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73-87 Chevy _ GMC Trucks => Performance => Topic started by: hogwild on April 09, 2008, 09:43:54 pm
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biulding a stock truck to race oval track, put brand new rochester 500 cfm 2bbl on and it runs like crap, 350 ci 450 lift 280 deg dur cam, headers, everything else is stock, anyone know good base line for carb ? screws at front of carb are now set at 2 1/2 turns out. it blows black smoke and has no bottom end power.i also locked dis adv , what should totle timing be set at?
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Those screws in the front are just for idle mixture, so all they will effect is idle.
Black smoke means unburned fuel... Perhaps your float is sticking. Try tapping on the side of the carb with a screwdriver handle and see if it clears up.
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those screws are for idle only and wont afect bottom end responce?. wich way is wich on them in more air or out ?, how about tottle timing 34 deg? taped on carb and it changed nothing.
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Where did this carb come from? You said it's brand new right?
The choke is open correct?
The idle mixture screws effect the idle circuit, and throttle response essentially. If they aren't set correctly you will have poor idle quality and perhaps load up.
Start by turning them both in all the way and back them out about 1.5 turns each. Do you have a vacuum gauge? If so try turning them out evenly until you get the highest vacuum reading. This will give you a good starting point. From there you can fine tune it to acheive the strongest idle quality. If it's still dumping too much fuel return it to wherever you got it from and get another one. It's overfueling.
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carb is brand new or maybe a reman, it dosnt have a choke, it is a racing 2bbl, i will set screws tomorrow and double check timing and see what i get. it has 373 gears and it wont even spin tires it is running so crapy, timing at 34?
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got her ideling good todaynow i have new problems, set idel at 850 idles good but soon as i put it in gear it stalls, when i do geter in gear i cant even hold it with brakes it wonts to go even at 700 rpm. also it idles good for 2-3 minutes then slows down then picks back up etc...
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do you have vacuum advance? If so hook it up to manifold vacuum.
Do you have good fuel volume?
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vacum adv is hooked to man vac, high perf carter mech feul pump. would vac line not being hooked to trany have somthing to do with it
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It's manifold vacuum not ported vacuum right?
The modulator hose is not hooked up but it's not unplugged right? As in vacuum leak?
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you say your timing is 34 degrees?
at what RPM is this at?
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thier are two vac lines tottle on this truck one from intake behind carb to vac adv , another from back of carb base to brake booster. 34 deg is at 3500 rpm ,trany modulator has nothing hooked to it . thier is no vac leaks i checked with starting fluid today
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put a vacuum hauge on it and post your results at idle, at part throttle, and when the throttle is snapped open.
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other than suggesting you try a known good carb, and go from there,
I don't know what to tell you
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cant do vac gauge till the am truck is at work ill post then
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Might as well check your timing at idle too
your are gonna need 10* to 14* btdc at idle with that cam
(a stock 8.5:1 engine is gonna be soggy on the bottom end with that cam)
Hopefully you mean 280 degrees advertised duration (like 230 @ .050"?)
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actualy it is 300 deg dur , it is a howards circle trac cam