hey guys as some of you know i bought a truck a week ago i went in to have it emissions and inspected and it failed my O2 test they said my air fuel mixture was off. so does anyone know the stock specs on a 20R motors carb? or what i can do to get it lowered. im pretty well stumped. do i need to rebuild the carb?

 

check all your vacuum lines and make sure there not clogged or disconnected

 

and if everything is good in working order, then check your jets. you may just want to rebuild the carb if its old or needs it. Mine wouldnt pass smog but after I rebuilt my carb when it was a 22r, it passed.

 

okay so what else could it be if ive already rebuilt the carb now?

 

I've come to learn when it comes to failed emissions it's tough to accurately diagnose it without the proper equipment. Not having a tach for one. Where I'm from they'll fail you if you're idling above 850.

If you have a compression tester, I'd check that first. Then I'd do a valve adjustment. Check your timing. Check to see how your plugs are burning; too rich - too lean? Exhaust leaks? Vacuum leaks?

Fiddling with the stock carburetor, as a few have already noted here, can be fruitless. It just doesn't respond the way other carburetors do. Rebuilt or not. That much more harder to tune if everything else isn't up to spec - all the things I mentioned above.



(Pulled from the web) Emission testing, emission failures and repairs.

The yearly emission test of your car is nothing more than a test to see how much fuel you are sending out the tailpipe that is not being used by your engine. Think of an emission failure as a huge gas leak, because that is exactly what it is.

To pass emissions, you must tune your engine to properly consume the gasoline that's being delivered to it and convert it to energy to move the car down the road.

The emissions test is an easy way to determine if the right mixture of air and fuel is being delivered. It also can check to see if the ignition system is working ok. As a matter of fact, when trying to fix a driveability problem, most good technicians will take a look at the vehicles tail pipe readings first. These readings will give them direction.

When you fail because of high limits of HC or hydrocarbons, it means there is raw fuel that is being sent out the tailpipe because your engine isn't converting the fuel to energy. The raw fuel actually washes the protective layer of oil off moving parts inside your engine and contributes to engine wear which can lead to premature failure. Some common causes are spark plugs, spark plug wires, misadjusted timing or vacuum leaks.

When you fail because your CO or carbon monoxide is too high, that means the fuel and air ratio is way off and there is evidence of incomplete combustion or burning of the air/fuel mixture. Most of the common causes relate to the carburetor or the fuel delivery system. A CO failure will always be rooted in the carburetor or fuel injection system. CO failures are not fixed with spark plugs.

If there is too much fuel and not enough air, a CO failure is usually seen. When there is too much air and not enough fuel, a HC failure is generally seen.

In 1976, the first year of vehicle emissions testing in Arizona, no one was forced to fix their car if it failed. The following year, 1977, it was mandatory that failed vehicles were fixed before renewal tags were issued.

The volume of tested vehicles over the last 10 years has steadily risen from 12 million vehicles to 18 million. The failure rate 10 years ago was 22% of the tested cars. In 1991 we were at 15%. We are clearly making positive progress towards cleaning up our air.

The failure rate by model year shows that the first year tested, 1967 vehicles, have a 35% failure rate. The failure rate declines to 28% for the 1974 year vehicles. 1975 Shows a failure rate of 31% and from there it shows a steady decline to 1996, where less than 1% of all vehicles fail. So much for old car enthusiasts (which includes me) saying their cars are not part of the air quality problem!

Let's group some vehicle years together and look at the failure rate in the state of Arizona:

1967-1980 have an initial failure rate of 33.6%.

1981-1996 Have an astounding low failure rate of 12.73%.

If we look at just 88-96, they have a initial failure rate of 3.8%

Talk about the progress of technology!

I don't know the name of the provider of centralized testing in Arizona, but their test equipment passed over 97% of the time it was tested for accuracy. Other emissions analyzers at fleet testing and certified repair facilities passed 86% of the time. This should eliminate your technician from saying that his equipment is right and "their's" is off.

So given the choice, I'm sure most of you would rather benefit from all the fuel you buy as opposed to having a large portion of it be wasted and end up polluting our air.

By the way, the best way to insure your car will pass its emission test is to make sure you hold the idle up to about 2,000 rpm while waiting in line of for at least 3 minutes prior to the test.

Here is an article we wrote about emissions failures for the
more advanced do-it-your-selfers:

As technology increases and more emphasis is being placed on air quality, many states and municipalities have instituted vehicle inspection/maintenance (I/M) programs, most of which include tail-pipe exhaust emissions testing. The center of the controversy is the internal combustion engine, which converts the chemical energy stored in fuel into heat. The heat is released by simple combustion. Perfect combustion changes hydrocarbons (HC) into carbon dioxide (CO2) and water (H2O).

Unfortunately, perfect combustion happens only under laboratory conditions. At any other time, the result is less than perfect combustion and undesirable emissions. The three major pollutants are carbon monoxide (CO), hydrocarbons (HC), and oxides of nitrogen (NOX).

