72 350 emmissions help needed - The 1947 - Present Chevrolet & GMC Truck Message Board Network

72stik · 05-30-2002, 06:22 PM

My truck passed all safety (in NJ) requirements. The engine has 97,500 on the speedo. The engine looks completely original, as did the cab interior, so I believe the speedometer. The engine has new air filter, plugs, oil and filter, and runs and idles fine.
At inspection the truck registered 812 HC where 500 was passing.
It failed. My question is, what are the first or most obvious things to check, and what are typical readings, or readings that can be easily obtained assuming the motor is in good shape. I appreciate your help. Dave

superchevy · 05-30-2002, 06:56 PM

They sniff '72's in NJ?
Geez, Texas has some of the worst air in the country but we exempt anything 25+ years old from the sniff test.
Might be time for a carb rebuild.
SC

cableguy0 · 05-30-2002, 07:44 PM

heres what u need to do. when u get within a mile or so of the emissions station lean out the carb and retard the timing till it runs kinda sh!tty.go and u will probably pass then just put everything back the way it was b4 u went through

70shortnphat · 05-30-2002, 09:37 PM

Ditto what CableguyO said!Changing the oil right before you go can help a little too sometimes.

mikep · 05-31-2002, 12:18 AM

A little ethanol in the gas does wonders for HC levels. Ive run several cars through the sniffer after pouring a $12 bottle of everclear down the tank. In 3 gallons of gas a fifth of grain alcohol will cause even the worst HC emmitter to run clean as a whistle. It'll be running leaner than hell though so dont get on it much. Knock your timing up a few degrees too but not too far or you will increase your Nitrous Oxide levels too far.

truckster · 05-31-2002, 02:22 AM

You don't say what kind of emissions test it is, but if it's an idle speed test, you might start with adjusting your idle mixture.

I've done the same as MikeP with denatured alchohol (about $8-9 per gal at a hardware store). An approx. 5:1 mix is good. You might need to bump up the idle speed, since alchohol is less volatile than gas. You'll be amazed at what it does for your CO levels too. If you bump up the idle speed, you don't have to mess with the timing, which would tend to increase NOx emissions.

72stik · 06-01-2002, 07:49 AM

Thanks for the advice. A little everclear, lean out the carb some, set up the idle a bit and advance the timing one or two degrees.
then reset after inspection. It was an idle test. No NO test was performed or CO. Had no vis smoke. Have a great weekend, be at Carlsile 6/28,29

jhwkns · 06-01-2002, 09:36 AM

High HC at an idle is usually an indication of a lean mixture. before you do anything else. Make certain that you dont have any vacuum leaks. If no leaks are found. Set youy basic timing at TDC and mack off the mixture screws a little. It shouldn't take much to get you below 500. Good Luck. John

mikep · 06-01-2002, 11:41 AM

If all they are checking is HC at idle then by all means increase the hell out of the timing. That will nuke all the unburned hydrocarbons at idle. What a goofy test if all they are testing for is HC. I used to run emissions tests back in the Peoples Republik of Kalifornia in the early 80's and even then we had to run the full gammut of tests for NOx, CO , and HC.

05-30-2002, 06:22 PM	#1
72stik Registered User Join Date: May 2002 Location: Freehold ,NJ Posts: 20	72 350 emmissions help needed My truck passed all safety (in NJ) requirements. The engine has 97,500 on the speedo. The engine looks completely original, as did the cab interior, so I believe the speedometer. The engine has new air filter, plugs, oil and filter, and runs and idles fine. At inspection the truck registered 812 HC where 500 was passing. It failed. My question is, what are the first or most obvious things to check, and what are typical readings, or readings that can be easily obtained assuming the motor is in good shape. I appreciate your help. Dave __________________ 72 C 20 350/4 sp, Ebay bargain! What's left is nice!

