Wow. It has been a long time since I've been on here. Hope someone has an idea to help. I run a QuickFuel 850 on my Riviera. Best pass is 12.71@106. The car runs great, except when I am cruising just off idle - like 35 - 40 MPH. Then it surges. Running 78/88 jets. A/F Ratio at idle is 14 or so. WOT is 13.0. When I am deep into the primary jets, doing basic around-town driving, the A/F Ratio is about 13.2 or so. When I am holding it JUST off-idle and cruising at 35 - 40 MPH it runs very lean - 17 or so. And my gas pedal is barely cracking open the primary blades at this speed. There is no need to put fatter jets in it, as it runs 13.2 when I am fully on the primary jets. So, is this an air bleed tune? Some kind of idle slot thing? It is pretty annoying to drive it while it is surging like it is. However, when it surges, if I JUST tick open the throttle a hair more, the A/F Ratio drops from 17 down to the 13's. It seems like the off-idle mix is very lean, but I don't know how to adjust that. Should I just run it really fat at idle and hope it covers the issue? Any ideas?
First, make sure your idle transition slot is mostly covered at curb idle; you may have to adjust the secondaries to let in more air at idle to allow this to happen (there's usually that little screw that's accessible underneath the base plate). You just want to see a little square of transition slot at curb idle. If that's OK, then you'll have to mess with the idle air bleeds; there are 4 on a Quick Fuel/Holley. In your case, you'll have to reduce the size to richen up the idle circuit. You could also increase the size of your idle feed restrictor, which is inside the primary metering block. Either way, if it's only surging at low speeds, your idle circuit is too lean. I think I'd start with .004 smaller air bleeds OR a .002 larger IFR. You can make your own out of brass set screws by using pin drills if you're so inclined. Good luck! EDIT! I just noticed that your idle is 14:1. That's pretty lean for a carbureted engine; however, idle mix can be inaccurate on a wideband. Always tune your idle with a vacuum gauge. You may just have to adjust the idle mixture screws in this case. That would be nice!
Thanks Aaron. So, if 14.7 is dead on, why is 14 too lean at idle with a carb? Is that just because the idle circuit needs to be richer to aid low speed cruising? Can you point me in the right direction to learn more about the air bleeds? Thanks!
Idle needs to be a touch richer. What is your idle speed in park, then in gear. If getting more than 150ish rpm drop try to richen it up a bit, then readjust base idle speed.
I'm certainly not a powertrain engineer, but camshaft overlap contaminates the idle with exhaust gases, meaning that the engine generally must be set somewhat richer than stoich to maintain a decent idle. With fuel injection, you have more precise metering, which is (I imagine) why you can run it a little leaner. But most carbed engines seem to like an idle AFR of 12-13.5:1. Once again, if you set it for max vacuum, you'll get the best idle (then you can turn the mixture screws in to get a 20 RPM drop or so, which is called lean-best mixture). Your mixture screws feed your idle ports and are adjustable, but your transition slots have no such adjustments, meaning that the idle air bleeds and idle feed restrictors determine the AFR for your transition circuit. You are correct in thinking that the carb is running on that circuit at low speeds, which means that the main circuit doesn't take over completely until about 2500 RPM or a little higher. Therefore, to change your light cruise AFRs, you'll have to adjust the bleeds or the restrictors. I'd start by readjusting your idle mixture screws.
I fattened up the idle and opened the secondary blades at ide a very little bit. Cruises great now! No more surging and the AF is in the 13's at steady cruise. Thanks v8buick family!!!!
