engine backfiring - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**MJN** · 03-22-2003, 03:14 PM

This began Tuesday night when the fuel pump died on the way home and ended up changing it in a parking lot, but now the motor will backfire when going up hill or accelerating when down shifted. The timing is right on, carb is set, new fuel filter, complete tune up two weeks ago. Also feels sluggish when under load but only backfires when it is kicked down into second gear. Any ideas? Thanks

HotRod929 · 03-22-2003, 03:19 PM

What kind of setup you running as far as carb./engine? It could be that the new pump is putting out too high pressure for your carb I dont know. Cut the Edelbrock carbs can only take like 6psi I think. Double check you timing as well because the exact same thing happened to me and it was because my distributer clamp broke so the distributor would torque itself around when I accelerated hard and under load.

HotRod

mtdave2 · 03-22-2003, 04:01 PM

are you sure you dont have a miss? or a miswired sparkplug wire?

**MJN** · 03-22-2003, 08:06 PM

It is a 350 with a mild cam and a rebuilt Quadrajet. I had it running beautifully before the fuel pump went out and now it just isn't the same. I've been through everything in the ignition and it all is new. Thanks

**MJN** · 03-22-2003, 11:48 PM

heres an update, now the backfire went away, but it still feels sluggish and underpowered when giving it moderate to heavy throttle. Will try carb adjustment again but could there be something connected to the fuel pump that I'm missing?

cdowns · 03-23-2003, 12:13 AM

try cleaning the fuel filter in the carb or take of the fuel supply line to the pump and blow about 10lbs of pressure back thru the tank [with the gas cap off]

Hooter · 03-23-2003, 12:17 AM

could you be having a problem with the vacum advance?, did you change brands or octane's of gasoline?

**MJN** · 03-23-2003, 11:09 AM

as far as I know the advance is working fine, I haven't had a chance to get gas yet, but I'll get some today and see what happens. What would blowing pressure back into the tank do? I've done that on diesels but not on gas stuff before.

cdowns · 03-23-2003, 11:39 AM

theres a screen on the fuel pick up tube that some times gets gooked up

O'l Buck · 03-23-2003, 11:48 AM

try changing fuel pumps again, with another new one. The last time I bought a new fuel pump for my truck I had the same problem.......not the backfire, but the miss and power loss were there. I was not happy with my new purchase. I pulled a used on off a junk car so I could use the truck and that used fuel pump is still running my truck.

A.P. · 03-23-2003, 12:14 PM

Your problem started after you replaced the fuel pump. Replace it again. Some times new parts don't work.

**MJN** · 03-23-2003, 03:41 PM

The fuel pump is actually a used one off my pickup that I threw on my Blazer but since the pump only has about 1000 miles on it and the truck ran fine I thought it should work. Depending on how it runs during the week I might change the pump again. Thanks

**MJN** · 03-24-2003, 03:05 PM

well on the way to school this morning, I was accelrating after turning onto the road and it backfired, then lost all power. Then after that I floored it and it sounded like it flooded out, started popping, coughing and stumbling until I let off to about idle. I might try another carb but if it was running fine last week, what else could it be?

Blue68 · 03-24-2003, 05:49 PM

I'd try a new fuel pump. Since it's the only thing you changed. Also, do the Quadrajets have a sintered bronze filter or are you running a filter after the pump before the carb? May be your pumpu puked into your fuel line. You may want to test your pump out by killing your ignition system (remove the coile wire) and turning the engine over to pump some gas into a jar to see if you are getting a good squirt.

Don Smith · 03-24-2003, 09:51 PM

If you are running an HEI, sometimes the wire to the pickup inside the dist. will have a cracked/worn spot on insulation and it will short to dist. body and will cause backfire and loss of power when vacuum advance mechanism moves .

**MJN** · 03-25-2003, 12:02 AM

My dad says it sounds like it isn't advancing, which could be possible since I was messing with the module and had to take off the rotor, I might have popped a spring off under that and that might be the whole problem. I'll have to check it out closer when I have some daylight to mess with that thing. Thanks

**MJN** · 03-30-2003, 12:15 PM

Yep, wasn't advancing the spark. The weights under the rotor got messed up from the night I put the rotor back on in the dark. Glad that was all. Runs great now. Thanks

krue · 03-30-2003, 12:24 PM

Glad to hear it was something simple (and cheap) to fix.

