Ignition timing effect on emissions

80Stang

80Stang

Well-known member
Sorry dumb questions but I never really learned this lesson before:

Q1: How does ignition timing affect on emissions, especially on O2 and HC?
Q2: Are they related?

What takes this on the table is that my engine failed on both these specs. High O2 probably is due to two small holes in my exhaust pipe welding (learning, learning...) and a bad clamp joint. These have been fixed now, so I assume O2 will now pass. Don't know if it affects the high HC though...

Ignition should alter HC, right? Which way I go first? I can go to a shop and tune it before I have to show it for inspection again. I'd just want to minimize time at the shop and maybe do things beforehand so it need no adjustments there.

Current figures are:
O2 about 8.20 (they allow 5.00)
HC 1138 (they allow 1000)

I've got Duraspark set at 17 degrees, centrifugals and vacuum advance don't add at idle (around 800rpm). Got Holley 350 with 61 jets, 0.25 nozzle...my A/F meter shows Stoich ratios all the time, maybe a little to the lean side. I can swap in smaller jets but 58s are too small for the application overall, might be good for inspection though.
 
Retard to reduce hc . Close to stock timing better for emission.
 
As mentioned. retard the timing will reduce the hydrocarbon emissions at idle. It leads to higher combustion temps. This is what the factories did in the mid-60's. They also went to ported vacuum sources to reduce the timing advance from engine vacuum at idle. In addition, they also started adding a second port on the vacuum advance motor that worked to retard the timing at idle when vacuum was higherst.
Never having had to do emission tests, I am a little surprised that they have a limit for oxygen.
Doug
 
Possibly he means CO, (carbon monoxide). Higher combustion temps will help this problem also. At higher temps the carbon molecule will join with two oxygen molecules, forming carbon dioxide, which is much more benign than its evil brother, carbon monoxide :twisted: The HC reading is from unburned hydrocarbons. A mis-firing plug will give really high HC readings, and will also allow excess O2 (oxygen) pass through. If this happens in an EFI engine that uses an O2 sensor it will sense the excess oxygen, which will cause the computer to send MORE fuel to correct the apparent "lean" condition.
Joe
 
HEY 80STANG

.....THE 8.2 IS TOO HIGH BECAUSE YOUR RUNNING RICH. DID YOU CHANGE THE JETS? IF SO PUT THE STOCK JETS BACK.

.....THE TIMING WILL RUN THE HC TOO HIGH. BUT IF YOU ARE 8.2 AND AT 17* THEN LOWERING THE TIMING BACK ...O2 WILL GO HIGHER.

.....YOU MIGHT LOOK FOR A LEAK IN THE CARB. IF THE JETS ARE STOCK. WEAK VACUUM WILL MAKE THE POWER VALVE TO OPEN AND YOU WILL RUN RICH ALL THE TIME.

.....I HAD A HOLLY YEAR 1983 AND THE POWER VALVE LEAKED. THE SHOP WANTED $150.00 TO TRY AND FIX THE CARB. OR $760.00 TO CHANGE THE CARB. I REPLACED THE POWER VALVE FOR LESS THAN $5.00. RETESTED AND IT PASS 50% BELOW LIMITS. MY NUMBERS HAD BEEN MUCH HIGHER THAN YOUR.

.....ALSO MINE WAS RICH ALL THE TIME . AT IDLE IT WAS VERY HIGH. THEY SAID IT WAS A GROSS POLLUTER AND WOULD TAKE IT OFF THE ROAD. I GOT ONE CHANCE TO MAKE IT RIGHT.

LIVE IN GRACE

LEROY POLL
 
Thanks fellas, way to go. Some notes/more info as an answer to your replys: vacuum advance is ported, and PV (7.5) should be fine (the carb was recently totally cleaned / rebuilt with all new parts; it is a '89 eco-model 350 with metering block from a 500 = yes it is a mix but it works fine). Stock jets....hmmmm, almost nothing is stock in this setup.

They measure 4 items at idle for a '80 vehicle. Here are all the results:

Item / Allowed / Measured
CO / 4.50 / 2.86
HC / 1000 / 1138
CO2 / all / 7.61
O2 / 5.00 / 8.29

I understand high O2 can also come from leaking exhaust pipes; that I already fixed.

Now I know I can get decent figures by
- retard timing to around 10-12
- check that PV is not open at idle, it shouldn't as the engine idles around 10-11 but retarding timing may lower that
- adjust the idle screws way in

If that won't do it I'll change to smaller jets, 58s...

I need to get this carb setup inspected just this time. Next year I'm running MS EFI. I'll let you know what comes out of the shop when I get there. Need to show good numbers by 22th day. Thanks.
 
I tend to like Joe's suggestion that it could be a bad plug or wire too. It sure would explain both a high HC and a high O2. I have a book on emission controls. Another things it says about high HC's is that it could be caused by super rich condition or a lean condition. It could be so lean that it causes misfiring, which would have the effect of giving a high HC & high O2 reading. So don't go too lean.
Doug
 
I went ahead and did the following:

- jets to 58
- swapped the PV to 6.5 unit. While I opened the carb there was fuel on the other side of the metering block too...which made me think if the bowl was not attached tight enough or the PV had died. Wsa111's LM-1 sessions inspired me too and I installed a .009 guitar string to restrict the 7448-500 metering blocks channels...
- ignition to 14* (my idle vacuum dropped to about 9)
- idle screws in, still runs decent idle
- brand new Motorcraft BSF82C plugs gapped .050

I don't really suspect faults with ignition stuff; everything there is new, installed this year. There is a small vacuum leak at the throttle shaft, it whistles a little.

This setup actually runs pretty well, very usable. I try to go to a shop in a few days to measure.
 
One thing you could do is something we would do on old carbed engines to get them thru emissions.......cheat. 8)

If you dump in a few liters of alcohol, the effect is to lean out the mixture. The car may run like crap, but the HC and CO emissions will drop dramatically.
 
Promised to report:

got to the shop before Christmas and it wasn't too easy to get it thru. As I screwed the idle screws in for a leaner mixture it dropped the CO to around 1100 and then started to go up again.

Finally we retarded to timing and had it idle at almost 1000rpm while it passed it pretty clearly. Unfortunately they took the printout from me at re-inspection and I didn't record the numbers...

Timing must have been very close to 0* as after the printout I advanced it back some (by hand, had no timing lights nearby). At home the day after I checked and it was 7*, then adjusted to 12* and now it is adjusted more (by ear once again) and maybe close to 17-18* initial.

I may have wrong bowl-to-body gasket in my carb combo, which might leak fuel directly to manifold. That is yet to be checked. I have MPG down about 10% from what it was during summer but that may be the difference between summer and winter (choke activity). But a leak like that might explain some richness while inspected and some other minor issues the engine is having.
 
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