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CarburetorsCarb discussion and questions. Upgrading your Third Gen's carburetor, swapping TBI to carburetor, or TPI to carburetor? Need LG4 or H.O. info? Post it here.
I'm familiar with (and successfully used) Sofa's Holley tuning guide some time ago. With a new direct replacement for the Holley 4160 carb I'd previously tuned, and using the same jets and power valve, I wonder where a reasonable starting point for the transition slot and secondary stop would be before I mount the carb. I'm considering .20 or .30 for the transition slot and maybe a 1/8 turn of the secondary stop screw. Advice?
I'd suggest a feeler gauge on the sec throttle blades and set the new one to open the same as the old, and the primaries to the same trans slot exposure. Should get you close initially.
Did you use an Allen, screwed in from the top, for your sec idle stop? If not, make that change now, it greatly eases sec idle adjustment.
Unfortunately, the old carb isn't readily at hand, so I'm working in the dark, so to speak. Yes, the Allen is something I did the first time around. I wonder what you feel about the "light gap method:
"The next thing to note is that the secondary transition slots on a Holley are often located higher in the throttle bores than on the primary side: If you try to obtain the .020” transition exposure on the secondary side of a Holley, you will often end up with the secondaries opened significantly more than the primaries.
To avoid this, use the following secondary throttle adjustment on the Holley carbs: After adjusting and setting up the primary throttle blade angle as described above (.020” transition slot exposure), hold the carb
up to a light and look up the bottom of the carb (with the choke blade wide open) – note the amount of “light gap” around the primary throttle blades. Now, adjust the secondary idle speed screw to duplicate the
primary “light gap” on the secondary side. This will usually get the secondary throttle blades close to the transition slot, but you may not have .020” exposure. Idea is to get the same blade angle and airflow
through all 4 corners of the carb – the “comparative light gap” method allows you to get this very close.
Once the primary and secondary throttles have been set to this initial idle speed setting (which should make your car idle very close to the correct idle rpm range), it is your job as a tuner to assure that any further idle
speed changes occur by adjusting both of the screws equally from this point on. Never adjust the idle speed by only adjusting the primary screw: if you adjust the primary idle speed by ¼ turn, you MUST adjust the
secondary idle speed screw ¼ turn as well. Keep the two throttles adjusted the same." by Lars Grimsrud
Colorado Corvette Crazies
JamesC
Last edited by JamesC; 11-02-2021 at 07:28 AM.
Reason: Additional Info
Not all 4160s have an idle and transition circuit in the secondaries. In fact, few do, especially the street ones, which is weird because that would seem to be where it's needed the most. 4-corner idle seems to be reserved for premium oval-track optimized models and the like. If yours doesn't, that tidbit (a VERY good one BTW) doesn't apply. They don't mention it, but it also requires perfecting the throttle blade match to the bores, prior to executing the procedure they talk about.
I threw caution to the wind and set the primary transition slot to .20 and the secondary stop to a 1/8 turn past the factory setting on a Holley Street Warrior VS (600 cfm with electric choke p/n 0-80457S). The float settings were good out of the box. Edit: After a second look, I determined that the primary was close but not perfect, so I adjusted. The secondary was good. I fiddled with the idle mixture screws, however, going from a vacuum of 15 to a steady 17 with an idle speed of 750. The car runs very nicely. I'm uncertain, however, about the choke. The manual mentions that the choke setting from the factory is on “index” or at center position, but that wasn't so (actually it was off by a couple of witness marks). The manual also states that the factory setting should give a 1500-1600 RPM fast idle speed but it jumped 3000 or so. EDIT: I've come to believe that that is not a carb issue. Purchased as new from Jegs. Anyway experimenting required in regards to choke settings.
JamesC
Last edited by JamesC; 11-26-2021 at 07:47 AM.
Reason: Additional Info
11-01-2021, 06:43 PM
