What is the deal with backfire? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

67chevemall · 05-04-2005, 11:29 PM

I'm trying to trouble shoot my back fire issues, and I can't fix it.
Ping Bing pop happens at idle and some levels of RPM's
But at a steady high RPM like 2500 it runs fine?

I always thought that backfires were from to much fuel, but I was
just reading my hanyes manual and it says "just taking the
air cleaner off sometimes will lean a carb enough to create
a backfire".
So I not sure if I'm to rich or lean.
The plugs are ok from all the "pops" but I HATE hearing that
noise. Next thing you know your trucks on fire and your running
down the road with grenades.

The book reads for trouble shooting a back fire:

1 Emissions system not functioning ( emissions?)

2 Ign. Timing Could be!

3 Sec. ign system. Plugs wire cap roter all new.

4 Carb neds adj or worn. Its a Qjet from a 78 C20 might need adj.

5 Vaccum leak Maybe the carb gaskets are still to loose

6 Valve issues Can't see it

7 crossed plug wires ? runs to good at 2500rpm

What do you guys think?? Thanks a TONN

cduster · 05-04-2005, 11:36 PM

A backfire is one indication of too lean of fuel mixture.

Dan

67chevemall · 05-04-2005, 11:42 PM

So I should try turning out my mixture screws?
As they are only 1 turn out.

cduster · 05-05-2005, 12:01 AM

I normally start with the Idle mixture screws out 2 turns and go from there.

jamis · 05-05-2005, 07:52 AM

is it a points dizzy? check the condition and the dwell then your timming. may also need a carb rebuild if you getting a lean condition

67chevemall · 05-05-2005, 08:35 AM

Its a HEI dizzy. The carb ran good on the other truck?
Tonight I'll try a few carb ajd.'s.
Would the timing change when changing carbs?

cduster · 05-05-2005, 10:20 AM

Timing shouldn't change. Check for intake leaks at the carb base. I would also double check the carb base gasket to make sure it is the right one. I have seen the gasket look right for the carb, but wrong for the intake, or vise-versa.

Dan

67chevemall · 05-05-2005, 11:48 AM

I have a square intake with the adapter for a Qjet. It may be the gaskets aren't sealed

The Qjet one has some wear on the two humps near the front.

I had a Holley on there two days ago with no adapter and it was backfiring too!????

cduster · 05-05-2005, 01:20 PM

Line the gaskets up on the adapter to verify they cover what they should. Possibly carbon tracking in the dist cap causing a misfire? Maybe a crossed plug wire. If the firing order is correct try re-routing the plug wires so they don't run right next to eachother. Still sounds kinda like a vacume leak to me, though.

Dan

RainFade · 05-05-2005, 03:14 PM

Might want to check your fuel pressure.
I recently went NUTS putting a known good carb on an engine that ran reasonably well with a clapped out carb and it was waaaay lean.
Finally ended up pulling the pickup 'sock' replacing some NASTY old rubber line and fuel pump and all is well.

If this truck has backfired with 2 different carbs, one known good I'd be asking myself what non carb problems could be causing this.

Has this truck ever ran well w/o backfiring for you ?

To look for vaccum leaks you can spray around the base of the carb with some carb cleaner and see if the idle speed goes up.

67chevemall · 05-05-2005, 03:36 PM

I bought a Holley 4150 carb 5 years ago to go on a 3000km trip which worked out great.
Then from sitting the gasket on the seat went causing the truck to run terrible.
So I rebuilt another 4150 that came with the truck.
It runs great but back fires. So I took a 78 Qjet got the adapter cleaned it up
and it will run great too but back fires?

I'll check for leaks tonight with some spray and see what happens.
I don't want to buy a new carb but it's getting tempting.

71meangreenc10 · 05-05-2005, 07:32 PM

the emissions they reference are the air injected systems, if there is a problem in the pump or diverter valve it could cause a backfire........

67chevemall · 05-06-2005, 08:32 AM

Holy fn shoot bags! Turned the mixture screws out 2.5 turns and BAMMMMMMMMKMMM
Were GONNNNNNNNNNNNNNEEEEEEEE~!
Thanks Guys
YEEEEEEHAAAAAA
To Love Thy Chevy

67chevemall · 05-06-2005, 12:44 PM

Oh ya I also tightened the carb bolts a whole turn.

CHEVIBUBBA · 05-06-2005, 07:37 PM

to get the idle mixture correct you need to put a vacuum guage on it.
adjust one at a time correcting the idle speed if it drops or increases.
shoot for the highest stable vacuum reading with it in p or n.