03-22-2003, 03:14 PM	#1
MJN Registered User Join Date: Feb 2000 Location: Maquoketa, Ia/ Orr, Mn Posts: 3,009	engine backfiring This began Tuesday night when the fuel pump died on the way home and ended up changing it in a parking lot, but now the motor will backfire when going up hill or accelerating when down shifted. The timing is right on, carb is set, new fuel filter, complete tune up two weeks ago. Also feels sluggish when under load but only backfires when it is kicked down into second gear. Any ideas? Thanks

03-22-2003, 03:19 PM	#2
HotRod929 Registered User Join Date: Mar 2002 Location: Bondurant, IA but in Worcester, MA for school Posts: 767	What kind of setup you running as far as carb./engine? It could be that the new pump is putting out too high pressure for your carb I dont know. Cut the Edelbrock carbs can only take like 6psi I think. Double check you timing as well because the exact same thing happened to me and it was because my distributer clamp broke so the distributor would torque itself around when I accelerated hard and under load. HotRod __________________ 1970 Chevy C-20 350 V-8 Turbo 350 w/ shift kit Dana 60 w/ 4.10 gears AND Powr-Loc POSI New Additions: Summit Headers 135amp alternator Proform HEI Edelbrock 1406 600cfm carb Sunpro Tach Powr-Loc POSI Edelbrock Performer RPM Intake MSD 6A Ignition Box AIM: HotRod929 Visit My Site: http://1970chevy.netfirms.com/

03-22-2003, 04:01 PM	#3
mtdave2 newly minted old timer Join Date: Dec 1999 Location: Hudson WI Posts: 3,511	are you sure you dont have a miss? or a miswired sparkplug wire? __________________ ____________________________________________ 72 c20 longhorn 65k org miles and counting! 69 k10 Suburban

03-23-2003, 12:13 AM	#6
cdowns Senior Member Join Date: Jan 2003 Location: daytonabeach Posts: 22,956	try cleaning the fuel filter in the carb or take of the fuel supply line to the pump and blow about 10lbs of pressure back thru the tank [with the gas cap off] __________________ 71c-10 350/2004r/4:11 lowered3/4 longbed/dead by hurricane MEANING OF DEATH::::: SOMEBODY ELSE GETS YOUR STUFF DONT BELIEVE EVERYTHING YOU THINK TAKE MY ADVISE;I DON'T USE IT ANYWAY

03-23-2003, 12:17 AM	#7
Hooter My other Love Join Date: Aug 2001 Location: Castlegar B.C. Canada Posts: 4,085	could you be having a problem with the vacum advance?, did you change brands or octane's of gasoline? __________________ Castlegar B.C.The great white North (Canada Eh!) Hooter_5@hotmail.com First generation Monte Carlo club pictures of my life

03-22-2003, 08:06 PM	#4
MJN Registered User Join Date: Feb 2000 Location: Maquoketa, Ia/ Orr, Mn Posts: 3,009	It is a 350 with a mild cam and a rebuilt Quadrajet. I had it running beautifully before the fuel pump went out and now it just isn't the same. I've been through everything in the ignition and it all is new. Thanks

03-22-2003, 11:48 PM	#5
MJN Registered User Join Date: Feb 2000 Location: Maquoketa, Ia/ Orr, Mn Posts: 3,009	heres an update, now the backfire went away, but it still feels sluggish and underpowered when giving it moderate to heavy throttle. Will try carb adjustment again but could there be something connected to the fuel pump that I'm missing?

03-23-2003, 11:09 AM	#8
MJN Registered User Join Date: Feb 2000 Location: Maquoketa, Ia/ Orr, Mn Posts: 3,009	as far as I know the advance is working fine, I haven't had a chance to get gas yet, but I'll get some today and see what happens. What would blowing pressure back into the tank do? I've done that on diesels but not on gas stuff before.

