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

Ackattack · 09-28-2003, 08:12 PM

Ok, I need a little help here

Sometimes when I'm cruising along at say 1/4 throttle, and then gun it to say 3/4 to full throttle, my truck will do something weird. It sounds like to me maybe it backfires through the carb, but i'm not sure. It kind make this weird noise like it didn't fire and will loose power, then when I let off it will continue to drive fine. It mainly does this when it's not quite up to full operating temp.

Does anyone know what it could be? I'm thinking either distributor/timing or carb. It has done it with both the 600 edelbrock and 600 holley I've had on it though.

It's kinda hard to describe exactly what it is in words..but hopefully someone can help.

jef5150 · 09-28-2003, 08:22 PM

i was gonna say choke,but you say it does it with 2 different carbs,,,,,maybe intake manifold leak.while the truck it idleing,take some carb cleaner and lightly spray it around the edges of the intake mainfold,,,,,if the rpm rises you have an intake leak,,,,,try it,then you can eliminate that as a problem,,,,,,,,jeff

**D.PASSMORE** · 09-28-2003, 08:51 PM

Probaly sucking air around carb on manifold.

Class of 69 · 09-28-2003, 09:57 PM

Is the choke operational? If not, that could cause the symptom you describe. After that, I second a vacumm leak.

griesel72 · 09-28-2003, 10:23 PM

Off topic, but Ackattack, did i see you in aggieville saturday? it was about 2:00 pm and i think that you were directly behind me. That is a pretty sharp truck you have there.

David

Ackattack · 09-28-2003, 10:43 PM

Choke on one carb is manual and was open, and the other is working just fine. I'll check for a leak.

Yeah, I was in Aggieville...had to try out Jimmie Johns. Were you in one of your 67-72s?

Ackattack · 09-29-2003, 07:30 PM

no vacuum leak around the intake or carb.

Any other suggestions?

1972chevy4x4 · 09-29-2003, 07:40 PM

I had a similar problem on my old drag car with a big block chevy. It would heisitate and make a kinda coughing sound and then take off. I had to adjust the vac. advance on the distributor and she took off like a stripped a**ed ape. Hope this helps somewhat.

**COBALT** · 09-29-2003, 07:44 PM

I was just going to say...you might want to check timing. Too much advance can do some wierd things. Same with a retarded state of timing.

What's the mark on the balancer say when you've got the vacuum advance plugged, and the timing light on?

Ackattack · 10-03-2003, 08:56 PM

Well I figured it out. The timing was WAY off. I got that fixed, but it created another problem.

I did a little research on the net about engine timing. Basically I'm trying run as much advance as possible without pinging.

Well now I'm having starting problems. The starter's not strong enough to turn over the engine.

On the good side though, I picked up a ton of low end torque, throttle response, and the sputtering and backfiring is gone.

When I first checked the timing it was at like 4* ATDC!!!!
I now have it set at about 10* initial.

09-28-2003, 08:12 PM	#1
Ackattack Senior Member Join Date: Aug 2002 Location: Valley Center KS Posts: 3,525	engine prob Ok, I need a little help here Sometimes when I'm cruising along at say 1/4 throttle, and then gun it to say 3/4 to full throttle, my truck will do something weird. It sounds like to me maybe it backfires through the carb, but i'm not sure. It kind make this weird noise like it didn't fire and will loose power, then when I let off it will continue to drive fine. It mainly does this when it's not quite up to full operating temp. Does anyone know what it could be? I'm thinking either distributor/timing or carb. It has done it with both the 600 edelbrock and 600 holley I've had on it though. It's kinda hard to describe exactly what it is in words..but hopefully someone can help. __________________ Project Bruiser: 68 Pro-Touring C-10 5.3L / 4L60E Project Ironhide: 97 GMC Crew Cab 4x4 Cummins SOLD: '69 GMC 1/2T SWB TBI'd: Project No Buck Assembly Manual Index Sign up Thread!!! Factory Service Manual Index

09-28-2003, 08:51 PM	#3
D.PASSMORE One of the First Join Date: Jan 2000 Location: St. Augustine,FL. Posts: 4,054	POW ! Probaly sucking air around carb on manifold.

09-28-2003, 09:57 PM	#4
Class of 69 Registered User Join Date: Mar 2003 Location: Stillwater MN Posts: 1,166	Is the choke operational? If not, that could cause the symptom you describe. After that, I second a vacumm leak. __________________ 69 GMC K10 ..some restoration required....still..

