SBC 350 Aches and pains
 

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.

Re: SBC 350 Aches and pains
 

Ouch!

Re: SBC 350 Aches and pains
 

You might need a new valve there...

Re: SBC 350 Aches and pains
 

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

Amazingly, no visible damage to the cast iron head

Re: SBC 350 Aches and pains
 

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

Re: SBC 350 Aches and pains
 

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

How would I know?

Re: SBC 350 Aches and pains
 

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.

Re: SBC 350 Aches and pains
 

I wonder why it burned in one spot like that.

Re: SBC 350 Aches and pains
 

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

Re: SBC 350 Aches and pains
 

Is it not normal to burn in one spot?

Re: SBC 350 Aches and pains
 

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

Re: SBC 350 Aches and pains
 

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

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

Re: SBC 350 Aches and pains
 

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

Re: SBC 350 Aches and pains
 

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

Re: SBC 350 Aches and pains
 

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...

Re: SBC 350 Aches and pains
 

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.

Re: SBC 350 Aches and pains
 

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.

Re: SBC 350 Aches and pains
 

HT wire ??

Re: SBC 350 Aches and pains
 

Guessing plug wire. Don is from another continent.

Re: SBC 350 Aches and pains
 

High tension - meaning high voltage.

Re: SBC 350 Aches and pains
 

HT wire = high tension = spark plug wire.

Re: SBC 350 Aches and pains
 

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

Re: SBC 350 Aches and pains
 

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:

Re: SBC 350 Aches and pains
 

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.

Re: SBC 350 Aches and pains
 

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.