Overly Rich Idle (Lars?)
Tech Contributor
Joined: Aug 1999
Posts: 14,377
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From: At my Bar drinking and wrenching in Lafayette Colorado
Don't be surprised if it won't idle at all. A four-corner idle has IFRs on both the primary and secondary metering blocks. It also has symetrical IABs at all four corners at the throat entry. A two-corner system has a fuel side only on the primary block. On a two-corner 750-sized carb, you'd be looking at an idle jet around .045 (feeds both primary and secondary throttle plate) which has an area of .0015. If you put a .017 wire (.00022 area) into that restrictor you knock the area down to .00128. Or an equivalent jet of .040 and that's a lot. A four-corner, which you do have, has four IFRs (two in each metering block) in the low to middle 30s, so lets go with a .033 which gives you an area of .009. That .017 wire will reduce your jet area down to .009 which come out to a .011 jet -and that pretty much closes up that hole and if it does, it will pretty much be running off the idle circuit still untouched on the secondary side. And that won't tell you very much other than such an imbalance is not a good tuneup.
I have run wires in the primary side only on a 4-corner carb to get cruise mixture leaned out without affecting WOT. But this is a band-aid fix only to identify a tuning problem.
Lars
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
Actually, if you go too lean, you'll get random lean misfire, which will drive HC way up, and you can get unburned fuel in the exhaust. Often, if you can "smell" the exhaust, chances are you have high HC from ignition-related issues and unburned fuel from a lean condition - CO (from running rich) is actually odorless. You may have other issues than a rich running idle circuit...
Thread Starter
Pro
Joined: Mar 2009
Posts: 703
Likes: 2
Actually, if you go too lean, you'll get random lean misfire, which will drive HC way up, and you can get unburned fuel in the exhaust. Often, if you can "smell" the exhaust, chances are you have high HC from ignition-related issues and unburned fuel from a lean condition - CO (from running rich) is actually odorless. You may have other issues than a rich running idle circuit...
Burning Brakes
Joined: Apr 2008
Posts: 825
Likes: 40
From: Glasgow, Scotland
Drilling the throttle plates is a technique that is used to allow adequate air into the engine to maintain a desired idle speed without opening the throttle blades far enough to expose more than .020" of the transition slots. If more than .020 of the slots is exposed, the engine will run rich due to fuel being discharged from both the idle fuel, transition slots, and in some cases, from the main discharge nozzles. The drilled holes do not lean out the carb by themselves - they allow a smaller throttle opening so the carb can function as it should. Fishndude only has .015" transition slot exposure, so drilling the plates will serve no purpose in his case. 
Lars
Larscheers
dave
Racer
Joined: Nov 2009
Posts: 423
Likes: 7
From: Utah
Fishndude,
Any luck yet? I'm interested because I just failed emissions in AZ for high HC at idle (possible lean misfire?). It idles great and pulls strong. I'm running the 750 Holley on a fairly tame 454. Trying to beef up my carb tuning knowledge before I turn the wrong screw...I'm also going to start looking for any vacuum leaks.
Shrekviper
Any luck yet? I'm interested because I just failed emissions in AZ for high HC at idle (possible lean misfire?). It idles great and pulls strong. I'm running the 750 Holley on a fairly tame 454. Trying to beef up my carb tuning knowledge before I turn the wrong screw...I'm also going to start looking for any vacuum leaks.
Shrekviper
Thread Starter
Pro
Joined: Mar 2009
Posts: 703
Likes: 2
Fishndude,
Any luck yet? I'm interested because I just failed emissions in AZ for high HC at idle (possible lean misfire?). It idles great and pulls strong. I'm running the 750 Holley on a fairly tame 454. Trying to beef up my carb tuning knowledge before I turn the wrong screw...I'm also going to start looking for any vacuum leaks.
Shrekviper
Any luck yet? I'm interested because I just failed emissions in AZ for high HC at idle (possible lean misfire?). It idles great and pulls strong. I'm running the 750 Holley on a fairly tame 454. Trying to beef up my carb tuning knowledge before I turn the wrong screw...I'm also going to start looking for any vacuum leaks.
Shrekviper
Racer
Joined: Nov 2009
Posts: 423
Likes: 7
From: Utah
Thanks. I'll keep watching, but just found out that I won't need emissions because I have collector's insurance...I'd still like to get mine running like it should. I read Lars' paper and will start tinkering once it cools down a little in AZ.
