I realize this has been discussed in multiple threads. Thought I'd revisit this because I had the problem again and figured it out. My setup had changed since this last happened. This time it was a problem with the BBK 52MM TB. A little backstory. My 87 ran a 32B mask and was an auto. Now it's a T5 running a 6E mask. That means the EVAP system tests differently and the purge parameters are also different from the 32B ABWT BIN. I'm running a tuned 6E APYP 1989 Corvette BIN.

I've also since done more engine mods. Aluminum heads. MSD, full roller 1.6 rockers, Lingenfelter 74211 cam, Beehive valve springs, 3" catback to a magnaflow parked under the tank. And the 52MM BBK. I thought all was well. Always had some pressure in the tank before and after mods. The last time I had this issue was a few years back. Turned out the nylon vacuum line was burned and welded close and the EVAP can was saturated. Rectified that and it seemed to be okay.

Fast forward to today with all the new mods. This is the first spring/summer that it's all working together since the new parts. Couple days ago went for a drive and almost blew my face off when I opened the tank. The giveaway was all the banging and popping in the tank and the fact the fuel pump sounded like a wood chipper. It's a racetronix. So down the rabbit hole I went. Started looking at the BIN. Made some changes to the purge conditions. Everything seemed to test fine. Went for a drive with live data and a light bulb tee'd into the purge solenoid circuit to see what the purge solenoid was actually doing. It all seemed to be working fine, until....

The BBK 52mm. This was the problem. The EVAP is supposed to use ported vacuum off the TB. Per the manual the system should get 15Hg of vacuum. The BBK Port on the bottom of the TB had 0. At most 0.5Hg. it actually was producing around 0.5 positive PSI upon throttle cracks. I removed it and had a look. It's a joke of a setup. That first pic is the stock TB. That large machined groove is the ported vacuum source. The BBK has that small hole under the throttle blade. Maybe 1/4 the size of the stock TB, throttle blade hole. The BBK Port hole is about the size of a sewing needle.That is not going to provide 15Hg to two valves. There's one between the tank and canister and one on the canister splitting that vacuum source. I switched to manifold vacuum and no longer have a fuel tank pressure issue. What a nice waste of time that was. Not sure how BBK managed to eF that up.

 

There's another big difference between stock and the BBK. The BBK bleed port is on the bottom, around the 4 o'clock position when looking at the TB. It's also right at the Blade closed point. When the blade opens it goes backwards and the port is now in front of the blade. GM's is at the 12 o'clock position and we'll back of the throttle blade. The blade opens forward and away from the port. I believe the BBK's smaller port is in a bad spot. Possibly very turbulent as I noted a pressure bump upon throttle crack. Not a vacuum. That should not be happening. Just wish I could test the stock one and see what it's doing.