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Old 03-14-2012, 05:37 PM
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carburetor question about idle and air bleeds.

Hello guys I have not posted in a long time here and my truck has been running pretty good for the last three quarters of a year since I have a lot of things replaced and done and checked on it. Ok here is my engine sbc 350 with rhs 180cc cylinder heads with a lunati voodoo roller cam 211-219 @ 50 and a holley 600cfm 1850s carburetor converted with electric choke.

Everything is all good as timing,plug wires,spark plugs etc and all is in tip top shape. Engine has ran good during the cold months with no apparent issues until the other day when it got up to 60 degrees outside. My current carb settings is idle is around 800 rpm and the idle mixture screws where about 3/4 or a turn out on both sides and that always seemed to give me the best idle with a spit and sputter here and there but not all the time.

Well took it out for a drive last Sunday and it started up ok with no issues choke worked fine and it drove off idle and cruised fine with not a problem but when got to stop lights and it would idle pretty erratic with stumbling like it was barely running and could't now figure out why when it did not do it before. So when I got it home I have each idle mixture screw a 1/4 or a turn richer on both sides and it helped some but it still still sputter's here and there still but will stay running better and it's not as bad.

Here is the real kicker I wondered if my idle air bleeds may have gunk in them or something so I took my air cleaner off and everything was spotless since I rebuilt it just a few months ago. Here is the thing I have a question about while it was running with the idle mixture screws just 3/4 or a turn out where it ran fine before if I put my fingers over both idle air bleeds on the primary barrels the idle would pick up just a hair and it would run real smooth and all the sputtering would go away.

What would cause this and how can I go about to remedy it to run good like that all the time. Turning my idle mixture screws out to 1 full turn helped it some but anything more then that just makes it to rich of an idle at that point. The cam is just a mild one and should not need that much idle to run correctly.

Even holding my foot on the gas just a hair at the stop light to keep it running back home it still would sputter like that even at 900 rpm holding it with my pedal. There are no vacuum leaks and all hoses are good and pcv is working correctly. It's new to me normally when you place your finger's over the air bleeds don't it normally make it quit or get real lean? I know about other things carb wise but just basic's on air bleeds.

thanks guys
Eric

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Old 03-19-2012, 06:42 AM
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I don't really have an answer for you but i want to ask how do you like the lunati voodoo cam? I want one myself because of the price
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Old 03-19-2012, 06:50 AM
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-reducing air flow into the IAB's with your fingers makes the idle AFR richer.
Blocking off the air flow kills the idle emulsion completely.

Blow out the air bleeds with compressed air and eliminate any all potential vacuum leaks before jumping to conclusions. Get a vacuum gauge and a AFR meter.
The ignition system needs to be in top shape. Many carb troubles are actually ignition issues.
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Old 03-19-2012, 07:07 AM
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I made my motor idle a whole lot better just by switching my distributor vacuum from ported to full
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Old 03-19-2012, 05:12 PM
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Hello Fbird thanks for input. As far as my ignition its in all tip top shape. My distributor is only about two years old and my coil cap and rotor was replaced last year along with my plug wires.

I have checked everything and had my vacuum gauge on it before and all was good. I readjusted my idle mixture screws a quarter of a turn more and it helped but it ran fine but still had the sputter here and there.

Well today after all this time as I suspected it was the carburetor itself. No matter how many fine tuning the idle mixture screws with a vacuum gauge and the primary idle mixture screw I could never get it to where it really needed to be outside of changing the idle feed restriction in the metering block. Holley blocks don't have the ability to change outside of drilling and I am not that advance of a carb expert.

Anyways the holley 1850s seems to have a very lean idle fuel curve and it's not really setup for performance engines with a more cam then stock and higher flowing heads. I sold some old parts I had laying around and bought a quick fuel slayer 600 vacuum secondary carb and hooked it up all today and it was a night and day difference between the carbs.

I hooked my vacuum gauge up and did my mixture screws for the best vacuum and now I have a perfect steady 20 inches of vacuum and the idle curve seems to be just enough richer then the holley to where now it idles just fine with no sputter like it had before.

Taking it out for a drive I have way better throttle response and feels almost like fuel injection and does a nicer job and it a lot more sensitive to tuning. Yes Fbird I know I could have gotten my holley to be like that too but it would take a lot more things to do to the carb such as drilling the metering block and without not wanting to go that route since i am not experienced in that are I am not wanting to drill idle feed restrictions in metering block and end up messing it up.

I also have checked all things before going to carb. Idle bleeds where blown out and carb was rebuilt just a month ago and everything cleaned out. Some day I will invest in a AFR meter so I can really fine tune my carb but for now I have to do like the old days just tune one thing at a time and read plugs and my carb tuning manuals etc.

Bottom line the quick fuel slayer carb is a lot nicer carb wise with all the features needed to really fine tune a carb with out needing to drill anything since you can change air bleeds, idle feed restrictions etc and if you go to far you can always put it back where it was. The next thing on my list will be a AFR meter so I can really dig into things and get it even better.

Also Chad I already have my vacuum advance on full manifold already and yes my lunati voodoo cam is a very nice cam and I have them in both of my chevy s10's and they are good cam's.
Thanks guys for in put appreciate it and things are all good now.
Eric
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Old 03-20-2012, 06:11 AM
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Thanks for the reply there man, i had almost the same problem with my quadrajet,but it would bog under heavy acceleration. I hated it. I don't care about the nostalgia or the "better gas mileage" with the Qjet, it was horrible. I invested in an edelbrock performer 600, best decision ever. Its way more tune able than Qjets and performs like you said, "almost like fuel injection". Also thanks for response on the cam. After i get my new wheels and tires mounted up tomoro im going after one of those cams for the 4.3liter.
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