305E Backfire out Carb and Exhaust

[boycec1]

My son and I recently bought a 1965 GMC C20 with a 305E. In an effort to troubleshoot a low compression issue on cylinder no. 1, I ended up adjusting the valve lash on no.1 and the rest of the cylinders. Turns out this didn't help no. 1 and it made things worse.

After the lash adjustment we now have a nasty backfire out of the right side exhaust and the carb. It idles fine but when I rev. it our try and drive the backfires begin.

I have double checked the lash many times and i confirmed that I have .012 on the intake and .018 on the exhaust. I checked this by manually turning the motor and waiting for the valve to fully close and the other one to start to open before I checked the lash.

If I pull the no. 6 plug wire the backfires go away so I am pretty sure the issue is with no. 6.

Do any of you guys have a suggestion on what I try next? The really frustrating thing is that we drove it around the block before we started down the valve adjustment road and it ran fine. I have been kicking myself all day as I am pretty sure I screwed something up but I can't figure out what.

[Jurassic-1]

Did you check to see if the rockers move up and down ? It could have a bad cam lobe. I do not know how common it is on GMC v6 motors but Chevrolet 305 and some 350s in the 70s had this problem.

[boycec1]

Wow, do you ever have one of those days when you are so focused on what you know the issue is that the over look something very obvious that could cause the same issue? Turns out, all I had to do was step away from the truck, get a beer, eat some food, and watch some football.

After doing all that I came to the realization that we must have mixed up the plug wires during all the fussing with the truck. Sure enough after setting the right order we are back to a running truck.

[Palf70Step]

I seem to have those days more than I want too.

Glad you got the thing figured out.

[Jurassic-1]

Been there done that. Glad the problem was a small one.

[TJ's Chevy]

Glad the problem was a small one....It reminds me of when I bought a new HEI for a 292 I6, I reinstalled the plug wires 180 degrees off....and I couldn't figure out why it would crank an crank an crank...then let out a tiny fire...then die...moves all the wires back a terminal...1/2 a crank an off we went. lol Funny things happen sometimes. All part of the fun.