09-28-2003, 10:23 PM	#5
griesel72 Senior Member Join Date: Apr 2003 Location: Howard, Kansas USA Posts: 1,997	Off topic, but Ackattack, did i see you in aggieville saturday? it was about 2:00 pm and i think that you were directly behind me. That is a pretty sharp truck you have there. David __________________ 1969 K10 Custom 10 1997 Z71 2006 Volkswagen Jetta TDI 2006 King Ranch 1980 Wife

09-28-2003, 10:43 PM	#6
Ackattack Senior Member Join Date: Aug 2002 Location: Valley Center KS Posts: 3,525	Choke on one carb is manual and was open, and the other is working just fine. I'll check for a leak. Yeah, I was in Aggieville...had to try out Jimmie Johns. Were you in one of your 67-72s? __________________ Project Bruiser: 68 Pro-Touring C-10 5.3L / 4L60E Project Ironhide: 97 GMC Crew Cab 4x4 Cummins SOLD: '69 GMC 1/2T SWB TBI'd: Project No Buck Assembly Manual Index Sign up Thread!!! Factory Service Manual Index

09-28-2003, 08:22 PM	#2
jef5150 Inline 69 Join Date: Apr 2003 Location: valleysprings,california Posts: 1,018	i was gonna say choke,but you say it does it with 2 different carbs,,,,,maybe intake manifold leak.while the truck it idleing,take some carb cleaner and lightly spray it around the edges of the intake mainfold,,,,,if the rpm rises you have an intake leak,,,,,try it,then you can eliminate that as a problem,,,,,,,,jeff

09-29-2003, 07:30 PM	#7
Ackattack Senior Member Join Date: Aug 2002 Location: Valley Center KS Posts: 3,525	no vacuum leak around the intake or carb. Any other suggestions? __________________ Project Bruiser: 68 Pro-Touring C-10 5.3L / 4L60E Project Ironhide: 97 GMC Crew Cab 4x4 Cummins SOLD: '69 GMC 1/2T SWB TBI'd: Project No Buck Assembly Manual Index Sign up Thread!!! Factory Service Manual Index

09-29-2003, 07:40 PM	#8
1972chevy4x4 Registered User Join Date: Feb 2003 Location: Wichita Kansas Posts: 223	I had a similar problem on my old drag car with a big block chevy. It would heisitate and make a kinda coughing sound and then take off. I had to adjust the vac. advance on the distributor and she took off like a stripped a**ed ape. Hope this helps somewhat. __________________ 1972 chevy k10 4x4 longbed 4-inch lift, 3.73 front/rears, frame off nearly complete 1972 chevy k10 4x4 longbed parts truck, known as the "ugly green machine" 1990 GMC 3/4 shortbed ext cab Always in need of 67-72 parts in central ks.

09-29-2003, 07:44 PM	#9
COBALT Senior Member Join Date: Sep 2001 Location: Seattle, WA, USA Posts: 5,817	I was just going to say...you might want to check timing. Too much advance can do some wierd things. Same with a retarded state of timing. What's the mark on the balancer say when you've got the vacuum advance plugged, and the timing light on? __________________ '69 3/4 ton C20 2wd-350ci/TH400 '69 3/4 ton Custom 20 2wd-350ci/4sp Manual '99 2wd 5.7 Chevy Tahoe Seattle, WA.

10-03-2003, 08:56 PM	#10
Ackattack Senior Member Join Date: Aug 2002 Location: Valley Center KS Posts: 3,525	Well I figured it out. The timing was WAY off. I got that fixed, but it created another problem. I did a little research on the net about engine timing. Basically I'm trying run as much advance as possible without pinging. Well now I'm having starting problems. The starter's not strong enough to turn over the engine. On the good side though, I picked up a ton of low end torque, throttle response, and the sputtering and backfiring is gone. When I first checked the timing it was at like 4* ATDC!!!! I now have it set at about 10* initial. __________________ Project Bruiser: 68 Pro-Touring C-10 5.3L / 4L60E Project Ironhide: 97 GMC Crew Cab 4x4 Cummins SOLD: '69 GMC 1/2T SWB TBI'd: Project No Buck Assembly Manual Index Sign up Thread!!! Factory Service Manual Index



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