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Discussion Starter #1
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?
 

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Sounds like the timing is off, could be only a tooth or two since the truck seems to be running fine otherwise.
 

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Timing is advanced to much.
 

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Discussion Starter #5
Both mechanical and ignition timing are dead nuts.
 

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It may be running a little bit lean. Did you replace the fuel filter? Is it the stock jetting for the carb?
 

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Discussion Starter #8
The #2 exhaust valve is not opening all the way, looks like a bad cam...
 

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sounds like an excuse for a bigger better cam :money:
 

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Discussion Starter #10
You're right. If I can figure out the head and block casting numbers on this re-man motor and size the cam appropriately...
 

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Discussion Starter #11
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.
 

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check the rods and maybe even the rocker arms. There's got to be some foul play in there somewhere.
 

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Discussion Starter #13
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.
 

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Lets keep it simple! You said it DID run fine when you bought it? It just started..Right?
Check the timing. Its earsy and fast. Probably OK ..I didnt read hearing a engine clatter or knock on accel.
Unless you know someone that can Rebuild your aftermarket carb. Buy a new one! Sounds like your starving fuel when you open the secondarys (the 4-barrel) Causing a lean out Back fire.
Also check the mechanical advance weights under your rotor...They stick with Old age!
 
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