67chevemall · 05-12-2005, 03:22 PM

chevibubba, Where does the vacuum gauge hook up to???

CHEVIBUBBA · 05-12-2005, 04:01 PM

any manifold vacuum source such as the tranny vacuum modulator port or the port below the throttle plates.

05-04-2005, 11:29 PM	#1
67chevemall Registered User Join Date: Apr 2003 Location: NB Posts: 3,367	What is the deal with backfire? I'm trying to trouble shoot my back fire issues, and I can't fix it. Ping Bing pop happens at idle and some levels of RPM's But at a steady high RPM like 2500 it runs fine? I always thought that backfires were from to much fuel, but I was just reading my hanyes manual and it says "just taking the air cleaner off sometimes will lean a carb enough to create a backfire". So I not sure if I'm to rich or lean. The plugs are ok from all the "pops" but I HATE hearing that noise. Next thing you know your trucks on fire and your running down the road with grenades. The book reads for trouble shooting a back fire: 1 Emissions system not functioning ( emissions?) 2 Ign. Timing Could be! 3 Sec. ign system. Plugs wire cap roter all new. 4 Carb neds adj or worn. Its a Qjet from a 78 C20 might need adj. 5 Vaccum leak Maybe the carb gaskets are still to loose 6 Valve issues Can't see it 7 crossed plug wires ? runs to good at 2500rpm What do you guys think?? Thanks a TONN

05-04-2005, 11:36 PM	#2
cduster Registered User Join Date: Oct 2002 Location: S.W. Michigan Posts: 403	A backfire is one indication of too lean of fuel mixture. Dan __________________ Good Judgment comes from experience. Most experience comes from bad judgment.

05-05-2005, 12:01 AM	#4
cduster Registered User Join Date: Oct 2002 Location: S.W. Michigan Posts: 403	I normally start with the Idle mixture screws out 2 turns and go from there. __________________ Good Judgment comes from experience. Most experience comes from bad judgment.

05-05-2005, 07:52 AM	#5
jamis F.A.S.T. president Join Date: Sep 2003 Location: Florida Posts: 1,883	is it a points dizzy? check the condition and the dwell then your timming. may also need a carb rebuild if you getting a lean condition __________________ F.A.S.T. President and Founder Fabricators for the Advancement of Sawsall Technology "A fear of the Sawsall is a sign of retarded sexual and emotional maturity." ~Sigmund Freud, "General Introduction to Psychoanalysis"

05-05-2005, 10:20 AM	#7
cduster Registered User Join Date: Oct 2002 Location: S.W. Michigan Posts: 403	Timing shouldn't change. Check for intake leaks at the carb base. I would also double check the carb base gasket to make sure it is the right one. I have seen the gasket look right for the carb, but wrong for the intake, or vise-versa. Dan __________________ Good Judgment comes from experience. Most experience comes from bad judgment.

05-04-2005, 11:42 PM	#3
67chevemall Registered User Join Date: Apr 2003 Location: NB Posts: 3,367	So I should try turning out my mixture screws? As they are only 1 turn out.

05-05-2005, 08:35 AM	#6
67chevemall Registered User Join Date: Apr 2003 Location: NB Posts: 3,367	Its a HEI dizzy. The carb ran good on the other truck? Tonight I'll try a few carb ajd.'s. Would the timing change when changing carbs?

05-05-2005, 11:48 AM	#8
67chevemall Registered User Join Date: Apr 2003 Location: NB Posts: 3,367	I have a square intake with the adapter for a Qjet. It may be the gaskets aren't sealed The Qjet one has some wear on the two humps near the front. I had a Holley on there two days ago with no adapter and it was backfiring too!????

05-05-2005, 01:20 PM	#9
cduster Registered User Join Date: Oct 2002 Location: S.W. Michigan Posts: 403	Line the gaskets up on the adapter to verify they cover what they should. Possibly carbon tracking in the dist cap causing a misfire? Maybe a crossed plug wire. If the firing order is correct try re-routing the plug wires so they don't run right next to eachother. Still sounds kinda like a vacume leak to me, though. Dan __________________ Good Judgment comes from experience. Most experience comes from bad judgment.

