Smogtips
CHOOSE YOUR CITY OR ZIPCODE:
Within:
SELECT SMOG CHECK TYPE:  

Join the community and post your question. Use our Ask-A-Tech for free now! ASE experts standing by.

Forum > General Smog Check > Question

1976 Chevy K5 Blazer 350ci has excessive Carbon Monoxide (CO) on both idle an 2500rpm.

First smog test w/ professionally rebuilt carburetor. Idle Readings @ 1087RPM - %CO2=8.0 %O2=2.5 HC(PPM)MAX=300 GP=550 MEAS=321 CO(%)MAX=3.00 GP=5.50 MEAS=9.03 2500RPM Readings @ 2643RPM - %CO2=9.4 %O2=2.1 HC(PPM)MAX=350 GP=600 MEAS=228 CO(%)MAX=3.50 GP=6.00 MEAS=7.11 Where is my problem occuring? The GVWR is over 6000lbs. so there are no Catalytic converters. it has long tube headers with true dual 2 1/2in exhaust with 40series flowmaster mufflers. Im at a loss with what is wrong the carburetor was just rebuilt and has a fresh tune up with new ignition coil, cap, rotor, sparkplugs, & spark plug wires. Compression is no less than 160 in all cylinders. No vacuum leaks that I can find. The idle in kind of rough but I believe it has a mild cam.
Answer:

CO on your Chevy is way to high. Forget all other emissions components and focus on the carburetor. If you plan on doing this work yourself you'll be needing a tool to measure CO output from the tail pipe while you may adjustments to the carb. At the smog station they use the smog machine set to "manual 5 gas analyzer". Engine performance shops sometimes have smog machines, but most often use a self contained 5 gas analyzer. This repair can not be done without a tool indicating how your adjustments are effecting emissions, unless you want to run back and forth to the smog station with every correction you make, which is not feasible.

 

Let me explain the smog test results... (both idle and 2500rpm are based on the same principle) 9.03 Carbon Monoxide (CO) is a direct indication the carburetor is dumping too much fuel into the engine. As a result the hydrocarbons (HC) have increased. HC is basically the raw fuel which the engine was unable to burn at all, and CO is the fuel that was partially burned. Very simple, not brain surgery.

 

I think the engine in your Chevy is sound... You only need to get the carb perfectly tuned (I mention tuned and not rebuilt - since it's already rebuilt, right?) and that will get you to pass the smog check. Most vehicle owners think when they purchase or rebuild a carburetor it comes tuned or gets tuned during the rebuild. Problem is every car, truck, SUV, van and motorhome has it's unique thirst for fuel, and the rebuild shop or manufacturer knowing this, make only major adjustments to the part, leaving minor/fine adjustments to the installer. Though minuet, one size does not fit all, and 99% of the time you'll need to fine tune a rebuilt carburetor after the installation, and with an emissions measuring tool.


posted by SmogTips Support

 

View SmogTips.com in: Desktop / Mobile