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#1 |
Registered User
Join Date: Feb 2019
Location: Centurion, Gauteng, South Africa
Posts: 228
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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.
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Best regards, Philip Chev: '70 C10 (350 V8 / 700R4 ) & '59 Apache ( 235, 3-sp ) Jensen Interceptor '74: Mopar 440(EFI'ed) / 4L60e Jaguar: '72 E-type Coupe V12 & '74 E-type V12 Convertible & '80 XJS V12 |
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#2 |
Registered User
Join Date: Feb 2012
Location: Ruskin Florida
Posts: 4,551
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Re: SBC 350 Aches and pains
Ouch!
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#3 |
Senior Member
![]() Join Date: Jan 2001
Location: Orem, Utah
Posts: 7,845
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Re: SBC 350 Aches and pains
You might need a new valve there...
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I have sworn upon the altar of God, eternal hostility against every form of tyranny over the mind of man. - Thomas Jefferson |
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#4 |
Registered User
Join Date: Feb 2019
Location: Centurion, Gauteng, South Africa
Posts: 228
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Re: SBC 350 Aches and pains
Yep, having tge heads "done", including new valve.
Amazingly, no visible damage to the cast iron head
__________________
Best regards, Philip Chev: '70 C10 (350 V8 / 700R4 ) & '59 Apache ( 235, 3-sp ) Jensen Interceptor '74: Mopar 440(EFI'ed) / 4L60e Jaguar: '72 E-type Coupe V12 & '74 E-type V12 Convertible & '80 XJS V12 |
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#5 |
Senior Member
![]() Join Date: Mar 2003
Location: Colfax-California
Posts: 8,438
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Re: SBC 350 Aches and pains
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#6 |
Registered User
Join Date: Feb 2019
Location: Centurion, Gauteng, South Africa
Posts: 228
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Re: SBC 350 Aches and pains
I've been wondering if these heads are ready for unleaded...
How would I know?
__________________
Best regards, Philip Chev: '70 C10 (350 V8 / 700R4 ) & '59 Apache ( 235, 3-sp ) Jensen Interceptor '74: Mopar 440(EFI'ed) / 4L60e Jaguar: '72 E-type Coupe V12 & '74 E-type V12 Convertible & '80 XJS V12 |
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#7 |
Registered User
Join Date: Jan 2013
Location: Calgary Alberta
Posts: 1,166
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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.
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#8 |
Registered User
Join Date: Feb 2013
Location: central California
Posts: 2,733
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Re: SBC 350 Aches and pains
I wonder why it burned in one spot like that.
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#9 |
Senior Member
![]() Join Date: Jan 2001
Location: Orem, Utah
Posts: 7,845
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Re: SBC 350 Aches and pains
Probably a metallurgy failure. One weak spot that failed and then the exhaust gases kept the erosion going.
__________________
I have sworn upon the altar of God, eternal hostility against every form of tyranny over the mind of man. - Thomas Jefferson |
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#10 |
Registered User
Join Date: Feb 2019
Location: Centurion, Gauteng, South Africa
Posts: 228
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Re: SBC 350 Aches and pains
Is it not normal to burn in one spot?
__________________
Best regards, Philip Chev: '70 C10 (350 V8 / 700R4 ) & '59 Apache ( 235, 3-sp ) Jensen Interceptor '74: Mopar 440(EFI'ed) / 4L60e Jaguar: '72 E-type Coupe V12 & '74 E-type V12 Convertible & '80 XJS V12 |
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#11 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,531
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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? |
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#12 |
Registered User
Join Date: Feb 2019
Location: Centurion, Gauteng, South Africa
Posts: 228
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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.
__________________
Best regards, Philip Chev: '70 C10 (350 V8 / 700R4 ) & '59 Apache ( 235, 3-sp ) Jensen Interceptor '74: Mopar 440(EFI'ed) / 4L60e Jaguar: '72 E-type Coupe V12 & '74 E-type V12 Convertible & '80 XJS V12 |
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#13 |
driving is in my blood
Join Date: Dec 1999
Location: Mesa AZ
Posts: 5,706
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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.