05-05-2005, 03:14 PM	#10
RainFade Registered User Join Date: Nov 2004 Location: Central Ohio Posts: 215	Might want to check your fuel pressure. I recently went NUTS putting a known good carb on an engine that ran reasonably well with a clapped out carb and it was waaaay lean. Finally ended up pulling the pickup 'sock' replacing some NASTY old rubber line and fuel pump and all is well. If this truck has backfired with 2 different carbs, one known good I'd be asking myself what non carb problems could be causing this. Has this truck ever ran well w/o backfiring for you ? To look for vaccum leaks you can spray around the base of the carb with some carb cleaner and see if the idle speed goes up. __________________ 71 K20 350/350/205, dealer air, 4" lift, Flowtech,Edel 1406 on 2101, procomp HEI,4.10's, 33x12.50x16.5 BFG MT's on slots. 4 wheel disc brakes. My lame K20 page My lame Eaton disc brake page

05-05-2005, 03:36 PM	#11
67chevemall Registered User Join Date: Apr 2003 Location: NB Posts: 3,367	I bought a Holley 4150 carb 5 years ago to go on a 3000km trip which worked out great. Then from sitting the gasket on the seat went causing the truck to run terrible. So I rebuilt another 4150 that came with the truck. It runs great but back fires. So I took a 78 Qjet got the adapter cleaned it up and it will run great too but back fires? I'll check for leaks tonight with some spray and see what happens. I don't want to buy a new carb but it's getting tempting.

05-05-2005, 07:32 PM	#12
71meangreenc10 Its a Truck Thing...... Join Date: Dec 2004 Location: Sierra Vista, Arizona Posts: 3,159	the emissions they reference are the air injected systems, if there is a problem in the pump or diverter valve it could cause a backfire........ __________________ 71 C10 283/3SPD Full Resto 71 GMC 1500 Sierra Grande http://67-72chevytrucks.com/vboard/s...d.php?t=518599 70 C10 Suburban Former military GSA truck. 72 Chevy Blazer 4X4, Sloppy Jo, Mountain Climber. Wife says no more trucks. http://67-72chevytrucks.com/vboard/s...d.php?t=707378 72 GMC 1 Ton Motor Home, wife said no more trucks until she saw this one. Gen 3 6.0/4L80E 4.10 http://67-72chevytrucks.com/vboard/s...d.php?t=761110 68 GMC Suburban was 3/4 ton, now 1/2.Wife shook her head 71 C30 Wrecker 71 C20 Scott-Bilt As weird as it gets..BB Cheyenne AC Truck 68 GMC Long Stepside. They keep following me home 71 C20 Longhorn/Ole Yellor 69 C30 Former Motor Home, Flat Bed time

05-06-2005, 08:32 AM	#13
67chevemall Registered User Join Date: Apr 2003 Location: NB Posts: 3,367	Holy fn shoot bags! Turned the mixture screws out 2.5 turns and BAMMMMMMMMKMMM Were GONNNNNNNNNNNNNNEEEEEEEE~! Thanks Guys YEEEEEEHAAAAAA To Love Thy Chevy

05-06-2005, 12:44 PM	#14
67chevemall Registered User Join Date: Apr 2003 Location: NB Posts: 3,367	Oh ya I also tightened the carb bolts a whole turn.

05-06-2005, 07:37 PM	#15
CHEVIBUBBA got boost? Join Date: Mar 2005 Posts: 123	to get the idle mixture correct you need to put a vacuum guage on it. adjust one at a time correcting the idle speed if it drops or increases. shoot for the highest stable vacuum reading with it in p or n. __________________ '70 chevy lwb w/'67 hood. 357 s.b.c. built, 4l60 w/vigilante lockup high stall converter, edelbrock pro-flo,brodix heads,comp cams rollerized w/ belt drive, one piece d-shaft, yada, yada, yada..... '82 honda v45 sabre"the black"



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05-12-2005, 03:22 PM	#16
67chevemall Registered User Join Date: Apr 2003 Location: NB Posts: 3,367	chevibubba, Where does the vacuum gauge hook up to??? __________________ 1968 Chevy C10 307 3ott fleetside 1967 Chevy C/10 V8, 3spd, fleetside lwb.Sold 1967 Chev C/10 step, 383, M21. SOLD

05-12-2005, 04:01 PM	#17
CHEVIBUBBA got boost? Join Date: Mar 2005 Posts: 123	any manifold vacuum source such as the tranny vacuum modulator port or the port below the throttle plates. __________________ '70 chevy lwb w/'67 hood. 357 s.b.c. built, 4l60 w/vigilante lockup high stall converter, edelbrock pro-flo,brodix heads,comp cams rollerized w/ belt drive, one piece d-shaft, yada, yada, yada..... '82 honda v45 sabre"the black"

Register or Log In To remove these advertisements.