Inline 6 Question: AIR vs. Non-Air Timing

[bluec10]

Having a tough time setting the timing for my '69/L6/manual. The truck came from the factory with the AIR pollution control. I've removed the AIR system, mostly to install a different alternator.

Anyway, the timing sticker states that timing should be set at 0 degrees. If I look in the shop manual it says that trucks with the AIR pollution control should be set at 0 degrees and trucks without it should be set at 4 degrees BTDC. Having removed the AIR pump, should I stick with the 0 degrees recommendation or go with 4 BTDC?

[geezer#99]

Not even close!!
Set your initial at 14 degrees.

[88Ironduke]

I might be wrong here but I didn't think pollution control came until later on in the 70s?

[CST10]

Originally Posted by 88Ironduke

I might be wrong here but I didn't think pollution control came until later on in the 70s?

Nope. My all original 68 had it on it too.

[Steeveedee]

Originally Posted by bluec10

Having a tough time setting the timing for my '69/L6/manual. The truck came from the factory with the AIR pollution control. I've removed the AIR system, mostly to install a different alternator.

Anyway, the timing sticker states that timing should be set at 0 degrees. If I look in the shop manual it says that trucks with the AIR pollution control should be set at 0 degrees and trucks without it should be set at 4 degrees BTDC. Having removed the AIR pump, should I stick with the 0 degrees recommendation or go with 4 BTDC?

If you want to set the timing to something other than the factory setting, the following applies-

1. AIR systems were used to reduce Carbon Monoxide emissions induced by rich mixtures by introducing fresh air into the exhaust during warmup, when the choke is partially closed.

2. AIR systems are usually set to a later timing value because the localities where the AIR system is used also have an interest in reducing oxides of nitrogen, which are reduced with less timing advance.

3. The distributor used for a truck with an AIR system may compensate (or not, I don't know the actual answer here) for the lack of initial timing by using a vacuum and/or mechanical that ends up giving back that advance. My suspicion is probably not, as total timing, if it is more advanced, will yield more oxides of nitrogen.

4. If you live in a region where the emissions of your truck will not be measured on a biennial basis, you can do what you please, as long as it doesn't destroy the engine.

Retarded timing obviously would be a performance/mileage issue. You should check out what your full timing advance is. This requires a timing light that allows you to dial back the indicated timing so that you can see what that total timing advance is. Once you know that, you can decide what you want to do. Over-advanced timing can lead to detonation and piston and bearing destruction.

Your best bet would be to look around the internet for a distributor for a non-AIR truck. That at least will give you the mechanical and vacuum advance that fit the parameters that your engine was expected to operate within. Note that there are likely internal differences, as well, like the cam and pistons (and thus compression ratio), that can affect how that distributor change will affect how you truck runs.

Also, emission controls began to appear in 1964, with the advent of the closed crankcase system, which eliminated the road draft tube. I was a SMOG mechanic, many a moon ago. When I graduated university in 1983, I went to work as an engineer, and any information that I may impart will be from that era.

[Steeveedee]

Originally Posted by geezer#99

Not even close!!
Set your initial at 14 degrees.

That may work in chilblain country, but if the OP is in a hotter climate, this could prove deleterious to his engine due to detonation from over advanced timing.

['68OrangeSunshine]

I don't think the distributor was different for A.I.R. models. You want to make sure all the holes in the head [6] are capped as that could give you a significant manifold [seeming] leak.