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09-20-2020, 07:15 AM
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#1 |
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Registered User
Join Date: Feb 2019
Location: Centurion, Gauteng, South Africa
Posts: 227
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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.
__________________
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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09-20-2020, 09:12 AM
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#2 |
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Registered User
Join Date: Feb 2012
Location: Ruskin Florida
Posts: 4,546
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Re: SBC 350 Aches and pains
Ouch!
__________________
70 swb 4x4 406sbc 700r4 203/205 d60/14blt locker yadda yadda http://67-72chevytrucks.com/vboard/s...88#post6935688 Yeller 72 Blazer 2wd conversion project "No Daggum Money" http://67-72chevytrucks.com/vboard/s...d.php?t=550804 LS1-T56 3.73 LSD super budget build Blanco-2014 Sierra SWB https://67-72chevytrucks.com/vboard/...d.php?t=810350 |
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09-20-2020, 10:42 AM
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#3 |
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Senior Member
 
Join Date: Jan 2001
Location: Orem, Utah
Posts: 7,804
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Re: SBC 350 Aches and pains
You might need a new valve there...
__________________
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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09-20-2020, 10:52 AM
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#4 |
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Registered User
Join Date: Feb 2019
Location: Centurion, Gauteng, South Africa
Posts: 227
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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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09-20-2020, 11:50 AM
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#5 | |
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Senior Member
Join Date: Mar 2003
Location: Colfax-California
Posts: 8,393
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Re: SBC 350 Aches and pains
Quote:
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09-20-2020, 12:03 PM
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#6 |
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Registered User
Join Date: Feb 2019
Location: Centurion, Gauteng, South Africa
Posts: 227
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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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09-20-2020, 03:03 PM
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#7 |
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Registered User
Join Date: Jan 2013
Location: Calgary Alberta
Posts: 1,164
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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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09-20-2020, 08:03 PM
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#8 |
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Registered User
Join Date: Feb 2013
Location: central California
Posts: 2,727
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Re: SBC 350 Aches and pains
I wonder why it burned in one spot like that.
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09-20-2020, 08:25 PM
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#9 | |
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Senior Member
Join Date: Jan 2001
Location: Orem, Utah
Posts: 7,804
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Re: SBC 350 Aches and pains
Quote:
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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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09-21-2020, 03:31 AM
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#10 | |
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Registered User
Join Date: Feb 2019
Location: Centurion, Gauteng, South Africa
Posts: 227
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Re: SBC 350 Aches and pains
Quote:
__________________
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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09-21-2020, 08:38 AM
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#11 |
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Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,516
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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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09-21-2020, 08:48 AM
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#12 |
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Registered User
Join Date: Feb 2019
Location: Centurion, Gauteng, South Africa
Posts: 227
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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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09-21-2020, 01:47 PM
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#13 |
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driving is in my blood
Join Date: Dec 1999
Location: Mesa AZ
Posts: 5,694
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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.
__________________
-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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09-25-2020, 09:42 PM
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#14 |
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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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10-04-2020, 02:38 PM
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#15 |
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Registered User
Join Date: Feb 2019
Location: Centurion, Gauteng, South Africa
Posts: 227
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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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10-04-2020, 03:39 PM
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#16 | |
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Senior Member
Join Date: Jan 2001
Location: Orem, Utah
Posts: 7,804
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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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10-04-2020, 03:53 PM
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#17 |
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Registered User
Join Date: Feb 2019
Location: Centurion, Gauteng, South Africa
Posts: 227
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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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10-04-2020, 05:24 PM
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#18 | |
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Registered User
Join Date: May 2011
Location: mich
Posts: 654
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Re: SBC 350 Aches and pains
Quote:
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10-04-2020, 08:01 PM
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#19 | |
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Senior Member
Join Date: Jan 2001
Location: Orem, Utah
Posts: 7,804
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Re: SBC 350 Aches and pains
Quote:
__________________
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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10-05-2020, 01:06 AM
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#20 | |
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Registered User
Join Date: Feb 2019
Location: Centurion, Gauteng, South Africa
Posts: 227
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Re: SBC 350 Aches and pains
Quote:
__________________
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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10-04-2020, 06:44 PM
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#21 |
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Registered User
Join Date: Jan 2013
Location: Eastern TN
Posts: 1,919
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Re: SBC 350 Aches and pains
Guessing plug wire. Don is from another continent.
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10-06-2020, 11:49 PM
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#22 |
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Registered User
Join Date: Jan 2013
Location: Eastern TN
Posts: 1,919
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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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