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running rich

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Old 03-19-2017, 07:07 PM   #1
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running rich

HU all finaly git the 305tbi out and the 350 in .problem runns rich fouls plugs and stinks of un burnt fuel .have tryed two difrent carbs a holly 750 and a qfi 600 no mater whot i do idel timing mixture difrent carb fuel pressure etc etc it runs rich .any ideas ?
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Old 03-19-2017, 07:38 PM   #2
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Re: running rich

Air/fuel ratio off gotta play with it.

try a smaller jet size maybe.
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Old 03-19-2017, 09:40 PM   #3
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Re: running rich

Does the 350 have a big cam in it?
Even if it does not, it sounds like you are running on the main jets
at idle instead of the idle circuit (throttle bores opened too much).
You need to play with the timing and carb adjustments some more.
If you flip the carb over, properly set up you should see "squares"
of the transition slots in relation to the throttle plates. My bet is
you have the throttle plates open too much at idle. Set the idle
speed screw so that you see "squares", put the carb back on, and
DO NOT touch that screw anymore. Adjust your timing from there to
get a good idle (somewhere between 0 - 20 degrees BTDC).

Also, float levels should be set to the BOTTOM of the plug/sight glass.
And make sure the fuel pressure is not greater than 6 psi.
You did switch to a non-computer distributor, correct?
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Old 03-20-2017, 11:19 AM   #4
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Re: running rich

How did you adjust your idle settings? Did you use a vacuum gauge? Air/ Fuel Meter?
Does the car have a cat?
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Old 03-20-2017, 08:07 PM   #5
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Re: running rich

Quote:
Originally Posted by midias View Post
How did you adjust your idle settings? Did you use a vacuum gauge? Air/ Fuel Meter?
Does the car have a cat?
I just set it by ear and road testing. You can use your tach and a vacuum gauge if you want to.

CAT should not matter, unless it is plugged and causing a restriction. It would only be a problem at WOT, not idle.

What is your calculated static compression ratio?

OK. Here is what you have to do. That XE274 is big enough for you have reduced manifold vacuum. 11" Hg @ 800 RPM & 14" Hg @ 1000 RPM w/ no load.
Are you you using a PCV valve with a breather in the opposite valve cover? You should be. PCV is a controlled vacuum leak, and will effect your idle.
What does your distributor advance curve look like?

Take the carb off and turn upside down. Set the idle speed screw for "squares" on the transition slots. Now back it off and count the # of turns it took, so that when it is back on the engine, you now know how many turns it takes to get back to that setting. Set it back to squares.

The following assumes that your distributor, carb, etc. are properly working and have no issues.

Set the mixture screws equally at a good starting point.

Disconnect and plug the vacuum advance. BTW, when you have it connected, it connects to PORTED. Start the car and try to set the initial timing to about 20*. It has to be done at a low enough rpm that the centrifugal advance is not kicking in. Don't have it advanced so far that the engine is hard to crank or kicks back. But you want enough that you can idle, even if the rpm is a little high, or low. You are trying to give the engine what IT wants.

Now everything should be in the ballpark, and you can begin to fine-tune it.

Because of that cam, your engine requires more air at idle. If you use the idle speed screw to raise the idle, you are also going right past the idle and transition circuits, into the main jet circuits. This is where some people's engines have that running rich at high idle, fouling plugs, burning eyes, etc. condition. Smaller main jets are NOT the answer here! You need more AIR - not fuel. How can you do that? Ever heard about drilling holes in the throttle plates? But if you do that, once you go too far, you can't go back. I'm going to explain how to achieve the same thing without doing that.
Your engine should require no more than a .250 vacuum leak hole. This would be equivalent to (2) .177 holes in each primary throttle plate, or (4) .125 holes if there was 1 hole in each plate (PRI & SEC).
So find a plug or port on the intake manifold or carb that would open up a 1/4" hole. YES, creating a vacuum leak, but allowing the engine to get more air WITHOUT going through the carb. GET IT? RPM increased, right? Now you can further retard your initial timing and fine tune your mixture screws. Try to leave the IDLE SPEED screw alone. If you must touch it, only deviate from the squares setting by maybe 3/4 of a turn in either direction. Keep going back and forth between the timing and the mixture screws.
You can make this "leak" adjustable by installing one of these: (attached), or something similar. Just be sure to keep it clean. If you install it into your air cleaner base (inside the filter area), and run a rubber vacuum line to it, the air filter will keep it clean.

You can read this also:
http://www.hotrodgenius.com/html/carburetion1.html
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running rich-bc-3-versa-pneumatic  

Last edited by NoEmissions84TA; 03-20-2017 at 08:12 PM.
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Old 03-21-2017, 01:20 PM   #6
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Re: running rich

The reason I asked about the cat is because without a cat odors are more noticeable. Also don't use ported vacuum stick to full manifold the car will run much better.

http://chevellestuff.net/tech/articl...r_manifold.htm

Read this.
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Old 03-21-2017, 01:47 PM   #7
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Let's get a little more basic. If two different carbs are doing the same thing, reason tells you it isn't a carb problem.

What are you doing for fuel delivery? Are you using the TBI electric in-tank pump? You mentioned trying different fuel pressures, what fuel pressure regulator are you using? What fuel pressure are you trying to run? How are you measuring the fuel pressure? Are you using a fuel return line?

What distributor are you using? Where is the timing set? How did you go about setting it?

What intake manifold are you using?

Let's get the basics established first.
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Old 03-21-2017, 09:01 PM   #8
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Re: running rich

Quote:
Originally Posted by midias View Post
The reason I asked about the cat is because without a cat odors are more noticeable. Also don't use ported vacuum stick to full manifold the car will run much better.

http://chevellestuff.net/tech/articl...r_manifold.htm

Read this.
I read. He's WRONG. And so are you if you believe that. You said, problem runns rich fouls plugs and stinks of un burnt fuel .have tryed two difrent carbs a holly 750 and a qfi 600 no mater whot i do idel timing mixture difrent carb fuel pressure etc etc it runs rich .

RICH, meaning too much fuel, correct?

Here is what he is talking about:

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected).

You are talking about RICH - he is talking about lean.

The method I described works. It's just that most people don't know how a carb's circuits actually work, and end up copying what the rest of the group does. Try it. It works. Then your friends will be the ones wondering how you got your engine to run so good.
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Old 03-22-2017, 07:52 AM   #9
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Re: running rich

Running retarded timing (not enough advance) will cause issues with a Holley that might make you think it is running rich. 95% of all carb problems are timing problems. The post Midias linked to is spot on.
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Old 03-22-2017, 08:30 AM   #10
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Re: running rich

The link I posted it 100% correct John Hinckley is a highly regarded automotive engineer he worked on and engineered many of those carbed systems in his 37 years at GM and Chrysler. I know how carbs work and have tried ported a few times after tuning I have never achieved results as good as full manifold vac. The idle should be set by a tach and vac gauge here is a good link.

http://www.superchevy.com/how-to/45638/
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Old 03-23-2017, 12:21 AM   #11
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Transmission: upgraded 700R4 3400 stall
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Re: running rich

In the end, you have to be happy with it.
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