Carb adjustment?

[olddog67]

I have a Carter AFB on my 383. It seem to be getting too much fuel as I get a light dusting of soot on the ends & sides of my tail pipes, they are straight out the back. I also get it lightly on the bumper. There are 2 screws on the front of the carb, what are they for? Can they be adjusted to fix this or do I need to open it up and change the jets or something?

 

[Billccm]

I've never had any luck with idle mixture adjustments unless I use a vacuum gauge. Have the car warm at idle, and adjust SLOWLY the idle mixture screws to peak the MAX vacuum. Perfect specification is 13-16 inches of vacuum, but on my 383 with a performance cam 11 inches was the best I could get.

 

[olddog67]

So those are the idle mixture screws? Sorry, Im learning as I go here lol.

I can get a vacuum gauge I believe.

 

[moparmarkk]

or use a tach/dwell meter,and get the highest idle on both sides,then back of on the idle screw on the side, cake

 

[67 GTX]

Remember that seeing soot on the inside of your tailpipes is a common occurrence, and not an indication of something being wrong.
Hook a tach up to the motor, and adjust the idle mixture screws with the tach. Adjust each screw looking to peak the tach rpm. Do both screws, then using the idle speed screw (drivers side) lower the idle to the desired rpm.

 

[coloradodave]

All of the above is done with the engine fully warmed up and the choke blade fully open. Good luck...

 

[olddog67]

Remember that seeing soot on the inside of your tailpipes is a common occurrence, and not an indication of something being wrong.
Hook a tach up to the motor, and adjust the idle mixture screws with the tach. Adjust each screw looking to peak the tach rpm. Do both screws, then using the idle speed screw (drivers side) lower the idle to the desired rpm.

No im getting soot on the sides of the tail pipes, the bumper, the rear panel where the tail lights are. Quite a bit too. If you rub your leg along the bumper it will be black.

What do you mean by 'looking to peak the tach rpm'? And are we talking about just the 2 screws in the front or is there another on the driver side for the idle, making 3 I will need to play with?

This is gonna take some youtubing I think but I will get it. I appreciate all the help and patience with me guys! What is a common term to most requires photos with big arrows pointing for me.

 

[67 GTX]

The two on the front are your idle mixture screws. They are effectively bleed screws, and what you are adjusting is how much air you let/don't let in.
The idle speed adjustment is the one on the side.

I doubt the soot build up is from the idle mixture though, as something that bad wouldn't really run at idle, or you'd notice it.

Could be your carb is just jetted was too rich. Do you notice any black smoke out the back as you rev the motor?

 

[66Satellite47]

Doubt that the idle mixture screws will relate to a soot issue on the bumper. Sounds like the float level has gone off. Find a good carb guy & have him go through it.

 

[olddog67]

Never noticed any black smoke no. I just put valves seals in a couple weeks ago and thought that was causing the soot on the rear of the car. It idles fine when warmed up as well.
I understand the screws we are talking about now.

 

[olddog67]

I live in a small town and no one is willing to or knows how to work on these cars anymore. I couldn't get anyone to do the valve seals after calling 5 different shops so bought the tools and it myself.
Here are some pics of the soot, although its kinda hard to see on the bumper in the picture. I wiped my hand along 6-8 inches and the tail pipes is one dirty and after wiping with a rag. I likely drove for 2 hours that day.

 

[moparmarkk]

Are you sure that not just left over oil from before the valve stem seal change?

 

[olddog67]

I have probably put 100 km or more on it since the seals were done. I guess it is possible though Mark.

 

[PurpleBeeper]

No problem Olddog.... I'll break it down for you.

1. Idle Mixture Screws - those two screws up front adjust the mixture of fuel & air (at idle or light cruising). Each screw does half the carb (& half the engine sort of). Out = more fuel, In = less fuel. At the optimum idle mixture, both the vacuum of the engine & the rpm of the engine (at idle) are their highest. Most people adjust those (as these guys are telling you) by adjusting the screws (one side at a time) until they get the highest idle rpm and/or highest idle vacuum. I use both, but not necessary. I've "ear tuned" many an idle mixture at least close.

2. Your problem - Well, if you really have THAT MUCH soot, then you "might" be running rich (too much fuel). I'd adjust idle mixture first. I'd also take a look at my spark plugs to see if any of them (or maybe one side of the engine) has plugs that are all black and covered in soot. That would be a pretty sure sign you're truly running rich. If you know a shop that still has one of the old emission "sniffers" that go up the tailpipe, you can adjust your idle mixture dead-on. If all this is done & you still have a bunch of soot, then you might want to find someone to tune your carburetor (different jets/rods).

For what it's worth, my car has a little soot and I think I'm pretty close to a perfect fuel mixture. Does your motor blow black smoke when you romp on the gas?

 

[olddog67]

Thanks PurpleBeeper. Can I hook the vacuum gauge to the port my vacuum advance is on? and better to be to rich than to lean correct?
I dont recall seeing any black smoke when I stand on to be honest.

I think I have a good understanding to get me started with things now and go from there.

 

[miller]

I'd adjust idle mixture first.

No. First, adjust timing, then adjust IDLE speed (idle adjustment at the throttle arm stop). Saying that because, it's a waste of time trying to adjust mixture (two screws up front) unless the idle speed is set with in limits.
The black crap on your pipes, can also be from crummy gas...needs to be a good grade! 92-93.

If idle speed is okay, letting the carb go into it's 'idle circuit', then you can adjust mixture. Playing with it, vacuum, or RPMs, once it's set...shut it down...and exactly match the screw settings for both screws the same.

btw...real good chance, once mixture is set right, you'll need to at least, check timing, and idle speed, since both can slightly change.
Gotta have starting points!

 

[oldbee]

Miller why are telling him to balance the mixture screws after being set? Ur assuming a perfect carb with every passageway being equal(?). I go with "purple" on this.

 

[AR67GTX]

I will suggest something that is a long shot but worth a try. Back in 1968 I bought my first car, a 1964 383 Sport Fury. As it was nearing the expiration of the 5 year powertrain warranty I noticed it was using a little more oil and I was getting a heavy film on my rear bumper from the exhaust. The dealer got permission to tear the engine down in the car and re-ring it under warranty - which I witnessed in their shop. But when I got it back nothing changed - still getting a heavy film on the rear bumper. Took it back in and one of their mechanics decided to try changing out the PVC valve. Problem solved - oil useage dropped and no more film on the rear bumpers. And I got a new set of rings as a bonus.

So, for the price of a PVC valve it may be something worth trying.

 

[oldbee]

Love this forum you get so many different angles to look at something that just fly by your own noggin.

 

[miller]

Miller why are telling him to balance the mixture screws after being set?

Right, or wrong, that's how I was taught, first starting to fool with 'em. Usually not much off, I've always looked at a four-barrel, like two two-barrels, side-by-side. Also depending on what manifold, how the mixture is fed...so, once mixture is set, I've always 'balanced' them, so one side is not leaner than the other...even.