How can I tell if I can go leaner on my carb?

[Secret Chimp]

The 440-spec 4966S Carter AVS I just installed on my 318 Coronet runs great. Really made the car a thrill combined with the new cam.

I'm not barfing black smoke or stinking at idle, but how can I tell if I can step down leaner on cruise? I know I can do a WOT blast and shut off the car to read my plugs for an idea of my power mixture, but I don't know how to get a read on my cruise mixture short of just buying some bigger metering rods and seeing when it starts to get a lean stumble.

I'm also kind of unclear on when my idle mixture stops being a major player in the mixture. Around town I'm burbling along at 1300-1500 rpm, on the highway I'm at around 1900-2100 rpm.

I want to be close to on the money for the best economy - I've been trying to get better mileage out of this thing since I bought it (best is like 14 mpg all highway, I'm going through a tank on this carb this weekend to see how I've improved) and I'm hoping having a carb I can actually tune will be the final ticket. Do I just need to experiment?

 

[mopar367]

Aren't the newer edelbrock carbs the same as what AVS carters used to be? I think you can get a strip kit for the Edelbrocks, if the parts interchange I would just play with the jetting. Keep reading plugs and see where you get the best economy and power. Just my 2 cents.

 

[Secret Chimp]

Aren't the newer edelbrock carbs the same as what AVS carters used to be? I think you can get a strip kit for the Edelbrocks, if the parts interchange I would just play with the jetting. Keep reading plugs and see where you get the best economy and power. Just my 2 cents.

Yeah, Scott from Harms Auto sent me some original AFB covers that let me use Edelbrock parts (Edelbrock AVS's don't use the long three-step rods the originals do). I was hoping there was some magic method beyond swapping and test driving...

 

[mopar367]

Right on at least you can use the edelbrock parts. I can't think of any other way to tune it other than trial and error. You said the carb was off of a 440, do you know the cfm? It could be too large of a carb for a 318.

 

[Meep-Meep]

Just about all the light throttle driving is on the transfer slot and that is fed by the idle feed restrictors. The metering rods play a roll here as well since the fuel that feeds the idle restrictors has to pass through the main jets.

You may find that carb is a bit big for a 318 unless your engine is built up pretty well. And even in that case low speed performance will suffer. Keep in mind that all carbs work better with a strong vacuum signal so sizing the CFM according to engine demand is the first step. You may try a 650 AFB because those have smaller primaries thus keeping the signal stronger. Also the emulsion circuit will function better with the proper signal. A common misconception is the bigger carb will run richer. This is not true! It can actually run leaner if the signal is weak. Air is much easier to move through a carb than fuel.

When dialing in mixture it's always best to find the lean point and bring it up from there rather then the opposite. Reason is an engine will run pretty well with a very rich mixture but it will barely run if it's way too lean, thus creating a good baseline from which to tune.

 

[Secret Chimp]

It's a 625cfm carb. It runs like a raped ape on my car under WOT and it's significantly stronger out of the secondaries just driving around versus my old BBD. It has 1 7/16" primaries, which supposedly is the same for AFBs of around the same cfm (? this is just according to other forum posts after a lot of Googling).
I've gotten about 17 inches of vacuum out of it at idle, which is pretty close to what my old BBD was pulling on the original iron intake and the same Summit 6900 camshaft. I get a secondary bog if I floor it at a really low RPM but I have door set so it's pretty much 100% gone at more typical RPMs. I haven't really had any oversize symptoms with it, surprisingly.

Though it does sound then like I need to get my idle mixture setting done right and my vacuum advance dialed in - I've tuned it by screwing in the mixture screws until the needle on my vacuum gauge drops a tick or so (or at least is jittering around in an area a tick lower than it did earlier) and then backing them off a quarter turn or so, which I understand is the preferred method. I cranked in my vacuum advance a full turn today and it runs the same save for a slight jittery almost-stumble at some speeds so I'm assuming I have that in the ballpark already.

Regardless I'm sure I'll get a strip kit in a few weeks and mess with it to see what more I can find in the carb. Is it really better to start by jetting down versus moving up to bigger rods? I was able to crack loose and retighten all of the original jets in this carb but I'm always afraid of some fatal cracking of brass that leaves me with a buggered jet stuck in there. Rods seem relatively mess-up proof by comparison.

 

[Powerflite]

If you have the $$, I would step up to the 21st century and purchase a wide band O2 sensor kit. This allows you to see the actual Air-Fuel Ratio (AFR) while driving, idling, WOT, whatever. You will need to weld the O2 bung to your exhaust, but you only need to purchase one of them to use for all your vehicles if you plug the bung after you are done.

Here is a link:
http://www.innovatemotorsports.com/products.php