SBC 350 Aches and pains

[BlouDon]

Starts well.
idles well.
drives well on light throttle.

But from about 30% throttle, she starts to "backfire" - but through exhaust,left side only. The more throttle, the more backfiring.

This is what I found on #3exh when I removed the heads.

[jjzepplin]

Ouch!

[truckster]

You might need a new valve there...

[BlouDon]

Yep, having tge heads "done", including new valve.

Amazingly, no visible damage to the cast iron head

[kwmech]

Originally Posted by BlouDon

Yep, having tge heads "done", including new valve.

Amazingly, no visible damage to the cast iron head

Machine shop should still install a seat if not all of them

[BlouDon]

I've been wondering if these heads are ready for unleaded...

How would I know?

[vince1]

As far as I know if they don't have hardened seats such as Stellite then they are old school and not so good for unleaded gas, but maybe I don't know very much. I could be just guessing on this one.

[AcampoDave]

I wonder why it burned in one spot like that.

[truckster]

Originally Posted by AcampoDave

I wonder why it burned in one spot like that.

Probably a metallurgy failure. One weak spot that failed and then the exhaust gases kept the erosion going.

[BlouDon]

Originally Posted by AcampoDave

I wonder why it burned in one spot like that.

Is it not normal to burn in one spot?

[geezer#99]

Looks more like something got stuck in there at one time.
What does the piston on that cylinder look like?

[BlouDon]

Nothing on that piston to set it apart from the others.

Not at home at present, so I can't post a pic.

[Tom]

They went to hardened seats when they changed away from leaded fuel.
This is a valve problem not a seat problem.

[gvw5400]

At least on US spec. engines, In 1971 GM dropped the compression ratio from 9.0-1
to 8.0-1 and started " Induction hardening" the valve seats in the heads to allow the use of
unleaded gasoline. Glad you are able to repair your engine backfire issue Philip!
All The Best
Dirk

[BlouDon]

Just an update on my woes:

Valve replaced, heads serviced, reinstalled.

Starts well.
Idles well
Drives well on light throttle.


But from about 30% throttle, she starts to "backfire" - but through exhaust,left side only. The more throttle, the more backfiring.

YEP, Same as per post #1!!

Inspected HT wires and found a bad one. Replaced it.

HAPPY CHEVY!!

Experiences like this keeps me humble...

[truckster]

Originally Posted by BlouDon

Just an update on my woes:

Valve replaced, heads serviced, reinstalled.

Starts well.
Idles well
Drives well on light throttle.


But from about 30% throttle, she starts to "backfire" - but through exhaust,left side only. The more throttle, the more backfiring.

YEP, Same as per post #1!!

Inspected HT wires and found a bad one. Replaced it.

HAPPY CHEVY!!

Experiences like this keeps me humble...

It's frustrating, but at least you know the work you had done was necessary anyway.

I'm glad you got it figured out and running right.

[BlouDon]

I shudder to think how long I would have driven with that valve had I found the bad HT wire first...

So yes, I'm not complaining but it was most disappointing after all that effort and money to have the same symptoms.

[hjewell2]

Originally Posted by BlouDon

I shudder to think how long I would have driven with that valve had I found the bad HT wire first...

So yes, I'm not complaining but it was most disappointing after all that effort and money to have the same symptoms.

HT wire ??

[burnin oil]

Guessing plug wire. Don is from another continent.

[truckster]

Originally Posted by hjewell2

HT wire ??

High tension - meaning high voltage.

[BlouDon]

Originally Posted by hjewell2

HT wire ??

HT wire = high tension = spark plug wire.

[4u2nv]

Someone educate me on how to check a spark plug wire to see if it's failing

[BlouDon]

Originally Posted by 4u2nv

Someone educate me on how to check a spark plug wire to see if it's failing

Here are a few (not a comprehensive list):

1) Running the engine in a dark environment and checking around the engine bay for blue-ish flashes - indication of spark escaping from the wires and jumping to "ground". NOTE: This may only happen at heavier engine loads - as was happening in my case!! Blipping the throttle rather deep may cause brief heavy load periods;
2) Pulling a spark wire off the spark plug and nothing changes NOTE: This could also result from plug or dissy issues;
3) Measure it with a multimeter. For most modern cables you should get a reading in the region of 8000 - 12000 Ohm (or thereabouts)
4) Visual inspection of the cables: This is what I found on my engine:

[burnin oil]

Different plug wire types have different ohm readings. Best bet is to compare 2 similar sized wires of the set and the ohms should be similar. When you look at aftermarket plug wire they rate it in ohms per foot.

[BigRandy]

Originally Posted by BlouDon

Here are a few (not a comprehensive list):

3) Measure it with a multimeter. For most modern cables you should get a reading in the region of 8000 - 12000 Ohm (or thereabouts)

I found this to be very important. I had a random misfire that I chased for days - replaced cap, rotor, plugs looked for arcing and saw none so I kept the same wires. They looked like they could be wiped off and put back in the box. Checked plug gap again on new plugs, checked cap for cracks and found nothing out of the norm.

Pulled out the multimeter and checked resistance in the wires looking for 12K ohms per foot and found #1 wire ranged from 8 to 24K ohms depending on how I held it. Swapped it out and haven't had a miss since.

Bottom line: VERIFY.