Classic cars forum & vehicle restoration.

[Ray White]

In this photo of a 1973 Triumph Stag engine bay it is clear that the air intake is separated from the grille. On closer inspection, the vacuum hose is missing from the flap motor. I am thinking this has been done for a purpose rather than just having been missed? .....but why?

My question is really more about how this emissions set up actually works?

I understand that there is a throttle operated spring loaded plunger in the side of the carb(s) that functions to open or close two ports (which I think may have something to do with float chamber ventilation?)

There is a vacuum pipe leading to a temperature sensor that works in conjunction with the aforementioned flap valve on the top of the air intake.

What I have never fully understood is how this emissions set up is supposed to work. If it operates as a kind of automatic choke (restricting or controlling air intake) then why is there also a manual choke fitted? ...or is that a later mod? I seem to remember these cars having a manual choke ...I simply can't remember.

Maybe someone who knows these cars better can remind me?

Thank you for your patience.

.

[ukdave2002]

Stags have a manual choke, the flap on the air intake is designed to keep the air intake temperature above 20 degrees, as you mention the vacuum pipe is missing, over time the diaphragm in the valve can leak, possibly why someone has removed it and presumably plugged the other end?

If its a US Stag, it may have additional emissions control, these may have included porting excess fuel fumes into something like a carbon canister, from both the tank and float chamber, this relates to the control on the float chambers that you mention, I have seen the mod on some UK built cars that went to the US, but honestly don't know if the Stag had them.

Incidentally the Stag was a massive failure in the US, with all the overheating issues that we had in the UK magnified many times over!

[Ray White]

Thank you Dave. You clearly explain how the operation works... but what I would like to know is how does the flap valve help with emissions?

Incidentally, I understand this is a U.K. car. I presume, therefore, that the float chambers are vented straight to the atmosphere?

[ukdave2002]

[alastairq]

[V8 Nutter]

My 1970 Cadillac has a similar arrangement. From a cold start a heat sensitive valve diverts vacuum to a valve that opens a door beneath the air cleaner that allows hot air from above the exhaust manifold into the engine to make warmup quicker and keep the emissions. As soon as the engine warms up the valve closes. At least that is the theory but after a few years it ceases to work, I don't think it made a great deal of difference. An MOT tester checked the emissions on the Cadillac just out of curiosity. No problems with the pre 1993 standards, and not quite scraping through the post 1994 standards. American emissions standards vary from state to state California being the strictest

[Ray White]

Some interesting replies. Thanks everyone.

I wonder why on this car the air intake has not been connected?

I can see how the flap motor might have been giving trouble and was perhaps ditched...but the air intake needs fresh air, surely?

[alastairq]

[ukdave2002]

[Ray White]

[ukdave2002]