Carbon monoxide (CO), is incompletely burned fuel or to be more precise are hydrocarbon molecules that split apart but don't burn in the combustion cycle. High (CO) is the result of one problem, a rich air/fuel mixture but may have several causes. Hydrocarbons (HC) is simply unburned fuel that escaped the combustion cycle.

Oxides of nitrogen (NOX) are a little more complicated. At engine combustion temperatures above (2500 F), nitrogen (which makes up 80% of our atmosphere) bonds with oxygen to form oxides of nitrogen (NOX). Normally these two gases are like oil and water, they don't mix. The exhaust gas recirculation valve (EGR) is designed to introduce metered amounts of exhaust gases back into the intake manifold, displacing a portion of the incoming air/fuel mixture to the cylinders. This process reduces high combustion temperatures and controls NOX.

Here's some common causes for the production of these three pollutants.

CO Failures (vehicles with carbs)

1. Engine not at operating temperature. Example, engine/cat cooled off while waiting in test line or has a stuck open or missing t-stat.

2. Bad or misadjusted float level.

3. Plugged air bleed passages or misadjusted main metering system, leaking fuel passages or gaskets.

4. Maladjusted idle air/fuel mixture screws.

5. Ruptured or sticking canister purge valve.

6. PCV plugged or drawing in fuel contaminated oil vapors.

7. Malfunctioning mixture control device.

8. Malfunctioning computer inputs. Example: O2 sensor defective, reading lean all the time. MAP sensor vacuum hose being clogged or broken (reading a heavy load all the time). Coolant temperature sensor having high resistance or open circuit (reading "cold engine" all the time). Throttle position sensor stuck or open ground, (reading wide open throttle).

9. Contaminated, restricted or bad catalytic converter.

CO Failures (fuel injected vehicles)

1. Any of the computer components listed above.

2. Leaking or bad injectors.

3. Mass air flow sensors (voltage or frequency being too high).

4. Fuel pressure too high (restricted fuel return line or stuck fuel regulator)

5. Ruptured fuel regulator (fuel leaking directly into intake)

HC Failures (all vehicles)

1. Engine was not at normal operating temperature.

2. Ignition system malfunction. Spark failing to occur for any reason will send unburned hydrocarbons (HC) down the exhaust pipe. Example: bad plugs, plug wires, distributor cap, rotor, coil wire, coils, etc.

3. An extremely lean fuel mixture that causes misfiring. Examples include disconnected, leaking or misrouted vacuum hoses, intake gasket leaks, EGR stuck open, low fuel pressure.

4. Over advanced timing. Insufficient spark duration.

5. Low compression or mechanical problems. Worn rings, burned valves, bad or misaligned timing gears.

6. Over rich fuel condition causing both HC and CO failures.

7. Contaminated, bad, or restricted catalytic converter.

NOX failures (all vehicles)

1. Improper operation of the EGR valve. Stuck open, obstructed, leaking, or misrouted EGR vacuum lines.

2. Plugged EGR passages. Carbon build up in passages.

3. Over advanced timing.

4. Too lean of fuel mixture.

5. Engines cooling system in poor condition, causing excessive temperature.

6. Malfunction of the electronic spark control (ESC), and knock sensor. Computer fails to retard timing.

7. Contaminated or bad catalytic converter.

What about Carbon Dioxide (CO2)?

CO2 is not a pollutant. CO2 is a byproduct of good combustion. CO2 is highest when the engine and catalytic converter are operating at maximum efficiency. The reason I bring this up is because the CAT can be a hidden cause for emissions failures in all categories. Testing of the cat would be the last step after looking at everything else. CO2 can only be measured with a four gas analyzer. Here is a fool proof method of testing CATs to see if they are capable of doing their job:

1. Warm the vehicle up to full operating temperature.

2. Shut the engine off and disable fuel and ignition systems.

3. Crank engine over at wide open throttle for 10 seconds, to purge fuel vapors from exhaust.

4. Insert analyzer probe into tailpipe.

5. Connect a propane enrichment device to a large manifold vacuum source, brake booster or PCV hose.

6. Turn propane half way on and crank engine injecting propane into the cylinders and then into the cat.

7. Note HC and CO2 readings after 10 seconds.

HC should be off the scale and CO2 should read 8 to 13%. If the CO2 reads less than spec, like under 5%, then the CAT is unable to convert HC to CO2. If the CAT is defective, replace it with a unit approved by EPA.

 

holy novel batman. lol. some people have helped themselves by running a cat for a vehicle that didnt have one from the factory. i think jd snake used one for a late 80s 4 cylinder mustang and got it to pass. might try that.

 

okay so i think i put the spring together wrong on this carb. the part that radiator fluid goes through to help the engine warm up had a spring in it that looks somewhat like a lawn mower pull string spring.. but i think i either over tightened it or under tightened it because the engine shuts off when the plate on the left side of the carb closes. any suggestions? im stumped.

 

yes I got it passed.

 

Ok, Brah-dah, here's your chance to shine.

What was the cause? How did you solve it?

 

in my quick stupidity i got the primary and secondary jets in the wrong ports so after two rebuilds i finally noticed my mistake. thank god. it runs like a brand new truck now with only 20k miles on the rebuilt engine.