Carbon canister on top of fuel tank Options

[356drb]

The fuel tank change occurred in April/May 1974. The carbon canister moved off the top of the fuel tank prior to this change. Was it at the end of calendar year 1973 or at the model change in the summer of 1973?

[wonkipop]

regarding above.

at first i thought there had to be errors in the manual.
contradictory information.

i did some research on 911 systems while i was trying to satisfy myself.
during the 70s 911s seemed to go through a similar evolution.
its virtually the same system, fan driven air is used to ventilate the cannister.
vw beetles used it as well, and i guess the rest of the air cooled vw models.
(i don't think water cooled cars used anything like this with a fan bleed pressure hose).

version 1 of the 911 was an air bleed line from fan into cannister with vapor line beside and line to aircleaner at opposite end.

version 2 was an air bleed line from fan into cannister and at opposite end vapor line in and line to aircleaner out.

version 3 deleted the air bleed line from the fan. system was truly closed.

1974 1.8 L set up is same as version 2 of 911.

made sense to me back in jan when i worked it out.
there is no valve (purge valve) in these early systems.
the part that is closing the system to the atmosphere are just charcoal filter particles.
(it is technically open to the atmosphere through the air bleed line from the fan),

in the earliest version they have the air bleed line directly next to vapor input line.
not sure what the cannister internals were like. but if they were in this arrangement then it would have meant a very short depth of charcoal particles for fumes to pass and escape via the air bleed line - especially during the phase when a car emits most vapor emissions, right after you have driven it for a while and then parked and switched the engine off.

the second version, as is definitely on the 74 1.8 means that any fuel vapors in that vapor line have to pass the full length of the charcoal cannister to escape out the fan air bleed line after a hot switch off. the full depth of the charcoal cannister contents is effectively the purge valve. the air bleed line to the fan is a weaker link for fumes to escape than the air cleaner intake line.

the fan link up via a bleed line peculiar i think to air cooled vws and porsches works to purge the cannister when your running the car. the longer and faster you run the car the more you purge the cannister. which might have been why they were progressively shortening the fan bleed hose throughout 1974? not sure.

there is a definitely a change in this cannister set up and hose layout design.
whether it occurs in 1974 or earlier i can't tell. but the 1974 1.8 L do not appear to share the hose set up into the cannister that much earlier cars used.