Fuel tank evaporative emission system blockage orifice

Hello Fellow Jag Lovers
I am sure there is an intro post for new people and I will do that very soon but I wanted to post this while it is on my mind. I have a 1975 Jaguar XJ12C that I rescued from Cheyenne WY late last year. I’m really digging into it and had to replace the fuel tanks as they were collapsed and full of rust at the bottom. One new tank is in at the moment and will be removed so that I can replace the rubber hose from the evap separator in the B-pillar and utilize the EVAP system. That is the issue/solution I want to share about the evap system flow path.
I could not blow air through the EVAP system. There was a blockage somewhere and everywhere and that is why the tanks were collapsed by the fuel pump.
I have at this time found 4 orifices (orifi?). One for each filler cap and one each for the hose going to the hard tube that eventually leads to the charcoal canister. All orifices were approx 3/4-1 inch inside the rubber hose (red tape or red mark) and the first orifice I found (the really corroded one) I had no idea what it was. I used my small drill bits to clear out the intact orifices and came to the conclusion that the are about .035-.037.
So hopefully this post will help fellow TWIN TANK XJ owners who want to utilize the evap system.
I am still clearing out the whole system (replace dirty blocked hoses, rusted tubing) but am confident I will be able to get it flowing. I am sure the orifices were used to limit flow and not have a huge vacuum leak so I am thinking of using a single orifice near the charcoal cannister, that I can get to easily and service. I hope you all understand the photos and please ask any questions.
I’m here to help and learn.

1 Like

Welcome on board, Monte…

These orifi/restrictors are not specifically mentioned in relation to the EVAP, except for one in the hose connecting the charcoal canister to the manifold - so it is an interesting addition to ‘our’ knowledge, thanks! It may be some early version - later omitted?

In any case, very little air flow is required for tank venting; it is just to ensure that air is getting into the tanks to replace fuel used, or relieve tank pressure if fuel is expanding when heated. Which of course happens slowly - but if tanks are left unvented for whatever reason; tanks will inevitably collapse, as you notice…

As an aside; you should also find a valve in the hose and to vent airnear the canister, which opens at some 1,5 psi tank vacuum to vent air into the tanks - maintaining this tank vacuum when the engine is shut off…

Thanks again…

xj6 85 Sov Europe (UK/NZ)