JanZverina Posted March 21, 2017 Share Posted March 21, 2017 Can anyone suggest what I need to do to lower the curb idle on a recently rebuilt (by a reputable source) Rochester 4GC carb on my '63 Riv? I turned down the curb idle screw at the left front of the carb to the point where it's no longer touching the baseplate - does that mean I have to bend one of those linkages above it or lengthen the rod from the accelerator pedal? I tweaked the two bleed screws following a recommendation to turn them out until highest idle is achieved and then screw them both in the same amount just before a rougher idle is noticed. But the idle remains a bit high. Any suggestions? Link to comment Share on other sites More sharing options...
PWB Posted March 21, 2017 Share Posted March 21, 2017 (edited) Tranny in drive? Check ALL linkage/spring between carb. and accelerator pedal. Look behind the pedal as well. Something must be bent or worn between all. Is it all stock? Floor boards replaced? Edited March 22, 2017 by PWB (see edit history) Link to comment Share on other sites More sharing options...
carbking Posted March 22, 2017 Share Posted March 22, 2017 (edited) Assuming that the "reputable source" knew what he/she was doing: (1) Disconnect the throttle linkage of the carburetor from the linkage to the footfeed. Does the idle change? If yes, determine where the linkage is defective. If no: (2) Visually inspect ALL vacuum lines for one that is cracked, or off. (3) Check the brake booster (if present) (4) Check the vacuum advance unit on the distributor (5) Don't know on Buick, but is there a vacuum controlled modulater on the transmission? Other possibilities, but if the linkage is OK, then next best possibility is a vacuum leak. Jon. Edited March 22, 2017 by carbking (see edit history) Link to comment Share on other sites More sharing options...
still looking Posted March 22, 2017 Share Posted March 22, 2017 Agree with carbking. Check for a vaccuum leak. Could even be at the intake manifold gaskets. Link to comment Share on other sites More sharing options...
JanZverina Posted March 28, 2017 Author Share Posted March 28, 2017 Update: Today I brought the Riv in to my trusted mechanic, who after checking all vacuum lines once more after I did, used a stethoscope all over the carb. He said it's definitely an internal air leak between the top of the carb base (not the manifold-to-carb gasket which is new) and the bottom of the air horn. So it's coming off again and to a highly recommended carb shop close to San Diego. I'm tired of shipping car parts out of state and back. So it's sucking in air somewhere where it shouldn't be, even after a top-to-bottom rebuild. Plus I had to ask for an exchange on an AC Delco voltage regulator (C635) after installing it, only to hear a strange hum coming from it and then seeing my dashboard "amp" light flash as if there was a 1,000-watt bulb installed upon turning the engine off. Sheesh! Link to comment Share on other sites More sharing options...
1965rivgs Posted March 29, 2017 Share Posted March 29, 2017 The 4GC is notorious (at least in my experience) for casting warping which results in the passages between the upper and lower carb body not sealing via the gasket. I have owned a handful of`62 Olds Starfires over the years and that is the standard carb. I had carbs I disassembled a dozen times before I realized what was happening. The good news is Riv owners can substitute an AFB for the 4GC without any other mods. If you know someone with an AFB equipped Buick it only takes a few minutes to substitute your 4GC for an AFB as a troubleshooting technique. I wish I had that option for the Starfires. Good luck, Tom Mooney Link to comment Share on other sites More sharing options...
RivNut Posted March 29, 2017 Share Posted March 29, 2017 2 hours ago, 1965rivgs said: The good news is Riv owners can substitute an AFB for the 4GC without any other mods. If you know someone with an AFB equipped Buick it only takes a few minutes to substitute your 4GC for an AFB as a troubleshooting technique. I wish I had that option for the Starfires. Good luck, Tom Mooney Been there, done that. Couldn't have been happier. Link to comment Share on other sites More sharing options...
carbking Posted March 29, 2017 Share Posted March 29, 2017 (edited) I probably shouldn't post because I don't understand what is going on or the diagnosis, but will insert foot in mouth anyway! First I agree with Tom about the AFB being an excellent carb. Now: There are two major gaskets between the three major castings. (1) The gasket between the lower cast iron casting (throttle body) and the center casting (bowl). The gasket used here has cutouts for hot idle compensation SO WILL NEVER COMPLETELY SEAL! This is by design! Reference: http://www.thecarburetorshop.com/Ventedgaskets.htm (2) The gasket between the center casting (bowl) and the upper casting (airhorn). The only area of concern for a leak between these two castings is the vacuum source passage for the automatic choke. If the factory "fix" were done by a competant rebuilder (the passage milled oversize in both castings and a bushing inserted) then the engine would run with this gasket removed! So now that I have inserted both feet in mouth, I will be reading this thread with interest to hear the final result! Just curious, did you remove the throttle linkage for a test? Is the choke butterfly completely open? Is the fast idle correctly adjusted? For a stock engine, the 4GC is also an EXCELLENT carb! Jon. Edited March 29, 2017 by carbking (see edit history) Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now