The Stovebolt Site Forums The Shop Area Engine Shop Rochester 4GC hesitation, stumble from idle. Fine at part throttle.

I have a 65 Chevy C20 with a 327 from a 66-67 Impala, that originally had a Holley 4160. I have no idea what the 4GC came from since the tag is long gone. I just got the engine running and I have a couple issues.

One, there is hesitation opening the throttle from idle, once that clears it seems fine. I've read on here that it's likely the secondaries opening too soon, and that I might need to adjust the spring. Is that the spring holding the auxilary valves closed and how do I adjust it?

The carb was also missing the link connecting the primaries to the secondaries, is there a specific length this link needs to be? I made one I could adjust, but it seems that the secondaries never quite return to fully closed after opening. Could this be causing the hesitation?

Thanks,
GrimGreaser

You want the secondaries to open at approx. 2/3 throttle so you drive on the first 1/3 using only primary side. It sounds like you are not getting the throttle closed all the way. try taking you home made linkage off and driving it. See what it does on primary only. Joe

I made sure that the link allowed the primaries and secondaries to close all the way. The secondaries only stay open when the engine is running, and even then it's not more than a few degrees till they close. Though I realize that can make a big difference.

Thinking back now, it had a hesitation when running on only primaries. When I would just slightly crack the secondaries by hand the idle would increase a bit and the hesitation would go away. Could this mean puddling? Too little air flow allowing fuel to condense in the intake until the throttle is opened?

When we're talking about the secondaries opening, we mean the upper, air velocity actuated, and spring calibrated auxiliary valves right? All the pictures and drawings I could find of 4GC carbs show a single link connecting the primary and secondary throttle valves.

I know my way around Edelbrock carbs (Carter AFB's) so this little guy is full of new gremlins to me.

GrimGreaser

If it hesitates just off idle it is probably the accelerator pump. With the engine off look down the throat & move the throttle linkage. 2 squirts of gas should go into the primary barrels each time. The pump piston may be bad even if new they can dry up. The linkage could be worn letting the throttle open before the pump starts. The secondarys must close tight or it will not idle right. The float levels may be low.

4GC's are notoriously hard to keep in tune, that's why GM switched to the QJet.

Thanks guys.

I checked the accelerator pump and I get four healthy squirts of fuel. I did pull the bowl/air horn off the throttle body and checked the auxiliary throttle valve, which was very loose. Tightened the spring up a 1/8 of a turn so we’ll see how it runs now.

GrimGreaser