I sure wouldn't say I'm an emissions 'guru', but I have had to battle this
for the last 20 years on my '69 2000 w/ Solex & 'b' cam.
In past years, I'd been able to make it pass by re-jetting the Solex's so
lean I nearly had to push it to the smog station, then change it back after
passing. I have tried retarding the timing, but this had minimal effect on
HC. To a certain point, (about 5 deg.) it did seem to help CO, but had very
little effect on HC.
I think that without the 'b' cam, it would pass easily without modification
or rejetting. WITH the 'b' cam, however, idle is just rough enough to
prevent complete combustion, resulting in high HC at idle.
What does help in all cases is handing the tester $40 and telling him to
slip the clutch just a little with trans in 1st (emergency brake on, of
course) and feather throttle to keep just below the rpm limit (whether the
'idle' setting or the 'high' rpm limit. The load always seems to help both
HC and CO. The testers I got this year, however, all turned down my 'offer'
to 'help'.
This year I failed miserably despite rejetting to what had worked in the
past. My Color-Tune showed intermittent spark, but not bad enough to keep
the car from running OK. I bought a Jacobs Pro-Street w/ Ultra Coil to go
with the GB distributer, increased plug gap to 50 thou, and now every little
bit of fuel has no choice but to burn - HC and CO both decreased
dramatically, and the car passed easily.
Recent Solvang trip from LV showed that my MPG increased from mid 20's to
low/med 30's. More power for sure, as well, and MUCH smoother idle.
In short, I never found retarding timing to have much effect on HC, and
actually had a negative effect if approaching 0 deg. - no doubt this would
not be the case if original air pump was still present. Jetting, on the
other hand, had a big effect.
I'm pretty sure that with the Jacobs unit, I'll not have to re-jet to pass
in the future.
Oh, one thing I did find out in years past, was to ignore folks suggestions
to use 'premium' gas instead of 'regular'. 91 octane has a higher flash
point to prevent pinging in high compression engines, and just exaserbated
the problems of unburned fuel (high HC readings) in my relatively low
compression U20.
John F
'69 2000 Solex
LV, NV
> Any smog gurus out there? Please, no guessing on the following.
> I need to know what really happens :-)
>
> What is the effect on unburned hydrocarbon emissions (HC)
> if the timing is retarded?
>
> Later (68+) cars were set a 0 degrees, making for poor performance
> but cleaner emissions. I had thought this dropped NOx, but some
> reading I was doing this weekend implies that retarding the timing will
> drop HC.
>
> So what happens at the smog machine if I back off from 16 degrees
> to say 4 degrees? Does the tailpipe clean up for the test?
>
> (for the astute: No, CA is not back to testing our cars)
>
> -- John
> John F Sandhoff sandhoff@csus.edu Sacramento, CA
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