Paulz Posted April 16, 2018 Report Share Posted April 16, 2018 Still trying to get my 1980 720 to pass a smog test. I have use of the sniffer at my college. Hydrocarbons way too high at idle, 900, needs to be under 200. CO good at .25, NOx good at 80. Everything good at high speed. Causes for high HC supposed to include improper timing, defective ignition components, lean condition, low compression, bad cat. Runs great on the road, zippy when you open the secondary. Compression equal, don't know about the cat, looks old. Pulls 17 inches vacuum at idle. I have all vacuum driven smog components blocked off right now. The vacuum advance can is bad so that's blocked off too. Idle mixture screw works but idle is rough. Oh and occasionally it smoke blue at idle, then quits completely, like flipping a switch. PCV valve checks out. What to do, besides move? 1 Quote Link to comment
Dolomite Posted April 16, 2018 Report Share Posted April 16, 2018 Fix vacuum components, but could definitely be the 37 year old cat. 4 Quote Link to comment
Charlie69 Posted April 16, 2018 Report Share Posted April 16, 2018 On my 86 750 Z24 I clean the EGR vale and install with new gasket verify the egr is working. If I fail I install a new oxygen sensor and catalytic converter. I am running a Weber carb so I can not help you on the stock carb. In general you are too lean at idle so you need to screw the idle jet screw out until it stumbles (runs ruff) put the sniffer back on it and see how your numbers are. Also check for vacum leaks these will cause lean condition at idle. Try running premium gas higher octane. How old is distributer cap rotor plugs and wires? Quote Link to comment
Crashtd420 Posted April 16, 2018 Report Share Posted April 16, 2018 I have all vacuum driven smog components blocked off right now. The vacuum advance can is bad so that's blocked off too. How much does this statement effect things.... ? You state you can't pass smog, yet you deleted smog components... Would these deleted components cause the issue your having? Quote Link to comment
Toxicrain Posted April 16, 2018 Report Share Posted April 16, 2018 I believe your issue is the cat as well. My truck when the PO did the smog he failed 3-4 times and then put new ebay cat on it and it passed with vaccum leaks and ignition wires out of place. it had stock carb at the time with all smog components still hooked up. Your idle mixture is messed up? your carb might be running rich at idle and I would suggest checking fuel pressure and your float as it might be dumping gas in at idle. and I would reset the screw by all the way in and 2 turns out and start the truck and adjust accordingly on cold/warm engine I dont remember... to the leanest possible until the motor bogs and back it out till it smooths out etc... Those smog components are probably what you need ideally but you should be able to work it out without them if you tune the motor down enough just long enough to pass. if all fails tac 3 cats on it and hope they dont look at the exhaust lol or move outta state. or into mariposa county (no smog there idk why california is a broken state) Quote Link to comment
Roadster-ka Posted April 16, 2018 Report Share Posted April 16, 2018 "Idle rough" Misfire? Miss will cause high hc. Pull plug wires to determine cylinder. Quote Link to comment
flatcat19 Posted April 16, 2018 Report Share Posted April 16, 2018 High Hc is rich. Dumping gas. Fix the fuel. Fix ignition. Since the rest of your readings are good, don't worry about the other things. Cats good. EGR good. If you're at school...what is your teacher not doing to help you? 1 Quote Link to comment
noramost Posted April 17, 2018 Report Share Posted April 17, 2018 open up your mixture screw on the carb to 1.5 and your HC will drop, you are running to lean. I am an emission guy and own my own Sun 5 gas analyzer 1 Quote Link to comment
flatcat19 Posted April 17, 2018 Report Share Posted April 17, 2018 Other things to check... Check for vacuum leaks. Ignition components (cap, rotor, plugs wires.) Clogged air filter. Hell, how's the oil? Quote Link to comment