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-78 c10 short/step: 388cid, M20, 5/5 drop, lots more. Playtoy and first vehicle. -98 c1500 x-cab: 5.7L, 17" rims, 5/6 drop, flowmaster, helper bags,NBS rear disk brakes. -02 Suburban 4x4: leveled front -CBR600F4i, CBR600RR, CBR1000RR, and standup skis DISCLAIMER: I cant spell for the life of me. |
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#14 |
Registered User
Join Date: May 2013
Location: Colorado Rocky Mountains
Posts: 263
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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 |
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#15 |
Registered User
Join Date: Feb 2019
Location: Centurion, Gauteng, South Africa
Posts: 228
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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...
__________________
Best regards, Philip Chev: '70 C10 (350 V8 / 700R4 ) & '59 Apache ( 235, 3-sp ) Jensen Interceptor '74: Mopar 440(EFI'ed) / 4L60e Jaguar: '72 E-type Coupe V12 & '74 E-type V12 Convertible & '80 XJS V12 |
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#16 | |
Senior Member
![]() Join Date: Jan 2001
Location: Orem, Utah
Posts: 7,845
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Re: SBC 350 Aches and pains
Quote:
I'm glad you got it figured out and running right.
__________________
I have sworn upon the altar of God, eternal hostility against every form of tyranny over the mind of man. - Thomas Jefferson |
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#17 |
Registered User
Join Date: Feb 2019
Location: Centurion, Gauteng, South Africa
Posts: 228
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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.
__________________
Best regards, Philip Chev: '70 C10 (350 V8 / 700R4 ) & '59 Apache ( 235, 3-sp ) Jensen Interceptor '74: Mopar 440(EFI'ed) / 4L60e Jaguar: '72 E-type Coupe V12 & '74 E-type V12 Convertible & '80 XJS V12 |
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#18 |
Registered User
Join Date: May 2011
Location: mich
Posts: 655
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Re: SBC 350 Aches and pains
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#19 |
Registered User
Join Date: Jan 2013
Location: Eastern TN
Posts: 1,921
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Re: SBC 350 Aches and pains
Guessing plug wire. Don is from another continent.
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#20 |
Senior Member
![]() Join Date: Jan 2001
Location: Orem, Utah
Posts: 7,845
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Re: SBC 350 Aches and pains
__________________
I have sworn upon the altar of God, eternal hostility against every form of tyranny over the mind of man. - Thomas Jefferson |
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#21 |
Registered User
Join Date: Feb 2019
Location: Centurion, Gauteng, South Africa
Posts: 228
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Re: SBC 350 Aches and pains
__________________
Best regards, Philip Chev: '70 C10 (350 V8 / 700R4 ) & '59 Apache ( 235, 3-sp ) Jensen Interceptor '74: Mopar 440(EFI'ed) / 4L60e Jaguar: '72 E-type Coupe V12 & '74 E-type V12 Convertible & '80 XJS V12 |
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#22 |
Registered User
Join Date: Oct 2018
Location: Ontario Peterborough
Posts: 749
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Re: SBC 350 Aches and pains
Someone educate me on how to check a spark plug wire to see if it's failing
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#23 | |
Registered User
Join Date: Feb 2019
Location: Centurion, Gauteng, South Africa
Posts: 228
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Re: SBC 350 Aches and pains
Quote:
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:
__________________
Best regards, Philip Chev: '70 C10 (350 V8 / 700R4 ) & '59 Apache ( 235, 3-sp ) Jensen Interceptor '74: Mopar 440(EFI'ed) / 4L60e Jaguar: '72 E-type Coupe V12 & '74 E-type V12 Convertible & '80 XJS V12 |
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#24 |
Registered User
Join Date: Jan 2013
Location: Eastern TN
Posts: 1,921
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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.
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#25 | |
Registered User
Join Date: Jun 2017
Location: San Antonio, Texas
Posts: 156
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Re: SBC 350 Aches and pains
Quote:
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.
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Repairing and driving old trucks is an analog experience. No computers or sensors and they can be fixed with basic hand tools |
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