03-23-2003, 11:39 AM	#9
cdowns Senior Member Join Date: Jan 2003 Location: daytonabeach Posts: 22,956	theres a screen on the fuel pick up tube that some times gets gooked up __________________ 71c-10 350/2004r/4:11 lowered3/4 longbed/dead by hurricane MEANING OF DEATH::::: SOMEBODY ELSE GETS YOUR STUFF DONT BELIEVE EVERYTHING YOU THINK TAKE MY ADVISE;I DON'T USE IT ANYWAY

03-23-2003, 11:48 AM	#10
O'l Buck Recovering Truck Driver Join Date: Mar 2002 Location: Orleans, NE USA Posts: 1,883	try changing fuel pumps again, with another new one. The last time I bought a new fuel pump for my truck I had the same problem.......not the backfire, but the miss and power loss were there. I was not happy with my new purchase. I pulled a used on off a junk car so I could use the truck and that used fuel pump is still running my truck. __________________ 67 K-20 350, SM465, Eaton rear, 4.56 no spin option 00 Dodge 2500 4x4, 24V cummins, 5 speed Chad South Central Nebraska

03-23-2003, 12:14 PM	#11
A.P. Black 72 Chevy Join Date: May 2002 Location: Los Angeles, California Posts: 662	Your problem started after you replaced the fuel pump. Replace it again. Some times new parts don't work.

03-23-2003, 03:41 PM	#12
MJN Registered User Join Date: Feb 2000 Location: Maquoketa, Ia/ Orr, Mn Posts: 3,009	The fuel pump is actually a used one off my pickup that I threw on my Blazer but since the pump only has about 1000 miles on it and the truck ran fine I thought it should work. Depending on how it runs during the week I might change the pump again. Thanks

03-24-2003, 03:05 PM	#13
MJN Registered User Join Date: Feb 2000 Location: Maquoketa, Ia/ Orr, Mn Posts: 3,009	well on the way to school this morning, I was accelrating after turning onto the road and it backfired, then lost all power. Then after that I floored it and it sounded like it flooded out, started popping, coughing and stumbling until I let off to about idle. I might try another carb but if it was running fine last week, what else could it be?

03-24-2003, 05:49 PM	#14
Blue68 Registered User Join Date: Apr 2000 Location: Frisco, Tx, USA Posts: 606	I'd try a new fuel pump. Since it's the only thing you changed. Also, do the Quadrajets have a sintered bronze filter or are you running a filter after the pump before the carb? May be your pumpu puked into your fuel line. You may want to test your pump out by killing your ignition system (remove the coile wire) and turning the engine over to pump some gas into a jar to see if you are getting a good squirt. __________________ http://home.comcast.net/~r.landrum/w...tos.html-.html '68 C-10 LWB, '98 Z-71 350 w/accessories (Serpentine), Edelbrock Performer, Edelbrock 1405 (600cfm) carb, GM HEI, CompCam Extreme 4x4 roller cam, Thorley Headers, Carter Electric Fuel Pump, Vintage Air, 700r4, 3.73, disc brakes and power steering upgrades.

03-24-2003, 09:51 PM	#15
Don Smith Registered User Join Date: Jul 2000 Location: Hampton, Ga, USA Posts: 217	If you are running an HEI, sometimes the wire to the pickup inside the dist. will have a cracked/worn spot on insulation and it will short to dist. body and will cause backfire and loss of power when vacuum advance mechanism moves . __________________ 68 Short Stepside, 350, quadrajet, 700r4, 3:73 posi, power front discs, HEI, Factory Air, Tilt, Tach, accelerator cable instead of rod, sway bar. 96 Impala SS 97 Monte Carlo



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03-25-2003, 12:02 AM	#16
MJN Registered User Join Date: Feb 2000 Location: Maquoketa, Ia/ Orr, Mn Posts: 3,009	My dad says it sounds like it isn't advancing, which could be possible since I was messing with the module and had to take off the rotor, I might have popped a spring off under that and that might be the whole problem. I'll have to check it out closer when I have some daylight to mess with that thing. Thanks

03-30-2003, 12:15 PM	#17
MJN Registered User Join Date: Feb 2000 Location: Maquoketa, Ia/ Orr, Mn Posts: 3,009	Yep, wasn't advancing the spark. The weights under the rotor got messed up from the night I put the rotor back on in the dark. Glad that was all. Runs great now. Thanks

03-30-2003, 12:24 PM	#18
krue Designated A-hole! Join Date: Mar 2003 Location: South Mississippi Posts: 36,450	Glad to hear it was something simple (and cheap) to fix. __________________ "If it ain't broke, fix it 'til it is!" Being stupid ain't illegal. We're Still Debt Free Except for the House!!! www.daveramsey.com 70 GMC SWB Stepside project "Green Booger" soon to be 6.0l/4l80e 93 S-10 "Poppaw's Truck"

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