More carb questions

[BB73Challenger]

I feel silly, but there is something not right about my carb still and to be honest it's driving me nuts.

I've picked CP's brain on more than 1 occasion about a "good" or right carb by his opinion, and the first thing that stands out is Chryco, you suggest more CFM than what I have now.

And here is where my lack of understanding starts to play in....

The last carb I installed was a Demon Jr, and I have to say right off the bat it had issues for a new out of the box piece, but that has been worked out.
I believe it is rated at 725 CFM, and my train of thought was, less CFM on tap, better gas mileage and also didn't want to over-carb.
That being said, I know it is insane to use the thought "MPG friendly" and I have a big block.


My biggest problem right now is, it is SUPER rich at idle. So rich that finally during a day this past week were it didn't snow for once and it got above 10 degrees, I decided to run the car. It becomes so bad in a 2 car garage with both bays open I can't stand in there with the car running due to my eyes watering and choking on exhaust.
It has been this way always, I think I'm getting cabin fever, and I know other cars don't run this rich.

When checking the carb for the best tune, I noticed that a cause for rich idle is having the butterflies open too far, but when I set the gap per book, I can't get it to idle enough, resulting in me turing the idle screw and increasing that gap.

With all that said, am I getting this problem because I'm trying to coax the right fuel amount from too small of a carb?
The rest of the carb "tuning" is done with a vacuum gauge and with RPM's, going for the highest readings.

Any input would be appreciated, as this board is always helpful!

[Bullitt-]

Don't want to get you off track but my '73-340 always seemed way too rich, I've had many different carbs, plugs, wires & new MP distributor through the years.  After I got her running again 3-years ago I started replacing ignition components, Bosh plugs, MSD 8mm wires, Accel coil & finally a new Standard ballast resistor. Somewhere along the way the eye burning fumes & light smoke went away. Now she starts easy & runs smoother than I can remember.

[Roppa440]

Why do you think that a smaller cfm carb will use less fuel?

The engine will use whatever air and fuel it demands. The carb you use has nothing to do with it as long as it is tuned correctly. Carbs flow air into the engine. Not fuel. The job of the carb is to add enough fuel to that airflow so that the engine gets the correct burn mixture. How much air and fuel you need mainly depends on how big your engine is and at what rpm it is running at. A smaller carb just means the air has to go faster through it.

Too small a carb will give you better throttle response and good low rpm drive-ability. Because airflow through a smaller opening has to be faster. But will not flow enough AIR at high rpms. It will choke.

Too big a carb will flow more than enough air at high rpms but at low rpms the air flow will be very slow due to the wide opening. So throttle response and low rpm efficiency will suffer.

So if you have a 440 for example a 750cfm carb would be ideal for a street car with a mild cam that rarely goes over 6000rpm. But a street/strip car with a wilder cam would want a 850cfm carb to take advantage of the high rpm power.

Now to your problem.
Your cam is demanding more air at idle. Which is why you have to open the throttle blades too much. This stops the idle circuits working and "turns on" the main jets.
The cure is to let more air in another way.
On some carbs (like Holley double pumpers) you can adjust open the secondary throttle blades a touch. Without this adjustment you have to drill a couple of small holes in the primary throttle blades. Start at 1mm.

Once you have let more air in you will find that the engine will idle at a higher rpm and be slightly leaner. Keep letting more air in (drill bigger holes if you have to) until you can get the throttle blades closed enough for the idle mixture circuits to work. I wouldn't go over 2mm though. Go bigger in 0.2mm increments.

An easy way to tell when you have the throttle blades closed enough is to put a vacuum gauge onto the vacuum port for the distributor timing advance. Any vacuum on this port shows the blades are too wide.

Once your rpm is right and the throttles closed enough you can put the vacuum gauge onto a manifold vacuum port and tune the idle mixture to just to the lean side of max vacuum.

[Roppa440]

Forgot to say. Make sure your timing is right to start with. To run with modern fuels you need plenty of initial advance. Which will also increase rpms a little.

[BIGSHCLUNK]

I had a rich prob years ago... had a bad power valve.  I also think along the lines of ROPPA . Do you have the correct adjustment on the secondary throttle blades?? Mine were closed when I bought my car and it took quite awhile to figure out the prob.

[BB73Challenger]

Got it guys, thanks for all the input!

The car itself runs well and starts well.
It has mild upgrades, including ignition system, coil and beefier wires. ( Bullitt )

It is just a street car, and I would not have thought to add more air with small hole into the primary blades, but after reading I do believe I have heard of that before at some other time. This seems right on ( Roppa440 )

I will also check the secondary blades to see if there is an adjustment ( BIGSHCLUNK )

Thank you guys very much again, again I don't have a very good understanding of carbs but this helps shine more light on it for me!

[Roppa440]

I had a rich prob years ago... had a bad power valve.

Yes a bad power valve will do that. So will a power valve that is not the right size for the vacuum the engine is pulling.

[Bullitt-]

I gained much understanding of basic carb. functions from this article


http://www.chevyhiperformance.com/techarticles/83118_carburetor_basics/index.html

[Roppa440]

Here is a good guide.

http://www.bob2000.com/carb.htm

[boydsdodge]

If you want fuel economy and power get a good 72-74 Thermoquad and get Demonsizzler to build it for you.
Best all around carb you could ask for on the street.
If it needs 800 cfm it will flow 800 cfm if it needs 600 it will be 600.
I can take a thermo quad off a 440 and tune it to run great on a 273.

[Chryco Psycho]

what vacuum are you making at idle , do you have the cam specs ?
 I agree with 2 things here smaller carbs will definatly not make better milage , it takes the same amount of fuel & air to make the X amount of power to pull the car down the highway at any given speed & it is how efficiently the carb will mix fuel & air to make that power 
 2 adding more air flow with holes in the throttle plates definatly helps with low vacuum to add more air flow while mainaining the ability to adjust idle mixture by keeping the throttle plate over the idle transfer slot
 The next point is long duration cams are ALWAYs rich at idle as they misfire cylinders & lope this pushes raw fuel through the engine , the carb may be tuned perfectly to run at RPM but will still be idling what seems rich due to unburned fuel . The next point is Demons in my experience are Not good carbs , I have seem some woirk excellent but I have seen more than a reasonable percentage that do not work well so I avoid them , If you are looking at buying a New carb I would lean towards the Holley HP or better yet the Proform HP series carbs with no choke , I have had great reuslts tuning these carbs

[Roppa440]

Does this Demon Jr have an idle air bleed? I was assuming it didn't. I know the Speed Demons do.

[nivvy]

You just need to learn to tune better,, drilling holes in the throttle plates is just a band aid IMO.... accidentally go to big and you cant go back.....
My 500"r runs a 1050 dominator and everyone says you cant run a dominator on the street..... My engine idled  at 800 rpm smoothly.... 

BUY A HOLLEY............... screw demons.....  If you want i can post a link from yellowbullet and Barry Grant gets his balls busted hard from all the complaints....... so bad he actually got on the site and replied himself...... It wasnt pretty...

[Chryco Psycho]

I don`t see drilling holes as a band aid fix , I have solved a lot of idle problems doing this & if you can weld nothing is permanent , even sqishing a small piece of solder into the hole will work
 I agree on the dominator carbs , they are aan awesome carb street or track I just wish there was better choice of intakes & air cleaners to support them

[Moparal]

Stroker, you put the guard under your carb incase the bottom falls out?  They used to drop parts into the intake. They make a screen for that