Spark Plugs What is wrong?

[53chevyforever]

Ok...really does not miss...just runs ragged. These plugs are in order as
I took them out. From left top..1 3 5 7 and top right 2 4 6 8. Three of the plugs look like have never been run. Why are the center plugs that clean?
I have tried two diff carbs...two hei's and two sets of wires that I know run well.
Also...adjusted valve lash and it does not have a flat cam. The 350has a small cam...and 15 inches of vacuum. Pcv valve is good and has a baffle. Any ideas?
I guess next is to pull intake manifold...?

[B. W.]

How long were the plugs in the engine?
You are running mis-matched plugs (AC Delco vs Autolite)
The four that are black are beside each other in the firing order (may or may not be an issue) 1,8 & 7,2
When does the engine run ragged? at idle, just off idle, etc.
What carbs are you trying?
Did you just put the cam in it & what cam is it?
Did the engine ever run good?

There's no need to pull the intake unless you know you have a vacuum/coolant/oil leak.

Back up & start with the basics. Make sure you are not trying to tune a dead engine.

do a compression test
do a COMPLETE tune up.
install new plugs ( all of the same mfg)
new cap, wires & rotor
confirm correct firing order
go from there (it may be a fuel issue)

[53chevyforever]

Okay...just bought the truck...running this way...compression test already done
and is good...I have had all ac plugs and made no difference...both carbs run
perfect on my other truck...1406 stock eddy and stock600 holley.
The cam already in engine...214 at .050 and 456 lift. One is a new msd street fire and the other hei is good I know runs perfect..been carrying as a spare...fires right up and idles well..its mid range and low on power almost like a wire is off...but not quite. Really does not smoke. Throttle response is good...fuel pump has 5 pounds of pressure. So you tell me?

[demian5]

Originally Posted by 53chevyforever

Okay...just bought the truck...running this way...compression test already done
and is good...I have had all ac plugs and made no difference...both carbs run
perfect on my other truck...1406 stock eddy and stock600 holley.
The cam already in engine...214 at .050 and 456 lift. One is a new msd street fire and the other hei is good I know runs perfect..been carrying as a spare...fires right up and idles well..its mid range and low on power almost like a wire is off...but not quite. Really does not smoke. Throttle response is good...fuel pump has 5 pounds of pressure. So you tell me?

Sounds mechanical:
That vacuum is quite low like a tired motor. Did vacuum gauge needle bounce or move while truck was running?
Do a leak down test to rule out rings seats, etc on those cycls. The autolite tells me that cylinder has had issues for a while and its not carb related.

Anything else is purely speculation and the lack of wanting to do any diagnosing.

https://mobiloil.com/en/article/car-...-leakdown-test

[B. W.]

Originally Posted by 53chevyforever

Okay...just bought the truck...running this way...compression test already done
and is good...I have had all ac plugs and made no difference...both carbs run
perfect on my other truck...1406 stock eddy and stock600 holley.
The cam already in engine...214 at .050 and 456 lift. One is a new msd street fire and the other hei is good I know runs perfect..been carrying as a spare...fires right up and idles well..its mid range and low on power almost like a wire is off...but not quite. Really does not smoke. Throttle response is good...fuel pump has 5 pounds of pressure. So you tell me?

OK, so you know what is good, you can eliminate those. It narrows it down to a timing issue or a mechanical issue. Do what Geezer said: bump up your initial timing (I'd dial in as much as you can, if it begins to "hard start", back it off a little. Also confirm your timing mark.

If it has run like this since you bought it then you are chasing someone else's rabbit. There is a possibility that the cam was installed out of time. What heads are on it? If the heads have dual springs I have seen one spring break & the engine runs fine but doesn't run well up top. (the remaining spring keeps the valve operating & quiet, but "floats" at higher RPMs)

I'm hoping it's a simple timing issue, sure you are too!

[53chevyforever]

Well today I noticed the water down just a little in the radiator...so added some...let it warm and open the thermostat...guess what big bubbles
in the flowing water....never stopped doing the bubbles....it has turn down
tailpipes so I couldn't see the exhaust.

[greasemonkey]

Originally Posted by 53chevyforever

Well today I noticed the water down just a little in the radiator...so added some...let it warm and open the thermostat...guess what big bubbles
in the flowing water....never stopped doing the bubbles....it has turn down
tailpipes so I couldn't see the exhaust.

Sounds like you know what this means...sorry man. Maybe it'll be the gasket between 4 and 6 like Aussie speculated. Not a terrible job to do.

