Rembrant Posted October 25, 2019 Share Posted October 25, 2019 Gentlemen, I finally got my AEM AFR sensor & gauge installed this evening and set out to check and set my idle mixture, etc. So I started the engine up (without choke) and just held the pedal while it warmed up. Idle AFR while cold was more or less bouncing between 15.5 and 16.5 sometimes bouncing to 17.0 or so. If I revved it a bit while cold, it would go to 13.5:1, right where it was set on the dyno day. After a few minutes when it started to warm up, the AFR maxed out lean, so it's above 17 or 18:1. Then I reset the idle mix screws, and decreased the idle stop screw as much as I could, and started adjusting the screws in. I could not get the idle screw adjustments to make any difference until I get in to where they're getting close to being closed and the engine wants to stall. AFR still pegged lean, and vacuum doesn't go any lower than about 16". If I rev the engine a bit, the vacuum shoots to 20" no problem. I had the idle stop adjustment down as far as it would go without stalling, so I don't know if I'm too far into the transition slot or not... Holley book says to try a few different things... Put toothpicks in the idle air bleeds to see if that richens it up. Maybe the carb needs a different idle jet?...If it's even changeable...I don't know. I need to go do some more reading, but thought I'd ask the experts for advice. Thanks gents. Link to comment Share on other sites More sharing options...
Gary Lewis Posted October 25, 2019 Share Posted October 25, 2019 I am not one of the experts, but I will comment. I would have thought you'd open the idle screws up rather than close them down. (There are some carbs that have the screws adjusting the air instead of the fuel, but I think those are just the emissions carbs, not yours.) Pretty sure yours are controlling the fuel going in, and you need more of it. Badly. Having said that, I don't worry about AFR at idle. I get the best idle I can get, so I adjust the idle mix screws to max RPM, which is also max vacuum - which should be up around 20" with your mild cam. Link to comment Share on other sites More sharing options...
85lebaront2 Posted October 25, 2019 Share Posted October 25, 2019 I am not one of the experts, but I will comment. I would have thought you'd open the idle screws up rather than close them down. (There are some carbs that have the screws adjusting the air instead of the fuel, but I think those are just the emissions carbs, not yours.) Pretty sure yours are controlling the fuel going in, and you need more of it. Badly. Having said that, I don't worry about AFR at idle. I get the best idle I can get, so I adjust the idle mix screws to max RPM, which is also max vacuum - which should be up around 20" with your mild cam. Gary is correct, starting setting on older Holley carbs is 1.5 turns out from seated, some of the emission ones need 2.5 turns out from seated. Only time you worry about idle AFR would be with a catalytic converter, and then there is a specific procedure either a lean drop or artificial enrichment. EFI systems control the mixture at idle, part throttle load and cruise, under WOT, they revert to preset tables. There are a few emission carbs where the idle mixture screws are actually air bleeds, these are ones where the richest idle mixture is still on the order of 14-16:1. Link to comment Share on other sites More sharing options...
Rembrant Posted October 26, 2019 Author Share Posted October 26, 2019 Gary is correct, starting setting on older Holley carbs is 1.5 turns out from seated, some of the emission ones need 2.5 turns out from seated. Only time you worry about idle AFR would be with a catalytic converter, and then there is a specific procedure either a lean drop or artificial enrichment. EFI systems control the mixture at idle, part throttle load and cruise, under WOT, they revert to preset tables. There are a few emission carbs where the idle mixture screws are actually air bleeds, these are ones where the richest idle mixture is still on the order of 14-16:1. Ok,..all understood gentlemen, thank you. I'm a 4bbl carb tuning rookie here (as if it wasn't obvious...lol). I haven't touched the carb since the engine came off the dyno. It worked fine. The Holley book I was reading said to start at 2 turns out, and adjust in from there, ideally ending up at 1 turn, but OK between 0.5 and 1.5 turns out. I have another Holley book here that says to start at 1.5 turns out. In any case, the carb as found was at 0.75 turns out. I'm done with it for tonight, but I'll be back at it again tomorrow. I'll set them at 1.5 turns and stick the air cleaner back on and go for a drive. Could be the gauge too I suppose. It was reading while the engine was warming up, but then once warm it went max lean. That was at .75 turns out (I think) so I'll try it again at 1.5 and see what the gauge says. Thanks for your help guys. Link to comment Share on other sites More sharing options...
