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I'd have to agree, it's like trying to put a diaper on an elephant.
Although you could make it a lot cleaner.
1) Fuel injection - Get D-jet hardware, and Megasquirt. Install it with (wideband?) O2 sensor feedback. Tune it very carefully, with emissions in mind, not power. MS hardware probably $400-ish, another $200 for a wideband, junkyard/eBay trolling for D-jet parts (head, intake, injectors, at elast some parts of the fuel system), lots of time and effort to put it in, make it work, and tune it.
2) Put a catalytic convertor on. Carbs are generally too erratic to proerly feed and care for a cat, but well tuned FI can do it. Cats can be fairly cheap, $100 - 150? But if the car isn't in pretty good tune when you put it on, it will die quickly. Either burnt out or clogged up.
3) Evap control systems, older Volvos (pre '68???) just have passively vented gas tanks. Probably one of the larger, unseen sources of HC emissions from a carb. Even if you replace the vented fuel bowl carbs on the motor with sealed fuel injection, the tank will still breathe a certain amount of HC out as the temps cycle. Not sure how hard it would be to retrofit something effective on an older car. My wife's '96 4Runner is a Cali emissions model, and when you shut it off, it actually has an electric pump which (some slight speculation follows here) runs for a while after shutdown to suck air out of the intake, presumably running it through a carbon filter to trap an unused gas that was injected right before shutdown. Makes a sort of frog-like periodic noise for an hour or two.
4) A tight engine with no leaking valves, no leaking rings, etc, is important. But there are some design considerations that will tend to make the old Volvo motors a little dirtier, even in the best circumstances. Modern engines are just designed from scratch to minimize emissions, since that is such an important part of certifying a new car these days. Combustion chambers have been optimized to burn lean. Even more subtle design tweaks make a difference. Like the piston rings, the Volvo's are further down from the top edge of the piston, which makes more of a little niche where unburnt HC's can hide out, waiting to sneak out the exhaust. Newer cars have the rings higher up, to minimize this.
5) Another sort of, ahem, HC emmission old Volvo's are known for is oil leaks. All those little drips add up to a significant environmental blight when rainwater washes it into the local streams. And if you have an earlier car, you probably have a crankcase dump tube which dumps all the crankcase blowby (unburnt mixture that squeezed past the rings, exhaust, oil vapor) out into the open air, def. convert that to a later style which recycles that back into the intake for another try.
Really, in the end, if you are truly concerned about the environment and your impact on it, it's a nice thought to keep an older car going perpetually instead of buying a new one periodically. It's just that the technology has come *SO* far in the last 10 years that the only truly environmentally friendly thing is to do that with a MODERN small car. One that will get better mileage, produce a tiny fraction of the emissions, and (at the risk of offending anyone) be safer to drive as well. And faster. And handle better. All you give up is style.
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'63 PV544 rat rod, '93 Classic #1141 245 +t
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