Paulz Posted April 17, 2018 Author Report Share Posted April 17, 2018 Thanks guys. I have found one thing: I held an unlit propane torch around the carb and at the base that faces the engine, idle speed increased. Sprayed some ether there, same thing. Pulled the carb thinking I'd find something but the gaskets and the insulator block look fine. Had to quit for the day but there is a vacuum leak there somewhere. It's not the throttle shafts, they are tight. My original post was confusing. When I first got this truck it wouldn't idle at all and vacuum was way low. By disconnecting some of the vacuum driven smog components I was able to raise the vacuum and get an idle, albeit rough. The diaphragms in those components, like my advance can, are shot. If I can get it to idle smooth and lower the HC then I'll start reconnecting the smog stuff. I'm not in the smog license class at the college, the teacher just showed me how to use the sniffer in manual mode. I'm taking auto body.. I'll try and get to the root of this carb base leak tomorrow and report back. Thanks again. Edit: Flatcat just saw your post. Since it runs well at high speeds and under load I have not suspected ignition components but I will have a look. Quote Link to comment
datzenmike Posted April 17, 2018 Report Share Posted April 17, 2018 Still trying to get my 1980 720 to pass a smog test. I have use of the sniffer at my college. Hydrocarbons way too high at idle, 900, needs to be under 200. CO good at .25, NOx good at 80. Everything good at high speed. Causes for high HC supposed to include improper timing, defective ignition components, lean condition, low compression, bad cat. Runs great on the road, zippy when you open the secondary. Compression equal, don't know about the cat, looks old. Pulls 17 inches vacuum at idle. I have all vacuum driven smog components blocked off right now. The vacuum advance can is bad so that's blocked off too. Idle mixture screw works but idle is rough. Oh and occasionally it smoke blue at idle, then quits completely, like flipping a switch. PCV valve checks out. What to do, besides move? Causes of running too rich or unburned gas... If the carb was rebuilt the primary jet could have been swapped with the secondary which is much larger. Idle mixture adjustment is way out. Choke not shutting off when engine is warmed up. Should move to completely open in under 10 min. May be stuck. Maybe choke heater not working or the relay for this. Spray all choke moving parts and linkages with WD-40. Engine off, hold throttle half open and open and close choke flap several times. Float level incorrect and fuel level in float chamber too high. Look at sight glass on front of carburetor. Fuel should be at the dot. May be flooding from dirty inlet needle... can't close. Poor ignition performance. Clean or replace plugs, wires, cap or rotor as needed. Set timing to 12 degrees BTDC or what ever it is in California. Air pump not supplying enough air so catalytic converter can burn the gas fumes. Catalytic converter worn out. Other... Set valve lash. Tight valves can cause loss of intake vacuum. Set correct idle speed. Take distributor cap off and suck on vacuum hose. Rotor should turn clockwise about an inch and should hold until you let go. Check the hose lengthe for cracks and splits and that it goes to the correct vacuum port on the carb and not the one for the EGR.* Clean EGR valve with a wire brush. If crusty with exhaust deposits and does not close perfectly at idle you will not only loose intake vacuum but will run lean and idle poorly. When idling pinch or remove and plug the vacuum hose to the power brake booster. Does the idle improve? * Both the vacuum advance and the EGR are ports above the throttle plate and have zero effect on intake manifold vacuum if the hoses are leaking. At idle there is no vacuum signal but as the throttle plate rises to and above them they are exposed to intake vacuum but any air entering has to pass by the venturies so no effect. Naturally a split or loose hose will delay or prevent the EGR or the vacuum advance from working. Quote Link to comment
noramost Posted April 17, 2018 Report Share Posted April 17, 2018 not running rich with a .25 CO, it is running lean at idle only; causing HC too go high @900. Open the mixture screw 1/2 turn to 1.5 CO and report back if HC drops enough to pass, if not give me new reading might have second issue. I speak fluent HC & CO, even Lambda and Air Fuel Ratio Quote Link to comment
Paulz Posted April 17, 2018 Author Report Share Posted April 17, 2018 Thanks again. I'm away until Thursday, I'll get the carb back on and report back. Quote Link to comment
Paulz Posted April 19, 2018 Author Report Share Posted April 19, 2018 I'm back. I put the carb back on today, ran it some more and believe I've discovered the vacuum leak is at the throttle shaft. Here is a video. That's starting fluid I'm spraying. The idle mixture screw does work, it almost stalls out when all the way in and picks up at a turn or so out but from there on out stays the same. Vacuum is about 16 at rough idle and 20 when I spray the shaft area. The funky sound is the unhooked smog pump valve but it does well do show the stumble. Quote Link to comment
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