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Discussion Starter · #1 ·
Its in a 68 Chevelle with a quadrajet carb. It runs rough under load. It starts fine and idles ok. Here is what I have done:

-Changed plugs, wires, cap, rotor, coil
-checked spark at all and getting spark at plugs

-compression check
  1. 150 psi
  2. 150 psi
  3. 150 psi
  4. 150 psi
  5. 150 psi
  6. 150 psi
  7. 150 psi
  8. 139 psi
-vacuum check (manifold port)
-20 -22.5 psi
-the needle bounces a lot @ idle

-examined rocker arms and they look fine

-took a temp reading @ header inlet and #8 is way cooler
-temp @ 8 is like 220 degrees vs. 500+ degrees on other headers

-I played with the timing a bit but didn't seem to change anything

Thoughts?
 

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I would pull the valve covers and measure lift with dial indicator and compair with cylinder next to it.

I would change oil and filter and run oil thru a screen, like for screening paint, to catch any metal flakes.

Then open filter and look for metal.

Thinking the cam maybe going bad.

Check for anything out of wack on intake valve, maybe bent push rod

Put cylinder on top dead center and connect air hose and listen for air coming out carb or exhaust

So bad cam, bent push rod, etc = intake not opening thus low compression

Bad valve or valves = burnt valve, thus air coming out.

Leak down test checks rings
 

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When you pull the pushrods go slowly. If they feel heavy put them back down.
if you’re unlucky you can pull the lifter right out of the bore.
i doubt your pushrods are bad.
You likely have a flat lobe.
‘Check rockers all move the same before removing anything.
 

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The lower cylinder pressure and bouncing vac needle moves me to suspect the cam has a wiped lobe and lifter on that cylinder.

Is the needle bounce rhythmic or irregular, rhythmic giving credence to cam lobe and lifter damage irregular not so much?

The SBC does have a reputation for eating lobes and lifters.

Bogie
 

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True Hotrodder
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I would take the passenger side valve cover off and just spin the engine over with a remote switch while watching the #8 cylinder. You can quickly tell if either rocker on that cylinder is not opening as much as #6 next to it.
 

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Discussion Starter · #8 ·
I would pull the valve covers and measure lift with dial indicator and compair with cylinder next to it.

I would change oil and filter and run oil thru a screen, like for screening paint, to catch any metal flakes.

Then open filter and look for metal.

Thinking the cam maybe going bad.

Check for anything out of wack on intake valve, maybe bent push rod

Put cylinder on top dead center and connect air hose and listen for air coming out carb or exhaust

So bad cam, bent push rod, etc = intake not opening thus low compression

Bad valve or valves = burnt valve, thus air coming out.

Leak down test checks rings
Ok new update:
My original compression test was faulty. My header was blocking #8 so I was kinking the braided hose on the tester. I removed the header to give me free access and the compression in #8 is fine. Matches the others at 150. So here is where I am:

-not a compression issue
-I have verified spark in all cylinders

Can it still be a cam issue (or valve issue?) if the compression is reading normal?
 

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Yes it can be a cam issue, check the spark plugs usually a failing cam lobe shows up with a wet plug or carboned plug. Next is remove valve covers either bumping it on the starter or with it running look for a rocker not moving as much as the others.

Also a miss that doesn’t go away can be tracking inside the distributor cap, a damaged or worn out plug wire or it’s terminal ends loose or corroded. A plug with a cracked insulator a common problem with installation and removal around header tubes. A plug that is conducting to ground down internal insulator instead of jumping to the ground strap.

Bogie
 

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Along with what Boggie said, I myself had a 350 do same thing as yours, my problem was an internal vacuum leak. I thought it was intake gasket, when I pulled it off, I found someone had ported the heads and broke thru the intake runner by push rod hole. A little metal epoxy sealed it back up. Once all back together it ran fine.

Here’s link to my post with photos at end

 

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'23 T-Bucket Pickup
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1,863 Posts
I would pull the valve covers and measure lift with dial indicator and compair with cylinder next to it.

I would change oil and filter and run oil thru a screen, like for screening paint, to catch any metal flakes.

Then open filter and look for metal.

Thinking the cam maybe going bad.

Check for anything out of wack on intake valve, maybe bent push rod

Put cylinder on top dead center and connect air hose and listen for air coming out carb or exhaust

So bad cam, bent push rod, etc = intake not opening thus low compression

Bad valve or valves = burnt valve, thus air coming out.

Leak down test checks rings
Can also back off the rockers on the offending cylinder and pressurize. I’ve done that several times. Sounds like #8 isn’t firing at all 220 looks like block temperature.
Yes it can be a cam issue, check the spark plugs usually a failing cam lobe shows up with a wet plug or carboned plug. Next is remove valve covers either bumping it on the starter or with it running look for a rocker not moving as much as the others.

Also a miss that doesn’t go away can be tracking inside the distributor cap, a damaged or worn out plug wire or it’s terminal ends loose or corroded. A plug with a cracked insulator a common problem with installation and removal around header tubes. A plug that is conducting to ground down internal insulator instead of jumping to the ground strap.

Bogie
I agree with Bogie. Don’t overlook the basics. I’ve seen brand new plugs that won’t light a cylinder even though they show a spark. I went into that in detail on a previous post.
 

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'23 T-Bucket Pickup
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Can also back off the rockers on the offending cylinder and pressurize. I’ve done that several times. Sounds like #8 isn’t firing at all 220 looks like block temperature.

I agree with Bogie. Don’t overlook the basics. I’ve seen brand new plugs that won’t light a cylinder even though they show a spark. I went into that in detail on a previous post.
Also most likely fine metal oftentimes doesn’t show up in the oil. As a heavy equipment mechanic we would cut open the filter canister with a filter cutter. ( yes it is a real thing). And then cut out a section of the filter media and fold it up and put rags on both sides of the media and crush it in a vise. Unwrap the media and stretch it out. A few tiny particles are normal. You can check with a magnet to see if they are steel or bearing material. I’ve used this method to diagnose more engines than I can remember. Just a tool for the toolbox. You can cut the filter can with a hacksaw, being careful not to dump the cuttings into the canister, and taking the media from the opposite end.
 
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