Hydrocarbons.....

smog

Hi Mike,
Well HC is a product of a misfire, basicaly unburnt gas.
Check your plugs and wires, make sure the plugs aren't fuel fouled.
Make sure the engine runs smooth with no misfire.
If your engine has sat for a long time change the oil and give it a good long highway test drive right before your test.
Depending on the GVWR you might not have a catalytic convertor so that could be a problem if you do. Vince

also look for vacuum leaks as they will cause a slight misfire that will run the hc levels up. while you are at it, check the choke for proper operation and replace the airfilter.

What are the readings and what is the limit?

I used to have to get my off road bugs to pass a sniffer test in the old days. I don't know about hydro carbons at idle, but I used to lean out the carb mixture, retard the timing and loosen the carb to intake manifold bolts (to draw in more air). It idle like crap but it passed the smog test. In our old company vans, the mechanics had to make them pass the smog tests, so they did the same thing, retard the timing and lean out the carbs.

my66coupe said:

.....but they had the car roll onto these dyno rollers, and ran it up to a speed (65-70) where it blew hard enough for the computer to get a good reading.

Mike

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When I did those things, all that was required at the time was a sniffer wand shoved up the tail pipe, no rollers or dynos. I haven't gone through a smog test in a few years now because of my old 67 coupe and new truck which doesn't require a smog test till it's 5 years old. I just remember my car just barely being able to idle at the smog test station and after the test I literally drove it around the block and did a tune up by ear to get it to run well enough to get home.

Okay, just asked my son about this, he's an enviromental chemist, and he said that to reduce hydro carbon emissions (after a bunch of chemical mumbo jumbo of which I didn't understand anything he said) yes, introducing more air into the air/fuel mixture should help burn the hydro carbons more. He said that excessive hydro carbon readings indicate that the gas isn't being burnt completely, so more air may help. Good luck.

smog

Hi Mike,
Well the thing is you can mess with the timing but it still has to be with in a certain spec. I think its 8' BTDC on yours. Also don't go to lean or you will cause a lean misfire that will raise your HC levels. Your best bet is to take it to a tuner...you might have hard to diagnose issues that is causing the grief. ie carbon on the valves, intake leak, EVAP system, EGR valve or carb problems. Your not running an AFB still? Please tell me its not a thermo quad. Vince

I have had good luck with "guaranteed to pass". Comes in a white bottle, put it in, with a full tank, drive it till its at or below 1/4 and then take it to test.

Dave-

Some basics:
High CO is caused by rich air/fuel ratios, but still shows that the fuel is being burned. HC is raw fuel, i.e. the gas sitting in your tank is HC. NOx is how hot the fuel/air mixture is burning. All things equal, the engine with higher compression will have higher NOx readings. If CO gets too low (too lean) then you start misfiring and HC and NOx shoots up. Too rich and your CO shoots up until the point where your plugs start fouling. Fouling means misfire, misfire means unburnt fuel, HC readings go up and since fuel has a cooling effect, NOx goes down. If your NOx and CO are within reason something is preventing all your fuel from igniting.

With that out of the way...
More air won't help if it gets so lean that it causes a lean misfire. Misfires, even inaudible ones, will shoot your HC readings sky-high. 350ppm at idle? That's a tough one especially with no cats. 650ppm is pretty high and sounds like there is a vacuum leak and/or a slight misfire. Removing the air filter element may help but not much unless the filter is really clogged. I'd do a resistance check on the plug wires, clean the rotor/cap contacts (maybe replace altogether) and replace the plugs. If either electrodes on the plugs are rounded off in any way the spark won't be as intense, as sparks like to jump to and from sharp edges. Speaking of plugs, did you check the gap? Try a slightly smaller gap than stock to make it easier for the plug to fire, for example .045 instead of .051.

Try a fuel system cleaner such as the Chevron Techron in a bottle. Also, the chambers, valves, runners, and whatever else is in the intake path might have enough carbon deposits to soak fuel instead of burning it. Give it a few good blasts down the highway in a lower gear (3rd instead of overdrive). Don't give it full-throttle under-load runs as you might soak the carbon with more fuel and raise your HC readings.

Good luck. My 5.0 failed the idle test two years ago but since then it's been in the shop and painted. I'm going to re-smog it and we don't have idle tests anymore. I'm hoping it'll pass this time.

all i have to say is good thing i live in florida where we dont have to do those stupid tests...hhahaha

Loosening the carb bolts can make the problem a whole lot worse if it is a non-holley type carb, as the fuel can bypass the idle circuit and go directly into the manifold causing a rich situation. My new edelbrock had that problem, and after tightening flew through testing.

We have to test annually here and it can be a real game at times. Adding a litre of methyl hydrate to the fuel helps it burn more completely as well as gets rid of any water that may have condensed in the tank (which can also lead to poor combustion).

Just a little info here - With no cats, an edelbrock 600, headers, pertronix, and accel wires (basically nothing special) my hydrocarbon reading is 212 ppm out of an allowable 885 ppm at idle. CO is .14% out of an allowable 4.19%.

Earlier that day my hydrocarbons were pegged at 2000 ppm just because of the minor vacuum leak around the base of the carb.

krash kendall said:

Loosening the carb bolts can make the problem a whole lot worse if it is a non-holley type carb, as the fuel can bypass the idle circuit and go directly into the manifold causing a rich situation. My new edelbrock had that problem, and after tightening flew through testing.

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You most likely had a lean misfire.

No. If you turned the idle screws in the engine would idle happily along as if you did nothing as it was still getting plenty of fuel. After cinching it down (I have a 1" spacer which equals double the mating surfaces) you could stop the engine with the screws.

Sorry, I'm not saying it wasn't running lean at that point, I'm just saying that a poor seal at the carb gastket can cause raw fuel to enter the system unregulated.