My 73 crew is running real rich. I pulled all the plugs and all were jet black. You can even smell it while idiling. It has a Carter AFB compitation 4 barrel carb. So my question is, the 2 screws on the front of the carb are the air/fuel mixture screws, which one is the fuel mixture screw? Would I just turn it in to lean it out or do I turn it out to lean it out? Also, would I have to adjust the air mixture screw at all? Thanks for the help.

Rob

 

There are no such things as separate fuel mixture screws, or separate air mixture screws. Just air/fuel mixture screws. There is one for each primary bore (left and right). Turning them in is lean, turning them out is rich.

You usually want to do this with a vacuum gauge. For most applications, you turn them all the way in with the engine off, then back them both out 1.5 turns. Start the engine, and slowly back them out until the vacuum gauge is at its highest reading. You may have to readjust the idle speed after the mixture is optimized; it usually kicks up a bit since pegging the mixture makes the engine run more efficiently.

 

Thanks for schooling me. I did not know each screw was a air/fuel mixture for each bore. I have a vaccum guage but where do I hook it up at? Would it be on the front of the carb where the vaccum advance from the distributer hooks up? And would I have to plug the line on the distributer? Thanks for your help.

 

The distributor usually hooks up to ported vacuum, which is above the throttle plates, which you don't want. You want manifold vacuum which is below the throttle plates.

That carburetor should have two small 1/4-inch ports on the front. Your distributor is probably hooked to the higher of the two. The lower of the two is manifold vacuum, which is the one you want. You are looking for 19 to 21 in. Hg. of steady vacuum.

You do not need to plug the line on the distributor.

 

My distributer is actually hooked to 1 of the lower ports on the front. I think its 1/8 inch and my PCV hose is hooked on the other lower port right in the middle 1/4 inch. So by what your saying, I have my distributer hooked in the wrong spot? I dont think I have another port to where I can hook my distributer to.

 

The two small ports are 1/4". The larger port in the middle should be 3/8". The higher of the two 1/4-inch ports is ported vacuum. Usually it hooks to the distributor. The lower of the two 1/4-inch ports is manifold vacuum, and is usually capped. The 3/8-inch port in the middle usually goes to the PCV valve.

Running your distributor off manifold vacuum isn't wrong, it's just less common. If you do, your initial timing usually needs to be lower. If you run timing in the 8 to 12 degree range, then usually ported vacuum is the way to go.

Whichever it is, use the lower of the two 1/4-inch ports. If your distributor happens to be running off that, then tee off of that line. You can't hook the vacuum gauge to the higher port because ported vacuum is zero at idle.

 

Ok I will do that. BTW, my timing is set a 6° BTDC. The way everything is hooked up is that way from the PO. The only thing I did to the carb was rebuild it but put everything back to the way it was.

 

I just went through a similar issue. Ended up being a vacuum and PCV issue. Spend the $200 and take it to the carb shop

 

[quote=fmc400;12567524]
Running your distributor off manifold vacuum isn't wrong, it's just less common. If you do, your initial timing usually needs to be lower. If you run timing in the 8 to 12 degree range, then usually ported vacuum is the way to go.

quote] Thanks for the great advise!! reps sent your way!!

I rebuilt my carb AFB on my 76 motorhome last summer and adjusted it with a vacuum gauge. And it runs good but still alittle rich. Which actually I am OK with. But now since you pointed out the timing, you have me rethinking bumping the timing up a little. I have a 460 and I think I have it set at 8 or 10 btdc. Also I'll recheck where I have the distributor vacuum hose plugged in. Since I gutted all smog stuff it had which wasn't much being a 1976. So if it is plugged into manifold vacuum I'll try it on ported vac to see if that helps.

So my question to you is do you think I would be able to help the rich condition by going to 12* btdc?? As long as I am plugged into a ported vacuum.

Rob if adjusting idle mixture doesn't help you might need to rebuild it. And if it is still running rich with the screws only one full turn out or less I would think it might have a dirty or bad needle and seat.
NEVER MIND I see you said you rebuilt it already.

 

Ed, I rebuilt it this past August. Its been running rich from the day I bought it back in 06 but now I have a little time on my hands I thought oi would try and make it run a little better.

 

Ignition timing does not affect the actual ratio of air to fuel that is presented to the combustion chamber; this is solely the job of the carburetor. The only reason ported vs. manifold vacuum and timing got brought up was with regard to where to plug in a vacuum gauge to set the mixture. It's still possible to have a strong vacuum signal in the presence of a rich condition. Also note that the vacuum gauge is only used to tune the mixture at idle. Off idle, at cruise and under acceleration use different metering circuits.

 

Have you checked to make sure the choke plate is wide open when hot.

Does this carb have metering rods???
If so, you could lower the rods to lean the mixture.

 

Make sure the metering rods and step up piston is free to travel with nothing but the spring holding them up, check the condition of the accelerator pump piston (they have a alcohol resistant one out) and check the floats to be sure they "float" and are dry.

Might check your fuel pressure .... and if higher than 5 or 5.5 psi .... add a regulator like Holley's 12-803. As I found out, and others said ... Mr Gasket / Spectre FPRs are wasted $$$..

Carter AFB / AVS / Edelbrock aren't as tollerant of higher fuel pressures as Holleys and Motorcraft Carbs, floats have different leverage ratios, longer arms, and often are bigger in the latter. Edelbrock tells you their (it is a copy of the Carter) carb is limited to 6 psi and they suggest 5.5 psi. The old Cars that had Carters also had only 4 - 5 psi, some in the 3 psi range (more psi is not more volume so much as it's harder to shut the door .... or "needle" on). Excess fuel pressure will cause a dribble into float bowls at low use times like idle that the flaot and inlet needle can't stop before flooding.

My '77 with over the counter 10+ year old no name fuel pump makes 7 psi and it was a problem when I switched from Holley to Edelbrock.

Just offering a suggestion?

 

Yes the choke plate is wide open when hot. Not sure what the metering rods look like or where there located in the carb.

 

They are toward the back and are accessed by a couple swing-away plates on top. I think you should look at your idle mixture with a vacuum gauge first, especially since you said you can smell it running rich at idle. Metering rods do not affect idle mixture. Once you get the idle mixture dialed in, you can determine if you still have a rich condition by reading the plugs over time; if you do, then a trial-and-error jet/metering rod investigation would be your next step.