 Custom Camper 350 Quadrajet, points ignition.

I'm half asking for help / suggestions, half pouring out my thoughts.

I have a wideband with the sensor after the driver's side header. I normally read about 14.6-14.8 at idle, lean out to as high as 15.5 at cruise, and hit somewhere around 12.5-13.0 at WOT.

Drove the truck to my parent's earlier today, and it was fine. When I left, it started out fine, then after about 2 minutes, it started running super lean. Anywhere from 16-19:1 at idle and cruise. As I opened the throttle, it would lean out more. I'm surprised it was still running at that ratio. The odd thing was, if I got the secondaries to open, it would go to about ~13.0:1 and drive fine. I got home mostly by cruising super lean, then going WOT any time I needed to accelerate.

I poked at it for a couple minutes, but didn't really find anything. Accelerator pump looks to be working fine. Checked for vacuum leaks at a couple places, but didn't find anything. Putting the choke on manually results in about 14.5:1 AFR. Choke full on normally gives closer to 12.5:1. I have a vacuum gauge on my wideband, teed into the vacuum advance line. It was reading about 15" at idle in park, which seems low. Pretty sure that is typically more like 18-21".

It seems almost like I am not getting fuel from the primaries. I would normally blame fuel delivery, but the secondaries seem to work fine. I don't expect an ignition issue, but it is always possible. I would think an ignition issue would have me running rich on the AF gauge. It's always possibble the AF gauge isn't reading correctly, but what the gauge reads, seems to me in line with the engine not running right.

I had the top off the carb a couple weeks ago to change out the primary spring, but i have driven it probably 50 miles since then, and it has been fine. This truck has been my daily for about 2-3 weeks, since I am doing suspension work on my Suburban.

Another thing - Last week when I got to my parents house, it was idling around 1300 RPM in park. I poked around a bit and it went back down to around 900 in park / 650 in drive. No idea what caused that. This has me thinking vacuum leak somewhere.

Plan for tomorrow:
Verify every vacuum line and vacuum device using a vacuum gun
Start it up and spray the carb to manifold and manifold to head areas with carb clean to attempt to find a vacuum leak.
Check timing
Fuel pump flow test
Change carb filter (last changed about a year ago. Maybe 500 miles since)
If I don't find anything else, pull the top off the carb to see if I see anything suspect.

Any other thoughts / ideas?

Thank you for reading