[77ChevyBigTen]

I have a 1977 Chevy Big-Ten pickup with a 305(.030 over) Rochester 4bbl and have a problem with it running rich at idle. The New Jersey inspection requires a 300HC level. The first time i went my level was at 4768HC, I have lowered it to 1902HC but need help with the rest. The test is ran around 1000RPMS.

I have fully rebuilt the engine as a high school project. The cam is a 224/224Dur .465/.465lift 114 degree cam and the carb has been rebuilt. The carb is from the original H.D. emission 350.

I checked for vacuum leaks, adjusted the time and the carb to where the air/fuel mixture is bottomed out but still runs. I have heard but have not tried yet to put in hotter spark plugs and adjust the fuel float level. I am young and still learning so any input would be appreciated. Ask if you need any additional info. Thank you

[smileys35]

What size carb do you have? Rule of thumb is one cfm for every horsepower. A recent study that was done by demon carbs ran a 500, a 650, and a 750, and the 750 only showed 8 horsepower difference from the 500 at 4500 rpms.
If it has a side site glass on it or a side screw to view adjustment, adjust the float level to were the fuel is just starting to run out of the hole, any more is too much. If it is a old carb, you may need to rebuild it or just change it. A new 600 holley is only about 200 dollars. Also check your timing, the old fashioned way, warm it up and move it around to it runs the smoothest. Shut it off and try to restart it, it should start right up and turn over quickly, if it turns over slowly, back the timing off, towards the driver side while standing in front of the vehicle. If this doesn't work check and make sure you have good ignition spark, maybe weak. Hope this helps.

[RHamm1320]

One thing that will help the HC is back the timing down a few degrees. It makes the engine run hotter. Beyond that, I don't know why it helps, but it does

[driveability]

Originally Posted by 77ChevyBigTen

I have a 1977 Chevy Big-Ten pickup with a 305(.030 over) Rochester 4bbl and have a problem with it running rich at idle. The New Jersey inspection requires a 300HC level. The first time i went my level was at 4768HC, I have lowered it to 1902HC but need help with the rest. The test is ran around 1000RPMS.

I have fully rebuilt the engine as a high school project. The cam is a 224/224Dur .465/.465lift 114 degree cam and the carb has been rebuilt. The carb is from the original H.D. emission 350.

I checked for vacuum leaks, adjusted the time and the carb to where the air/fuel mixture is bottomed out but still runs. I have heard but have not tried yet to put in hotter spark plugs and adjust the fuel float level. I am young and still learning so any input would be appreciated. Ask if you need any additional info. Thank you

When we are talking about HC we are not normally rich. The CO is much more a Lean/Rich indicator. For example, if you were to turn in your idle mixture screws in until the engine starts to run at a little slower and unstable idle you would surely be raising the HC in the exhaust stream. There is a balance needed when adjusting the idle screws, I surely have found turning in the idle adjustment jets will lean out the mixture thus normally raising the HC in the exhaust stream. I might look at moving them out a bit. If you have a tachometer set the base idle and adjust the idle adjustment jets/screws to get the fastest idle increase with still having a smooth idle, then screw them in until the engine just starts to loose rpm. A good drop is around 50 to 75 rpm. That way your in the ballpark,not to rich or lean. Again leaning out the mixture will raise the HC because of the presence of a "lean misfire" along with higher HC you will also have higher NOX readings with a lean mixture. If you get close but still fail you can always replace the cat-converter that will always drop the HC a heck of a lot. Its a guessing game so good luck.. John

[77ChevyBigTen]

Being a H.D. half ton in 77 i don't have cats and i have true dual exhaust but i have been thinking of putting cats on just to pass inspection. But i will try adjusting the carb. again. The standard CO% is 3 and my truck is running at 0.60

[driveability]

Originally Posted by 77ChevyBigTen

Being a H.D. half ton in 77 i don't have cats and i have true dual exhaust but i have been thinking of putting cats on just to pass inspection. But i will try adjusting the carb. again. The standard CO% is 3 and my truck is running at 0.60

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High hydrocarbon (HC) readings usually indicate excessive unburned fuel caused by a lack of ignition or by incomplete combustion. Concentrations are measured in parts per million (PPM). Common causes include a faulty ignition system, vacuum leaks, and fuel mixture problems. Circumstances that can lead to a high HC emissions are:

* Incomplete combustion due to fouled spark plugs.

* Improper timing or dwell

* Damaged ignition wires

* Poor compression

* Vacuum leak

* Ineffective or faulty air management system (ECM control of air/fuel ratios)


Thanks for the heads up on the smog equipment, given the Co is so low it just may help to move it up. A lean mixture can cause lean misfires and that will make the CO go through the roof. Surely a misfire at idle will as well. If they are testing it at 1k you may have timing issues as well, maybe the timing is too advanced at that rpm causing a misfire. If it advances too far you can get a misfire and surely that will kill your readings. The mixture being a bit more rich (higher Co) will normally help lower the HC.