Had an interesting conversation with an engine builder today. He said that most "68" original cast iron manifolds have cracks! He showed me pictures and the cracks develop in the webs right where the quadrajet mounts. It apparently causes rough idle when the engine warms up and other engine related symptoms. Anyone else on here can confirm? Never heard of this before.
x3 never heard. Iron manifold is a pretty tough customer. That doesn't mean it never happens, or couldn't happen. But why 68? And these cracks in cast iron are big or create an uneven surface enough to let in enough air to cause rough idle? I don't know about all that. Plus it would be a known thing here... like make sure you check for cracks.
Might be, the cracks I saw in the picture were hairline cracks, nothing real visible unless you focus on that area. I have a spare 4bbl manifold with a late 67 date code that I checked with a magnifying glass last night and sure as hell....one crack right by the secondary bores....I was on a GTO website last night and saw reports for 69 cast iron manifolds developing cracks and folks attempting to braze them. I don't know if a tiny crack really would make a difference on the car in terms of performance or idle as the carb base gasket goes there. My car runs fine, and I just rebuilt the carb, so I'm not going to take it back off to check for cracks.
The exhaust crossover does run right underneath the carburetor so it does make sense that thousands of heat cycles would cause some cracking eventually.
Alright wise guys, here ya go,lol. Not easy to take pic of it, but it IS there! I encourage you guys when feasible to take a close look at the Webs of your manifold, this is the 3rd one I see that has hairline cracks.
A crack in the casting between bores is of no real consequence. Only if it were to allow any outside air to enter below the carb mounting might it have any kind of effect on fuel mixture. Then it would amount to a vacuum leak. Don't think I've even seen that scenario in a stock iron intake.
I just would find it nearly impossible to believe that a very small crack in that location would make any difference at all. I even seen some heavily modified intakes... that made very little difference.
No air leaks possible there unless goes all way through bottom. Had one back in the 80's like that. Wouldn't of known if hadn't tried a Performer intake with Holley carb.. Ended up putting original intake and Q-jet back on cause ran better and faster.
