Tuning a Carter AFB carb with a big cam?

First off, I have no experiece adjusting a carb with a big cam. I have a chevy 350 in my 57 truck with what I was told is a 290 duration cam. No info on lift. IT will idle, but vac is low and adjsting the idle screws does not change vac. Intial idel screw position is 1.5 turns out. Timing is set at 10 BTDC. You can smell a rich mixture during idle. Any suggestions on how to adjust. HEI ignition, new plugs, new wires. THanks in advance.

 

Connect your vacuum gauge to a source of full manifold vacuum (not ported vacuum) so you can see if you're making the right changes. Then start with some lighter tension springs under the power pistons. The parts may be the same as used in the Edelbrock Carter style carbs. With the low manifold vacuum the power valves are trying to open up at idle and the idle mixture screws won't have much effect.

Also more initial timing will help keep the vacuum up a bit but you may have to take some mechanical advance out of the distributor so you don't end up with too much total ignition timing.

Might want to check your fuel pressure too. Apparently these carbs don't like more than about 5 lbs. or they tend to flood.

 

What dist is in it?

 

Big cams like a lot of initial timing. Like 15-18*. Limit the total initial + mech advance to 35*. Also like compression but not much you can do there. The advertised duration(290) is not enough to guess at. Better to know the duration @ .050 tappet lift. That gives you a better feel for how it will run.

Buy a power piston spring kit from EDB. Usually the performers come with orange springs 5 IH/HG. You will want 4" to start with (don't recall the color).

 

Thanks for the responses.i will adjust timing and change out metering springs and look for improvement. After followup research, those two items appear to be my problem. You guys are great!

 

Yep start with more initial timing and if it doesnt have alot of static compression it may like even more than 35 total.

 

The best tunning advise for a larger cam..... Buy a Holley!

 

At idle, the metering rod position is not going to make a difference. If the carb was designed to be used w/ a PCV valve and you are not using one, you might need to make some modifications. Before you do that, make sure both the air bleeds in the primary clusters are clear (no blockages or buildups)...
...the 1st is at the top of the cluster above the small br*** tube, facing the rear.
...the 2nd idle bleed is on top of the cluster.
Usually, a rich idle is just something blocking the metered air from entering the idle mix.

Drilling a small .125 hole in the primary throttle blades will also lean out the idle and will help if a big cam's high idle rpm is causing the transfer slots to be overly exposed.

Here's a link to an actual Carter tuning manual, better than the Edelbrock "owners manual"... http://www.mymopar.com/downloads/carterafbtuning.pdf

 

Running the distributor advance to full vacuum will also help smooth out your idle. The lower of the 2 ports in front of the carb will be full vacuum.

Now if you get detonation with the advance hooked up it will be time to look for an advance with a shorter stroke of that requires higher vacuum to operate. Cesar

 

Regarding power valves, There isn't any in a afb

 

The Carter Tuning Guide (link provided by weedburner) is a great source. However, use it with a grain of caution. This guide is specifically written for use with the then current production aftermarket Carter AFB carburetors. Much (but certainly not all) of the information is also applicable to other Carter AFB's. In the Guide, Carter themselves state that all AFB's are similiar but different.

It is the responsibility of the user to understand these differences.

Offering the answer to an unasked trivia question; Carter produced 505 different model AFB's. This does NOT include any of the F/M, Weber, and Edelbrock clones. There were reasons to produce these different carburetors, other than simply denying investors from dividends for half a dozen models.

Jon.

 

I kind of understand your thinking, but the Carter AFB manual that you link to states the following:

LOW SPEED CIRCUIT

"Fuel for idle and early part throttle operation is metered through the jets in the low speed circuit. The low speed circuit is located on the primary side only. Gasoline enters the idle wells through the main metering jets."

The real problem is that after a certain amount of engine modification you end up with throttle blades open further than normal in an attempt to stabilize the idle to some degree. After a certain point of opening you start to pull fuel from the main nozzles with the engine idling under no load and wanting a leaner mixture.

Drilling the throttle blades is one way to help lean things out but it's the kind of permanent modification that I might only do as a last resort. Some carbs may allow you to make an adjustment that just slightly opens the secondary throttle plates also helping to lean things out at idle. This is a less permanent and more easily reversible change that you might try. Just make sure the secondary throttle plates aren't so far open that it now wants to pull fuel from the secondary fuel nozzles at idle.

 

The main jets with the metering rods in them are not the same as the low speed jets. Fuel entering the idle wells p***es thru the main jets / rods, but idle fuel is actually metered by the low speed jet, a much smaller hole.

 

Great responses and a ton of info. These kinds of posts are the reason I log on here. Thanks for sharing your knowledge guys.

 

Drilling the throttle blades is not as permanent as you might think. Just last nite i welded a couple up so i could drill a smaller hole. A couple quick bursts from my mig and a little touch from a flap wheel allowed me to start all over from scratch. I had used solder before, but a nitrous backfire last weekend melted it out. Melted a little aluminum off the bottom of my booster skirts too...

 

See Picture,I have what you need PM sent

 

Give that man a cigar!!!..^^^

 

Oh...And thanks for the link!

 

Thought i might mention that the "full time" vacuum port on the front/driver's side of the main body may not get vacuum at idle, as most i deal with are connected to a small slot right at throttle plate level and are blocked at low idle speed settings. The "ported" vacuum connection on the p*** side of the carb is connected to a slot that is only slightly higher. To get a true manifold vacuum source, you might need to connect to one of the large PVC/brakebooster ports or directly to the manifold.

Just because the Edelbrock manual says the left side port is the full time vacuum source, you better check...