[davepl]

Does your intake manifold by chance distribute fuel to the inner ones from one side and the outer ones from the other? Then you could have a problem on one side of your carb. But you'd have to look at the layout of the manifold to know for sure...

[geezer#99]

Timing!!
What's initial timing?

[BugzC10]

What's the gap on the plugs?...should be 45

[68is4me]

Do a leak down test sounds more like a mechanical issue. Also check and make sure that the power feed for the distributer is a full voltage and not the factory resistance wire.

[AussieinNC]

Cyl 3,4 and 6 all look like water contamination within the combustion chamber...

Compression test and leak down test...most likely blown head gasket between 4 and 6...

Could also be a lean out due to manifold crack...is this a two barrel or four barrel carb?

Open plenum manifold or closed ?

[Grumpy old man]

I would get a correct set of matched plugs gapped correctly and run it again before you get too crazy trying to diagnose whats wrong , the threads don't even look the same length let alone the heat range can't be the same .

[53chevyforever]

Originally Posted by AussieinNC

Cyl 3,4 and 6 all look like water contamination within the combustion chamber...

Compression test and leak down test...most likely blown head gasket between 4 and 6...

Could also be a lean out due to manifold crack...is this a two barrel or four barrel carb?

Open plenum manifold or closed ?

Manifold is edelbrock performer bought new from oriellys by the guy I
bought truck from....carbs are not the problem both that I have tried run
perfect on my other truck.

[53chevyforever]

Originally Posted by BugzC10

What's the gap on the plugs?...should be 45

GAP is .045 for the hei distributor.....

[53chevyforever]

Originally Posted by geezer#99

Timing!!
What's initial timing?

Timing is around 8 degrees with advance unplugged....

['68OrangeSunshine]

Is it a trick of lighting or camera, that Plug #2 looks 1/4'' shorter than Plug #1?
On closer look, you have 5 ACs and 3 Autolights. Running ragged, I wonder why?

[53chevyforever]

Originally Posted by '68OrangeSunshine

Is it a trick of lighting or camera, that Plug #2 looks 1/4'' shorter than Plug #1?
On closer look, you have 5 ACs and 3 Autolights. Running ragged, I wonder why?

I have replaced numerous plugs...at first all were ac plugs.

Just bought a few autolites to try...made no difference.

[geezer#99]

Bump your initial timing up 6 degrees.
Your timing marks might be wacky.

[mike16]

its easy to mix up 5 and 7 when hookin back up the wires.

[tjc]

I think they are the wrong plugs, you might need long reach plugs, I ran across same issue, took me 2 years to figure it out. use a thin wire with hook on it and put in hole for plug and see if it needs a longer plug.

[AussieinNC]

Do the spark plugs screw into the heads at 90 degrees to the head or are they angled off center ?

Taper seat or (peanut) plugs are meant for heads that have angled spark plug threads....if the spark plugs thread in at 90 degrees to the crankshaft, these plugs are the wrong type...they may be the same as what was in there, but that doesnt mean they are correct...

Post a pic of the side of the head showing the plug without the boot ....thanks

[Steeveedee]

Originally Posted by AussieinNC

Do the spark plugs screw into the heads at 90 degrees to the head or are they angled off center ?

Taper seat or (peanut) plugs are meant for heads that have angled spark plug threads....if the spark plugs thread in at 90 degrees to the crankshaft, these plugs are the wrong type...they may be the same as what was in there, but that doesnt mean they are correct...

Post a pic of the side of the head showing the plug without the boot ....thanks

I'm pretty sure that beginning in '71, taper seat plugs were used on SBCs, and went straight in, not in angled seats.

[Coley]

...plugs look like a mix of brands and not the original sizes.....
Sounds like your basic parts group otherwise is in good shape so far....but you have some troubleshooting work to do.
I think the 15 inches of vacuum is probably ok, depending on what rpm you were at.
More important was the 'needle behaviour'...ie: steady? bouncing? intermittent drops? or?
...do the timing first....as Geezer said, bump it up.
Then I would look like mad for a vacuum leak somewhere....brake booster?...intake manifold?....carb insulator gasket?....open port or line?....broken vacuum hose?
...more pics?

All good
Coley

[MagmaJct]

While everyone has already mentioned the differences in plug brands and sizes, there is also a chance they are of different heat ranges too.

I've used hotter running plugs to compensate for poor oil control in the past.