A few days back I posted about a friends car that was popping and backfiring through the exhaust after he had reinstalled the heads. (had leaking head studs) I like many on here suspected incorrectly adjusted valves because I know they didn't use the EO IC method. Well, I went out last night and gave them a hand. We first re-adjusted the valves and fired it up. It started right up but started popping through the exhaust. I felt the header tubes and #8 was cold. We suspected a bad wire or plug. Changed these and tried again. Still popping through the left bank exhaust. We were scratching our heads trying to figure out if #8 wasn't firing why would it pop through the other side. (no H pipe on exhaust)I was just going to pull #8 plug again when I noticed both vacuum lines at back of carb (booster and pcv were not connected. I put them on and started it again. It fired right up and ran like a dream. :confused:

Can someone explain how a major vacuum leak like this would cause 1 cylinder not to fire and pop through the exhaust on the other side? I am glad we got it but sure would like to know how and why

Does the intake have an exhaust crossover in the middle? If it also has an EGR valve it may have been not been working properly for a lack of vacuum signal.

With such a massive vacuum leak you were lucky to get it running at all! I honestly think you had a number of things going on at once so it's kind of hard to tell what was causing the popping to be just on one side. The number 8 cylinder was probably not firing due to the vacuum leak than a bad plug wire. Did you test the wire you removed? It probably wasn't bad in the first place. If I remember correctly, all things working properly #8 tends to run on the lean side so factor in a massive vacuum leak and it likely won't fire at all.

In other words, I have no freakin' idea. :D

Rich, the plug wire and plug were both good. We even swapped wires and plugs from other cylinders firing to check. I am pretty new to all this stuff and figured I was getting a good handle on it, but when I see a problem I like to know the reasoning behind it.My pea brain can't make any sense out of this.

A huge vacuum leak into the intake runner makes #8 too lean to fire. Also affects the cylinder on the other end of that runner, seems like #5 if I recall correctly.

Tom

I had a similar problem once when I plumbed the PCV hose into one of the runners, rather than a point near the carb. Leaned it out badly.