Hi folks, I'm new here. I hope to get some great networking here.
I just bought a '75 K5 Blazer. I drove it for about a week and it ran great, then it started getting a popping or backfire through the carb. It idles fine. When I blip the throttle hard or run it at 2500rpm and above it starts popping constantly. This is a reman 5.7l with a 1406 Eldelbrock 4bbl.
What I've done:
Replaced timing gears and chain.
New plugs
New cap, rotor and coil.
I've checked the compression. 150psi across the board.
Vacuum reads 15+ inches at idle. At 2500rpm the needle flutters a little.
I looked at it at night and no light show.
I have taken one plug wire off at a time to see if it was a particular cylinder, but it still popped.
Any ideas?

 

Welcome to the site.

Its been a while since I worked on a carb'd engine.
However you seem to have covered the basics, timing and ignition.
Is your distributor vacuum advanced?
Still got points?

 

sounds like a bad camshaft or timing out of whack....... take off the valve covers and watch the lift of the rockers, you can put your finger on them one by one and your finger should go crazy with the movement, if theres one thats not moving fast then i say theres a round lobe on the cam... theres a chance that when the motor was reman the rockers were tightened too much and rounded the cam lobes

 

Sounds like a lean condition missfire.
As 2002XTREME says, possible camshaft. It could also be as simple as the carb is to lean either by the air/fuel adjustment or the jets are to small...or a weak fuel pump or plugged fuel filter.

 

Carburetor shouldn't be the problem as this condition just developed. I'll check pump and filter. Distributor is vacuum advanced and does not have points. I'll check the valve lift and see if the cam is the problem.

 

Well sure enough. #2 exhaust valve is not opening all the way. Geez, not how I wanted to spend my weekend.

 

That's not good. Hopefully it is just a collapsed lifter and not a wasted cam lobe...

 

Ahhh! I pulled the intake off, removed the push rods, then rotated the motor. Sure enough, the lifter was not coming up as high as the other exhaust lifters. I continued to pull out the cam and now I'm buffaloed. The lobe on the cam is not worn. So I stuck it back in to make sure I saw what I saw. The lifter is not coming up as much as the other exhaust lifters. The #2 lifter is the exact same height as the others. I'm missing something, but don't know what.

 

Good Lord, I am such a dummy. I was driving to the parts store to look at a new cam, when I realized that the cylinders are offset and I was looking at the wrong lobes on the camshaft. The lobe for #2 cylinder was worn so smooth and round that I thought it was a bearing surface. I'm such a rookie, a lesson very well learned. Got all my new parts and will be driving with the top down by tomorrow. The parts guy sold me oil for a diesel for the break-in. Sound right?
Thanks for all of your help.

 

Well, seems you found your problem! Hope the reassembly goes well!

The oil will work. Use plenty of assembly lube.