Throttle Position - no vacuum at idle

Joined
Sep 9, 2019
Messages
224
Location
LA, CA
Hey all - having a tough time with my throttle positioner and not getting vacuum from VSV port 1 at idle. Truck is a 10/76 build 1977 California 2F. I bought a "new" VSV thinking this might be it, as the '77 emissions control manual pointed to that as the issue, but to no avail. I've also reflowed solder on all of the joints on the back of the emissions computer circuit board, replaced all hoses, and checked for other vacuum leaks. Additionally checked that the VSV is receiving power (it is) and checked all of the pins as per the emissions control manual. I have not tried the speed test, as I don't know if there's a point in doing so if there's no vacuum at idle; i.e., no way to see if it's switching off if doesn't turn on to begin with.

I've also checked all fuses and they look fine, however I've only looked at the fuses under the dash (if there are others I don't know about them?). Last thing this points to is a wiring problem, potentially. Any other ideas / thoughts?

TIA!
 
Joined
Sep 9, 2019
Messages
224
Location
LA, CA
SOLVED - this was an issue with the carb rebuild kit that I used and the very specific 1977 CA spec carburetor. Port 14 on the carburetor is ported vacuum, sitting just below the primary throttle plate. However, the inlet for it in the carb throat is toward the center of the carb. In order to get the vacuum over to the passenger side-facing port, there’s a small channel in the EGR block, shown below, that links up two holes on either side of the primary carb throat.

873F4581-7476-481D-9328-0F4E68D6C28F.jpeg


216EE705-CE27-4AF3-9968-5C96803F2E71.jpeg


The holes in the general use gasket that are included in the carb kit are for the slow jets, NOT the ported vacuum. As a result, the gasket blocked off the two holes shown above, and resulted in no vacuum (and, by extension, no throttle positioner). Whoever had the car before me didn’t spend the time goofing with this, and just backed the high idle screw out entirely to compensate. See below for how the gaskets come to you from the kit:

6EAB0445-401D-40D6-8BED-AAB00C9B0DF4.jpeg


The solution here was to create two additional holes in the bottom gasket that line up with the holes in the carb, being very careful to not drop gasket material down into the hole that leads to the carb throat. The inlet is tiny and it’s worth blowing that out with extra carb cleaner to clean it out.

Once I finished this, my throttle positioner works great. Still need to test the speed control computer on it, but this now pulls vacuum at idle through the new VSV. Most likely my old VSV was fine, so will probably throw that back on there and list the additional ‘77 CA VSV for sale. Ah well.
 

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