Emissions and Timing Question.

[68 Suburban]

Does advancing your timing, as long as you don't get detination, help reduce emissions?

[walker]

As far as I know the more you advance your timing the higher your emissions, although I can't remember for sure if it is HC or CO that goes up. If you don't pas the sniffer test then reset your timing to about 6* btdc or so.
BTW was that your Suburban at Poore Brothers, off of MC-85?

[68 Suburban]

Originally Posted by walker

As far as I know the more you advance your timing the higher your emissions, although I can't remember for sure if it is HC or CO that goes up. If you don't pas the sniffer test then reset your timing to about 6* btdc or so.
BTW was that your Suburban at Poore Brothers, off of MC-85?

Thanks for the info. No that was not my burb .

[68 Suburban]

Originally Posted by walker

As far as I know the more you advance your timing the higher your emissions, although I can't remember for sure if it is HC or CO that goes up. If you don't pas the sniffer test then reset your timing to about 6* btdc or so.
?

So I guess running the vacuum advance off the full ported port on the carb could increase emissions?

[walker]

I will eventually see your burb somewhere. I see so darn many of our trucks, I just keep looking and looking.
As far as the vaccum advance goes that is a whole new raging war! I run mine off the ported port. This provides higher vaccum as the engine revs higher. If you install it on the other port the theory is that you advance then is only on the lower RPM range and goes away at higher RPM. I have put a vaccum gauge on my motor and have found that not to be true, at least for a stockish motor(no huge cam). The vaccum at higher RPM would need to be equal to or less than idle vaccum for that advance to go away, and that is just not the case, again for a stockish motor. Even at WOT the manifold vaccum is higher than idle manifold vaccum. There is a dip when the throttle is first opened but then it climbs up. I run a 1405 edelbrock on a GM crate with stock heads. So you could do what I did and put on a vaccum guage and a timing light and play with it to see what your engine does. It is an interesting exercise if nothing else. I am sure that many other people have many other biases in this area, there will be those who swear on way and the other, I just call 'em like I see 'em.
As far as an emission test goes, I would check you total timing(initial+vaccum+mechanical{unless you are of the school that is connected to the manifold vac port and believes that vac adv goes away with higher RPM in which case it is initial+mech}) and see that is not much more than about 30* or so. Connect the timing light rev 'er up and see what the total timing advance goes to. Might stop in at Lopers(I think checker even has it now too, in their speed shop area) and pick up a timing tape to make it easy to read totlat timing.
If you want to play with it I also have one of those Color Tune thingys you could use too. Let me know.

[walker]

Another thing, if you are having trouble passing, post your numbers here. You will get some good advice, and have a pretty good idea what to do to pass. You will also hear to put in that alcohol, or gas drier before you go. Be careful with that advice when testing this time of the year, as it lower the boiling point of your fuel, leading to vapor lock. Run the lower grade of fuel to as it burns quicker/more complete, unless of course you need to run the higher grade due to high compression.

[68 Suburban]

I am just missing on the Idle HC. I rembered after I took this test, that after the last test I passed, I had switch to plugs with a 60 gap, instead of the 45 gap with my HEI. I wonder if this could have an effect? I just put some 45 gapped plugs back in.


Here are my numbers:

Load HC: 449
Load Standard: 450

Idle HC: 582
Idle Standard: 450

Load CO: 1.38
Load Standard 3.75

Idle CO: 0.08
Idle Standard: 5.00

[86SILVERADO]

Chris

just retard your timing as far as you can and you should be fine. Also use higher octane not lower. the higher the octane the more complete the combustion reaction not the other way around.

[walker]

Actually the high octane, which has more complete combustion at higher compression levels does not burn completly at lower compression levels. This is due to the fact that it has yummy chemicals in it to slow down the burn and inhibit preignition. Do a google under octane ratings, it will show a lot of discussion on the subject. Higher octane gas does not burn as fast.
You are right, '86, on the timing issue though.
Try this web site for some info on Octane http://chemistry.about.com/cs/howthi...a070401a_2.htm

[shortbed67]

It looks like you're slightly lean at idle.Very low CO will bring up HC.

[walker]

HC's are related to fuel delivery to the engine. Co is related to the engines capacity to burn the fuel. For high CO you would want to adjust your timing, plugs, wires, etc. For high HC you would want to check/adjust your floats, main/idle jets fuel pressure. You may want to check your float level, perhaps leave it a little on the low side. Depending on the carb you have you can also change your metering rods/jets.