Gary Lewis Posted October 26, 2019 Share Posted October 26, 2019 Ok,..all understood gentlemen, thank you. I'm a 4bbl carb tuning rookie here (as if it wasn't obvious...lol). I haven't touched the carb since the engine came off the dyno. It worked fine. The Holley book I was reading said to start at 2 turns out, and adjust in from there, ideally ending up at 1 turn, but OK between 0.5 and 1.5 turns out. I have another Holley book here that says to start at 1.5 turns out. In any case, the carb as found was at 0.75 turns out. I'm done with it for tonight, but I'll be back at it again tomorrow. I'll set them at 1.5 turns and stick the air cleaner back on and go for a drive. Could be the gauge too I suppose. It was reading while the engine was warming up, but then once warm it went max lean. That was at .75 turns out (I think) so I'll try it again at 1.5 and see what the gauge says. Thanks for your help guys. The # of turns out are a starting point. I would for sure try turning them out farther to see what happens. Your goal is to get the best possible idle, regardless how many turns open the screws are. Having said that, if you have to turn the screws way open to get the best idle then something is wrong. Maybe not "bad wrong", but wrong. It might be that you have a small vacuum leak, like a cap off a fitting. Or it might be that something in the carb is plugged. But I wouldn't stick with the number of turns open just because that is what the book says it should be. Link to comment Share on other sites More sharing options...
Rembrant Posted October 26, 2019 Author Share Posted October 26, 2019 The # of turns out are a starting point. I would for sure try turning them out farther to see what happens. Your goal is to get the best possible idle, regardless how many turns open the screws are. Having said that, if you have to turn the screws way open to get the best idle then something is wrong. Maybe not "bad wrong", but wrong. It might be that you have a small vacuum leak, like a cap off a fitting. Or it might be that something in the carb is plugged. But I wouldn't stick with the number of turns open just because that is what the book says it should be. Understood Gary thank you. One more thing... The idle mix screws. Do they have internal stops when turning them out? I assume yes...wouldn’t want one falling out.. Link to comment Share on other sites More sharing options...
Gary Lewis Posted October 26, 2019 Share Posted October 26, 2019 Understood Gary thank you. One more thing... The idle mix screws. Do they have internal stops when turning them out? I assume yes...wouldn’t want one falling out.. No, they'll come out, but not until well after the spring goes..... Wait, this is a Holley and the screws are in the metering block. No springs, just the gasket. But they'll come out. However, before they do nothing will be happening as you've already gotten past the "wide open" point. I think. Link to comment Share on other sites More sharing options...
grumpin Posted October 26, 2019 Share Posted October 26, 2019 No, they'll come out, but not until well after the spring goes..... Wait, this is a Holley and the screws are in the metering block. No springs, just the gasket. But they'll come out. However, before they do nothing will be happening as you've already gotten past the "wide open" point. I think. Glad the experts chimed in! My first thought was a vacuum leak. Something I check quite often on my F250 is vacuum lines and caps. Link to comment Share on other sites More sharing options...
Rembrant Posted October 26, 2019 Author Share Posted October 26, 2019 Glad the experts chimed in! My first thought was a vacuum leak. Something I check quite often on my F250 is vacuum lines and caps. Hey Grumpin! Well, I've been playing around with this for a couple weeks and searching for vacuum leaks is one of the things I've been doing. I might as well run this by you guys in case I'm missing something. My Holley has 4 vacuum barbs. There are two 3/8" barbs...one in front which I have piped to PCV, one in the rear which is piped to the brake booster. Then there are two 1/8" barbs, one that is full vacuum up front that I previously had capped but now (normally) have it connected to the air cleaner assembly, but currently it is piped to my vacuum gauge. The other 1/8" barb is the ported vacuum source that I have piped to my vacuum advance, which is also currently plugged. All tubing is new. I even disconnected the PCV last night and plugged it for a test. The brake booster seems to be OK and I pulled the fitting out of it a couple days ago and it was still holding a vacuum after the truck had not been started in 3 days. Currently running an aluminum 1" spacer under the carb with new gaskets. Everything is still brand new really, but I checked the intake and the base of the carb for flatness, and they both appear to be fine and look like new. So, for the time being I've kinda given up on looking for any further vacuum leak unless you guys point out something that I'm missing. Link to comment Share on other sites More sharing options...
Rembrant Posted October 26, 2019 Author Share Posted October 26, 2019 Having said that, I don't worry about AFR at idle. I get the best idle I can get, so I adjust the idle mix screws to max RPM, which is also max vacuum - which should be up around 20" with your mild cam. Out of curiosity, why don't you worry about AFR at idle? Link to comment Share on other sites More sharing options...
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