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72Z/28
07-03-2007, 03:04 PM
Hi guys
Does anybody by any chance know how a proper way to adjust the carburator in order to get a good fuel air ratio..

I have holly 750 CFM, and I am sure there are some CO and HC values that need to be considered when adjusting the carburator to make sure the car is not running rich or lean.

Thanks in advance.

MonzaRacer
07-03-2007, 09:08 PM
OK,
first of all if its running you need to setup yout timing. I assume your running a sbc from your sig so here is my way.
I get it running then on regualer 350ish(or any for that matter)engines I give them 14 to 18 degrees initail, on street engiens with limited build up (ie its not all aftermarket set on kill type) when I use HEIs I get either good used ones or good aftermarkt new and use stock weights and mix and match centtrifical springs from lightest to mixing lightest and next step.
Then I hook up vacuum advance to maniifold vacuum. Make sure all your vacuum aaccesories(brake,pcv,heater control feed etc) then I get a good vacuum/fuel pressure guage, and get it running.
then I getthe idle back down so it will idle smoothly in gear no lower, and then adjust for the highest vacuum and then give a 1/4 to 1/2 turn out (this allows the carb to be compensatedd for barometric changes later on). Then open the float bowl sight plugs and the fuel should only drip out or you may have to rock car to get fuel to tip out.
It takes a wide blade screwdriver and a 5/8 end wrench to adjust the floats.
I balance everything several time till I am sure its all good THEN with a couple of 3/8 wrenches (or what ever fits the accelrator pump arm screw, the one with the spring(s) on it) and I tighten it up till it only has jsut a slight amount of slop , spec is .015" but I just get it loose.
And this is how you do it.
Using a 4 or 5 gas analyzer is ok but I prefer to use the vacuum trick, I have that around pretty much anytime i need it.
also you can use a tach/dwell meter on a low rpm setting and dial the mixture for a tiny rpm drop then back up and then the 1/4 to 1/2 turn out.
tuning the base on a Holley is a delicate balance and takes a few tries to get it but once you understand how to and practice its so simple, and it works for most carbe except for the float and accel pump adjustments. AFB/Edelbrocks you change pump a rod holes and bend the rod and , move the arm or bend a rod on Qjets.
Good luck and ask any questions if you need it.
Also it depends on your cam you may need to install a lower rating power valve if your idle vacuum, in gear is very low. Most times I just install a 4 or 4.5 power valve and call it good unless your idle vacuum is much lower then you gotta order real low number ones or plug it off if its below 2 in of vacuum , I generally never have much trouble with using lower power valves but if your engine has no vacuum in gear at idle its not gonna need a lot of tuing as its only good for 1/4 mile blasts.
Good luck
Lee Abel
AFTERMARKET PERFORMANCE
Now sometimes I

GMachineDartGT
08-27-2007, 05:57 PM
I agree with most of what was said. However, I never hook vacuum advanced up to manifold vacuum. I find that the 14-18 initial works for cams up to approx 240* @.050, but for larger cams even more advance is needed for crisp response. Adding the vaucuum advance on top of this does not make sense to me. I put in a timed port, so when the blades are cracked at light throttle, the vacuum kicks in, then dissipates as vacuum drops off (more throttle).

I am a firm believer in wide band and tuneable carbs. That 1/4 - 1/2 turn out can make a huge difference if you were to see the actual change of the AF ratio in real time.

Its as complicated as you make it. Its the difference between good and great...

MonzaRacer
08-27-2007, 06:36 PM
Well if you want a dog runner with a spark knock on tip in use ported vacuum.
If you want a smooth runner and good mileage, use manifold vacuum.
If you set up for proper power making timing then add in vacuum advance with high vacuum, yoour car will run much smoother, get better mileage and be a lot chrisper,,,, Just retuned a car for a fella the other tuner used ported vacuum and it would rattle like a pig on tip in.
Using manifold cauum also lets it retard for power and not spark knock under power.
Been doing this for 20 plus years and ported vacuum is only for a 70 smog era POS, not performance engine/car.
As for the AFR I agree on using a AFR meter for tuning or even a 4 or 5 gas analyzer.
But the 4 or 5 gas isnt portable unless its on one of the OTC Genisys scanners, and that makes it portable.
Anyway, do what you want.