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Discussion Starter #1 (Edited)
got my lil single s480 ls 5.3 runnin and its jus pig rich at idle, talking in the mid 10's jus sitting on the idle circuits. it came with 76 up frt and 86 in the rear so i jetted down to 72 and 82 and it helped alil, then i tried 68 up frt and 76 in the back, still pretty rich(12ish afr).with the 68/76 setup i can only back out the mixture screws bout 1/2 turn in the frt and a qtr turn in the rear screws. still seems to rich of a idle at 12.3.im thinking bout jetting down again cuz a 68/76 combo on jets jus seems way to much fuel n.a. for 325 cubes.what are you guys using these 750 ban's running for jets?i dont wanna go to far jus idle tuning the ls motor then when i get a chance to put a load on it, i find out i went wayyy to far and lean melt my motor lol. this is in a jetboat btw
 

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Main jet doesn't have a ton of effect on the idle unless the blades are way open. Most of the fuel is coming through the idle circuit or transfer circuit! If you screw all the idle mixture screws in will it turn the car off from leaning it out? If not, you are getting too much fuel from somewhere else (usually the transfer circuit). The rear slot for the transfer circuit should be completely covered and the primary side should only have a small portion showing (.020-.050ish) below the blades. If the blades are cracked open too far and into the transfer circuit, you have to drill small holes (.060ish) in the front two butterflies so you can close the blades and the engine will still idle.

If the transfer circuits are not showing and the blades are aligned properly. You will need to restrict the idle feed restrictors some. I usually start about .010 smaller than what is in there now. It will probably be too lean that way, but then you gradually open them back up a couple thou at a time till it gets close.

The way I generally tune mine, is get the WOT tune up there so I don't burn the engine up. Then worry about the idle quality and driveability issues!
 

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Main jets don't effect idle. You need to make sure your throttle blades are set correct in relation to t-slots, then if it's 4 corner idle set the idle screw 1 to 1 1/8 turn out. Now start playing with IAB's at operating temp. The right size IAB will bring the idle AFR where it needs to be and it will give a nice idle rpm. In some extreeme cases you may need to adjust the IFR.

Also make sure your float level is set correctly, to high can cause a booster to dribble at idle.


And for what it's worth I set my rear t-slots to .020...
 

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Discussion Starter #4
thanx for the info guys, i was alil high in the rear on the slots so i fixed that and ran it on the trailer and its still too fat, 12.3ish at 800rpm idle.i pulled out the 70 primary idle air bleeds completely and off course it went lean nto the 15's, so im thnkn i need to go to either smaller idle circuit jet or try a 80 idle air bleed.what yall thnk?
 

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The idle air bleeds are easy to change for sure. I would go with a .075 first and see where you are at and then slowly drill from there! Be careful, in my experiences if I got it way lean on the idle, it would have some stumbles when first hitting the gas.
 

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Discussion Starter #7
right at 27* at idle.you can run a pretty aggressive timing and map advance curve in dragboats, they flash to high rpm almost effortless like a bigggg stall convertor.at atmosphere im giving it 25* and retarding a * per lb till 10lbs where it jus sits on the rpm timing table at 15* locked basically above 10lbs.
 

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I run one of those carbs on a procharged 355 at 25psi...

i set the t-slots to .020 then adjusted the idle speed on the rear screw
i run stock ifr's when i tried adjusting them i found them too responsive and went back to stock
As for airbleeds, .074 idle air bleeds... it cleaned it right up and starts real well, it sits in the upper 13's low 14's at idle and i blocked the hsab's off.

I am running 70 primarys 96 secondarys and .078 pvcr's. It sits at 11.0-11.2 wot non intercooled with a bunch of water meth.. as long as i cruise on the primarys is stays in the 13's at cruise if i dip into the secondarys when the motors still making vacuum it goes really rich
 

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As far as timing goes an LS will be a lot different but i run 32* locked + 15* vaccum advance and pull .6* per lbs after 10psi
 

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Discussion Starter #11
Wow thats not much timing at all...

I run 40 degree base, and at 24 psi i'm down to 25 degrees. In a car, not a jet boat though.
its a ls engine tho, i dont think i'd ever get away with that much timing on boost.i can see giving it a shitload at idle,but ive got my max timing set on the msd 6012 aT 30* jus in case shit ever happens, my map sensor takes a dump etc..

thanx for the info sheldon!
 

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Sheldon... You aught.to open up youre secondary pvcr and take some of that secondary jet out. Would clean that minor going fat during cruise with too much throttle position issue right up.
 
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