Topic: engine backfiring update!
in Forum: C3 Engines
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Former Member
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Willow Grove, PA - USA
Joined: 10/27/2006
Posts: 232
Vette(s): 1974 Stingray Coupe LS4 454 Dark Brown Metallic with Deluxe Saddle interior
Well, we fired the car up... it started relativly easily and with no real drama.
74stingray454 2007-03-29 17:51:59
We got it to idle smooth, set the timing.... she purred like a kitten...
then when we bring up the RPM's to the higher end and do some rapid thrown on the throttle, apon deceleration the engine intermittently pops out the carb one good pop on deceleration.
we also seem have have a issue with one of the new lifters not staying pumped up and the drivers side bank has a tap.. so we adjusted the valves, tap went away, but then the backfire happened.... and it happens with no rhyme or reason.... and not all the time, rev the piss out of it, shes fine, then next time "POP out the carb....
I imagine its nothing too big, but it is big enough of a problem that we just gave up for the day...
see also elec fan issue in cooling system
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Lifetime MemberModerator
Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
Make sure none of your plug wires are touching, first. With the higher output of the HEI, the wires can cause a cross-fire if they are too close to one another, especially # 5 & 7. Also, make sure the wires are not arcing out to ground on anything. They can arc from the cap to anything close, even a vacuum hose, and of course, the exhaust manifolds/headers. It may be easier to see this in the dark.
Might be a carb issue, too, but I would check all the ignition sources first.
Might be a carb issue, too, but I would check all the ignition sources first.
Joel Adams
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Former Member
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Willow Grove, PA - USA
Joined: 10/27/2006
Posts: 232
Vette(s): 1974 Stingray Coupe LS4 454 Dark Brown Metallic with Deluxe Saddle interior
OK, will do! checking the wires tomorrow, , my buddy seems to think its a intake valve hanging up..... but its only on decel after a hard throw on the throttle....
74stingray454 2007-03-25 17:08:27
thanks again!
Lifetime MemberModerator
Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
Is this a "fresh" engine, or has it been sitting a while?
The sticky intake idea may have some validity if the engine has been dormant for a period of time, or, if a fresh rebuild, the valve guides may not be clearanced properly(too tight).
Something else to consider...if this is an older(not rebuilt) engine with some miles on it, the timing chain may be stretched enough to create a backfire thru the intake. The question is, was it doing this before you installed the ign/dist?
Joel Adams
C3VR Lifetime Member #56
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
Former Member
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Willow Grove, PA - USA
Joined: 10/27/2006
Posts: 232
Vette(s): 1974 Stingray Coupe LS4 454 Dark Brown Metallic with Deluxe Saddle interior
well, its a older engine with about 88k on it...
I installed a fresh cam and lifters over the winter and installed a new double roller timing chain.... thats about it!
the intake valves did have a good bit of build up on them when i took a peek at them..... now, before I bought the car, it sat for about 10 years, but once it was back on the road, it was driven about 500 miles.... i do have paperwork saying that when the car was redone, it had a "sticky valve" issue..... and was taken care of... but from the moment i bought the car, it always has a slight ign miss....
Lifetime MemberManteca, CA - USA
Joined: 11/20/2005
Posts: 3623
Vette(s): 1978,two tone,Metalic Rootbeer & gold
1975 L48 4 speed
Any chance the timing chain might be a tooth off ? I don't know how the marks are on GM motors, but the last Ford I changed the cam on was difficult to set the gears right the first time by eye. ........Just guessing.Can't wait to change over my experience to GM motors.
Jimmy B.
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Former Member
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Willow Grove, PA - USA
Joined: 10/27/2006
Posts: 232
Vette(s): 1974 Stingray Coupe LS4 454 Dark Brown Metallic with Deluxe Saddle interior
we checked and double checked before putting the timing cover on, so I feel pretty confident that the gears were set right
Lifetime MemberModerator
Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
[QUOTE=74stingray454].... i do have paperwork saying that when the car was redone, it had a "sticky valve" issue..... and was taken care of... but from the moment i bought the car, it always has a slight ign miss....[/QUOTE]
Uh-oh...
Maybe their idea of "taking care of" was a bottle of "Marvel Mystery Oil"!
Wow...that's info we didn't have before. By golly, you might be lookin at a valve problem, still.
Now ya got me scared...I think I'd be pullin the heads at this point. You sure don't wanna be droppin a valve on a big block at speed...btdt...
Uh-oh...
Maybe their idea of "taking care of" was a bottle of "Marvel Mystery Oil"!
Wow...that's info we didn't have before. By golly, you might be lookin at a valve problem, still.
Now ya got me scared...I think I'd be pullin the heads at this point. You sure don't wanna be droppin a valve on a big block at speed...btdt...
Joel Adams
C3VR Lifetime Member #56
My Link
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
Former Member
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Willow Grove, PA - USA
Joined: 10/27/2006
Posts: 232
Vette(s): 1974 Stingray Coupe LS4 454 Dark Brown Metallic with Deluxe Saddle interior
Hum, stoopid thought.... maybe we set the timing wrong... I know for a fact we set it to underhood decal specs @ 10 deg BTDC, guess we should have used the cams spec sheet istead huh????
Lifetime MemberModerator
Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
That's possible, but, it would have to be quite a bit off to cause a spit-up...ya never know, tho.
Joel Adams
C3VR Lifetime Member #56
My Link
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
in Forum: C3 Engines
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