05-30-2002, 06:56 PM	#2
superchevy Got big-block? Join Date: Jul 2000 Location: Austin, Texas Posts: 2,453	They sniff '72's in NJ? Geez, Texas has some of the worst air in the country but we exempt anything 25+ years old from the sniff test. Might be time for a carb rebuild. SC __________________ '68 CST SWB factory 396/T400 buddy buckets, a/c, ps, pb '72 Corvette convertible 454/4spd/ac, ps, pb '73 GMC Sierra Grande short wide 454/T400 ps, pb, ac '77 K5 Blazer black on red, 350/T350/NP205 ps, pb, ac all original '78 GMC Sierra Classic short wide 454/T400 ps, pb, ac '72 GMC Sprint SP454. 1 of 38 built. '73 GMC Sprint SP454. 1 of 71 built. '75 GMC Sprint SP454. 1 of 25 built. SOLD! '72 GMC K25 4x4 468/4L80e/NP205 build thread http://67-72chevytrucks.com/vboard/s...23#post4324423 SOLD!'55 Chevy gasser 496/4spd build thread http://67-72chevytrucks.com/vboard/s...96#post4324396

05-30-2002, 07:44 PM	#3
cableguy0 Senior Member Join Date: Apr 2000 Location: Delta,Pa Posts: 14,948	heres what u need to do. when u get within a mile or so of the emissions station lean out the carb and retard the timing till it runs kinda sh!tty.go and u will probably pass then just put everything back the way it was b4 u went through __________________ Owner of North Point Car Care in Dundalk Md. We specialize in custom exhaust on both modern and classic vehicles. We are a full service auto shop from classics to modern vehicles. Feel free to contact me with questions. I will give a 10% discount to any board member.

05-30-2002, 09:37 PM	#4
70shortnphat "See the U.S.A....." Join Date: Apr 2002 Location: Richmond TX. Posts: 269	Ditto what CableguyO said!Changing the oil right before you go can help a little too sometimes. __________________ 1970 short bed fleetside 1949 all origional chevy 3600 1974 camaro Richmond , Texas(just S.W.of Houston)

05-31-2002, 12:18 AM	#5
mikep Used to have a truck Join Date: May 2002 Location: port orchard WA Posts: 1,552	A little ethanol in the gas does wonders for HC levels. Ive run several cars through the sniffer after pouring a $12 bottle of everclear down the tank. In 3 gallons of gas a fifth of grain alcohol will cause even the worst HC emmitter to run clean as a whistle. It'll be running leaner than hell though so dont get on it much. Knock your timing up a few degrees too but not too far or you will increase your Nitrous Oxide levels too far. __________________ No truck :-(

05-31-2002, 02:22 AM	#6
truckster Registered User Join Date: Jan 2001 Location: Orem, Utah Posts: 7,998	You don't say what kind of emissions test it is, but if it's an idle speed test, you might start with adjusting your idle mixture. I've done the same as MikeP with denatured alchohol (about $8-9 per gal at a hardware store). An approx. 5:1 mix is good. You might need to bump up the idle speed, since alchohol is less volatile than gas. You'll be amazed at what it does for your CO levels too. If you bump up the idle speed, you don't have to mess with the timing, which would tend to increase NOx emissions. __________________ I have sworn upon the altar of God, eternal hostility against every form of tyranny over the mind of man. - Thomas Jefferson Last edited by truckster; 05-31-2002 at 02:25 AM.

06-01-2002, 07:49 AM	#7
72stik Registered User Join Date: May 2002 Location: Freehold ,NJ Posts: 20	emmissions tests Thanks for the advice. A little everclear, lean out the carb some, set up the idle a bit and advance the timing one or two degrees. then reset after inspection. It was an idle test. No NO test was performed or CO. Had no vis smoke. Have a great weekend, be at Carlsile 6/28,29 __________________ 72 C 20 350/4 sp, Ebay bargain! What's left is nice!

06-01-2002, 09:36 AM	#8
jhwkns Registered Cruiser Join Date: May 2002 Location: Las Vegas NV Posts: 2,230	High HC at an idle is usually an indication of a lean mixture. before you do anything else. Make certain that you dont have any vacuum leaks. If no leaks are found. Set youy basic timing at TDC and mack off the mixture screws a little. It shouldn't take much to get you below 500. Good Luck. John __________________ I intend to live forever. So far, so good.

06-01-2002, 11:41 AM	#9
mikep Used to have a truck Join Date: May 2002 Location: port orchard WA Posts: 1,552	If all they are checking is HC at idle then by all means increase the hell out of the timing. That will nuke all the unburned hydrocarbons at idle. What a goofy test if all they are testing for is HC. I used to run emissions tests back in the Peoples Republik of Kalifornia in the early 80's and even then we had to run the full gammut of tests for NOx, CO , and HC. __________________ No truck :-(



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