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davester Avatar
davester Dave Diamond
Berkeley, California, USA   USA
1965 Austin-Healey Sprite
1971 MG MGB GT "Dad's Car" ~ For Sale ! ~
In reply to # 3681050 by refisk Hi Bev,

US cars of that vintage have quite a system for capturing evaporative emissions. The crankcase, carb float bowls, and the fuel tank are all vented through a charcoal adsorbtion canister that captures hydrocarbons trying to escape into the atmosphere. If any part of the system fails the car doesn't run properly.

I always thought the system is rather silly because while it is very effective at capturing the tiny amount of emissions from the carbs and tank it does absolutely nothing for the emissions resulting from simply filling the fuel tank. If you have a seven gallon tank that is nearly empty and you fill it with fresh fuel you just pumped nearly seven gallons of very heavily hydrocarbon laden air back into the atmosphere. And yes, I know a few US states have vapor recovery systems. But most don't. Where I live the filling station tanks don't have a vapor recovery system either. When the tanker drops 5,000 gallons in the station tank 5,000 gallons of vapor go right into the atmosphere. Makes the stuff on our cars rather pointless.

Definitely some misconceptions in this post. First of all, the car will run fine unless a line is plugged. Second, the emissions from the carbs and tank constitute a very large fraction of the smog created by a car. "Tiny amount" is wrong. You are partially correct in that in areas without fuel pump vapor recovery systems pumping a gasoline tank full will put smog into the atmosphere. However, that periodic smog contribution is small compared to the 24/7 emissions from an unvented gas tank. You're also correct regarding the tanker. If there's no vapor recovery system then filling the gas station tanks produces smog. Note that rural areas with little traffic can allow some smog to be emitted from cars and gas stations because it is diluted to the point of not being noticeable by all the clean air in those locations. The reason for emission systems and regulations to exist is that more urban or developed areas cannot afford to allow unrestricted smog emissions without severely impacting public health and the ugliness created by large smog concentrations.

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