Originally Posted by gr8ridn
Check to see if the check valve is installed between the "T" and atmosphere. Also the factory check valve at the rear of the BMW tank should have been removed. You have to be careful about overfilling the TT Tank. The cool stored gas from the underground tank will expand once it is filled into your tank. That check valve to atmosphere needs to have the arrow pointing in toward the tank so it restricts flow out but ready allows air flow in to equalize pressure in the tanks as fuel is consumed. Any of this wrong will result in pressure build up in the tanks. Done right the tanks remain equalized with atmosphere.
OK, finally got sufficient motivation and time to check on this. I see the "T" fitting, but there are two check valves, one between the "T" and the TT tank and the other between the "T" and the main tank, both pointing toward their respective tanks. The base of the "T" runs to what I'm assuming is a charcoal canister? Are you saying there should be one check valve between the "T" and the canister? Seems like the setup I've got is valid, unless the thing I want is for the tanks to equalize with each other...
(Btw, I do have the overfill tube issue where the fitting and airbox are fighting for the same space, and the LBS made a mess out of the equivalent tube on the main tank. But those tubes should only come into play right after filling the tank and spilling a bit around the filler neck, right?)
Never mind, I think I've found what the likely culprit is. The TT instructions say to run the vent line from the stock tank up front, and to put the T there, then run the line up and over the instrument cluster, which would provide a nice air gap I'm thinking. The only problem is that this bypasses the charcoal canister entirely. The LBS tried to keep the canister with a creative routing involving putting the T under the seat. I'll re-do that and the overfill lines tomorrow.