TBI vacuum leak at manifold

[Eds]

I have a vacuum leak at the TBI adapter to intake manifold interface. Last night I replaced gasket and if anything made problem worse. I can spray carb cleaner in one specific spot at that interface and RPM dips noticeably (with IAC unplugged it will kill it).

my setup in a nutshell: 383 with vortec heads, GMPP vortec intake for q-jet spread bore, stock TBI to 4-barrel intake adapter from a 87-90 BBC TBI, 46 mm rebuilt and re-bushed TB, 454 74 lbs/hour injectors.

I’m using stock style gaskets for the BBC application both above and below the adapter. They’re both very thin, but I’m wondering if I should be using a traditional q-jet bas gasket which is much thicker? Any other suggestions?

Here’s a pic. The leak is right below the pipe plug, center front. Leak is not being caused by the plug - the adapter has coolant passages to improve cold weather operation; I’m not using that feature.

7DC004F9-92DA-4057-AD3F-42F5C02F62ED.jpg

You say the leak is not being caused by the pipe plug but did not say how you verified that. The adapter is hollow (coolant passage) and pipe plugs are tapered. I am wondering if the plug when tighten, is distorting the adapter? If you were to remove the pipe plug, do you note any changes?

[GreaseMonkey210]

Have you thought about trying a different adapter for your TBI. TransDapt, SPR Performance, Holley, and a few others make them. You might check and make sure the base on your adapter is flat and not warped a little bit or cracked and you can't see it. You can also run a raised base air cleaner should clear the distributor, at least it does on mine. Hope this helps.

[Daveo91]

Yeah, I switched to a different one tonight and it solved my problems. There's a link to the one I used in post #7. The reason I used the OEM one originally instead of this one is the OEM one has a vacuum port big enough for a brake booster. But I just used the one on the back of the manifold for brakes and tee'd into it for the small HVAC control lines. Also had to cut my own gasket - All the q-jet gaskets I found used the inside bolt pattern on the manifold, but the adapter uses the outside bolt pattern. And the gaskets I found that matched the bolt pattern were for other applications that didn't match the TBI bores or the manifold bearing surface.

[Daveo91]

You say the leak is not being caused by the pipe plug but did not say how you verified that. The adapter is hollow (coolant passage) and pipe plugs are tapered. I am wondering if the plug when tighten, is distorting the adapter? If you were to remove the pipe plug, do you note any changes?

I was using carb cleaner with a straw to direct the spray and it was pretty apparent exactly where the leak was at the interface between manifold and adapter. Possible that the pipe plug was causing distortion, but it was not in there tight and the other night I used a straight edge to check the bottom surface of adapter every which way and it was perfectly flat. The mystery of the leaking adapter may remain permanently unsolved - I switched to a different adapter tonight and problem solved.