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Username Post: 350 engine backfiring        (Topic#358316)
Valued Contributor
Posts: 2806
Loc: what does this part do?
Reg: 05-26-02
10-05-19 07:35 AM - Post#2777125    

hiya folks.

My 350 carbureted engine in my '65 C10 is backfiring. As in, I'm on the highway for awhile, engine is up to temp, a little over 185, when I let off to slow down for some reason, like to get off at a ramp when it drops from 65-70 to around 45 or so it goes KABOOM...

Yesterday I swapped out the spark plugs and put new wires on, seems to run better but it still back fired.

Anyway, looking for reasons why a engine backfires, more things to check. This is not something I have come across before in my automotive adventures so thanks for any information.


This was all in there when I bought it, as things have broken I've had to figure out what year is what.


Engine: 5.7 L Chevrolet – truck / 1992 / 350cid / 210 hp / manual trans / RPO# L-05 / TBI(throttle body injection) vin K(Canada assembly plant)

The distributor is 1985-1987 style, based on when I had to replace the engine module under the cap the last time... that's what it worked out as...

Carburetor: Holley – Model 4160 – single pumper 600cfm.

Intake: Edelbrock Standard Performer – Dual Plane

exhaust is stock type cast iron exhaust manifolds, from there the exhaust was rebuilt at a shop about 2 years ago.


Frequent Contributor
Posts: 2153

Loc: Lake George, NY
Reg: 11-11-15
10-05-19 10:21 AM - Post#2777135    
    In response to Woogeroo

So this is out the exhaust, not back up thru the carb?

Super Senior Member
Posts: 5696

Loc: paradise
Reg: 09-05-03
10-06-19 03:29 PM - Post#2777194    
    In response to Woogeroo

sounds like your timing is too retarded

70 L camino, grampa engine, g-force 5 spd, road rage suspension. Pray first before all else fails.

Posts: 335
Loc: seligman,arizona
Reg: 05-14-09
10-06-19 08:53 PM - Post#2777227    
    In response to bobb

Could be an exhaust valve ? I would hook a vacuum gauge to the engine and see if it's a steady reading or jumping all over the place.

Member #271 Senior Chevytalk Moderator
Posts: 25264
Loc: Austin, TX 78748
Reg: 05-09-00
10-07-19 03:56 AM - Post#2777236    
    In response to pauldian

The backfire is unburned fuel. What kinda of plugs are in it? My truck blew out a cat once doing the same thing. Started fire with the fireball on grass.
As pauldian states, grab a vacuum gauge from Harbor Freight. Connect to the full vacuum port. Watch the needle at idle.

'83 Silverardo XST - ZZ4 powered
'95 Jimmy SLT (Bought @131,814)
'96 GMC Jimmy LS Ret. @236651 miles
'08 GMC Acadia 3.6L SLE

"8th Year" Gold Supporting Member
Posts: 401
Age: 60
Loc: Albany, eNZed
Reg: 11-04-12
10-08-19 08:10 PM - Post#2777420    
    In response to gchemist

an exhaust backfire can be caused by an exhaust leak. When you back off, air can be drawn into the exhaust through the hole & ignites unburnt fuel in the exhaust. Kaboom.

Check for leaks

1966 Caprice 396 496 496mkII TH400

Valued Contributor
Posts: 2806
Loc: what does this part do?
Reg: 05-26-02
10-10-19 01:20 PM - Post#2777575    
    In response to Crusty66

Just replaced the spark plugs/wires with the ac/delco jobs for the engine.

Thanks for the replies everyone, I'm still checking things.


Ray P W 
Posts: 524

Reg: 09-30-15
10-10-19 07:25 PM - Post#2777610    
    In response to Woogeroo

"Could be an exhaust valve ?"

Pressurizing each cylinder with a compressed air adapter while the valves are closed will produce an audible hiss at the exhaust pipe end if there is a burned exhaust valve. It's a quick, simple and inexpensive way to isolate burned valves and other problems.

Ray W


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