'85 GMC Popping & Backfiring when engine is cold! - The 1947 - Present Chevrolet & GMC Truck Message Board Network

chargedstang89 · 02-28-2003, 06:21 PM

Ok...I have an '85 GMC 4x4. It has a BRAND NEW 5.7L 350, a brand new edelbrock 1904 carbureator, and a new tranny. Anyways, when I start it to drive it in the morning or evening, it will backfire through the carbureator, the truck will buck...the RPM's will dip waaayyyy down and then it will die out. What is weird, is that it ONLY does this when the engine is in gear under a load. If I put the engine into neutral, it will revv smoothly. It will keep doing this until the engine is warm. I took it to a shop and they put a new EGR valve on it, but that didn't have any effect at all. They also tweeked the carb a little and made it just a tad richer. I can't figure out why the heck this is happening. It just recently started doing this.

Can anyone pllleeeasseee help me????

josh

chargedstang89 · 02-28-2003, 06:28 PM

I should also add that it misses bad too during this time. Also, after the engine is warm, it runs perfectly. No problems at all. It seems to be a cold problem...any suggestions???? josh

LOHRTBT · 02-28-2003, 08:34 PM

If you find out let me know...I have the same thing with my brand new motor. Have since it was new. Had it running like a top in the summer..then it got cold...runs like chit!

I think maybe the power valve is messed up.

Will try and fix it soon

chargedstang89 · 02-28-2003, 11:05 PM

power valve????

low 84 · 02-28-2003, 11:08 PM

timing could be too retarded causing back fire. i would check for vacuum leaks too.

chargedstang89 · 03-04-2003, 11:01 PM

Ok...here is a new twist. It seems that if I just jump in the truck and go as soon as it is started, it will drive okay. However, if I let it run for a few minutes before I take off and it's not fully warmed up to operating temperature, it will spit, sputter, backfire and die. The carbureator is an Edelbrock 1904 from Summit. I had to return one carbureator as the well plugs were leaking into the engine after it was shut off. Is it possible I have yet ANOTHER defective carb? The guy at the shop I took it to said his father had to get 3 different Holley carbs from Summit b/c he kept getting defective ones. Anyone got any guesses?

82 Silverado 4X · 03-05-2003, 12:39 AM

The Jet replacement Q-Jet did the same thing on my old motor but I just figured it was because the compression was down and the motor was tired. After installing it on my new 350 it acted the same exact way. I found on mine it was a combination of the choke housing setting and the choke pull-off dimension. Since it was basically nothing but a stock replacement carb I was able to cross reference the carb # in my Haynes manual to readjust it with the proper settings. My choke housing was set to allow the choke to open the choke far too quickly and the rod going to the choke pull off needed rebent to close the choke plate just slightly more.
We went through some nasty cold weather here in January and February and after the adjustments I did nothing but start it and warm it just long enough to circulate the oil then I was able to drive off with no problem.

chargedstang89 · 03-05-2003, 04:07 PM

hmmm....I have a chilton's manual for this truck. maybe i'll take it to the shop and have them look into what you suggested. They owe me some free work since i spent $200 there and the damn thing still didn't run right. I just haven't been back to them yet since i've had the truck back for about a week. I've been too busy. Ill have them check what you suggested and I'll keep updated.

josh

02-28-2003, 06:21 PM	#1
chargedstang89 Registered User Join Date: Oct 2002 Posts: 90	'85 GMC Popping & Backfiring when engine is cold! Ok...I have an '85 GMC 4x4. It has a BRAND NEW 5.7L 350, a brand new edelbrock 1904 carbureator, and a new tranny. Anyways, when I start it to drive it in the morning or evening, it will backfire through the carbureator, the truck will buck...the RPM's will dip waaayyyy down and then it will die out. What is weird, is that it ONLY does this when the engine is in gear under a load. If I put the engine into neutral, it will revv smoothly. It will keep doing this until the engine is warm. I took it to a shop and they put a new EGR valve on it, but that didn't have any effect at all. They also tweeked the carb a little and made it just a tad richer. I can't figure out why the heck this is happening. It just recently started doing this. Can anyone pllleeeasseee help me???? josh

02-28-2003, 08:34 PM	#3
LOHRTBT BeWarE tHe eViL CrAnkTOe! Join Date: Oct 2002 Location: Yukon, OK Posts: 3,605	If you find out let me know...I have the same thing with my brand new motor. Have since it was new. Had it running like a top in the summer..then it got cold...runs like chit! I think maybe the power valve is messed up. Will try and fix it soon __________________ ~Dave~ You can call me Mike, Mike Oxlong - 1986 Chevy SWB - Stock for now - *1986 Chevy Custom SWB* *Stolen 6/26/05* - 1985 Chevy SWB SOLD - 1999 T/A Some mods www.cardomain.com/id/ramairdave AIM Screenname - Jester84

02-28-2003, 11:08 PM	#5
low 84 Registered User Join Date: Jan 2002 Location: Houston, TX Posts: 2,144	timing could be too retarded causing back fire. i would check for vacuum leaks too. __________________ '51 F100, backhalf with a narrowed 9 and coil overs, 18.5" mickeys, monte front clip, +400" sbc in the works '05 1500 Crew Cab RIP '84 swb 5/7 drop, solid cammed 408 w/ fully ported dart heads, th350 w/ 10 inch hughes, 12 bolt w/ 3.73 richmonds and a locker '80 swb 4x4 in progress: 7" lift, 350 th350/np205, d44 and 1.5" ORD tie rod, 14BFF w/discs , armored diff covers, 40" MTR's

03-05-2003, 12:39 AM	#7
82 Silverado 4X Genuine Chevrolet Join Date: Feb 2003 Location: South West Pennsylvania. Posts: 582	The Jet replacement Q-Jet did the same thing on my old motor but I just figured it was because the compression was down and the motor was tired. After installing it on my new 350 it acted the same exact way. I found on mine it was a combination of the choke housing setting and the choke pull-off dimension. Since it was basically nothing but a stock replacement carb I was able to cross reference the carb # in my Haynes manual to readjust it with the proper settings. My choke housing was set to allow the choke to open the choke far too quickly and the rod going to the choke pull off needed rebent to close the choke plate just slightly more. We went through some nasty cold weather here in January and February and after the adjustments I did nothing but start it and warm it just long enough to circulate the oil then I was able to drive off with no problem. __________________ George 82 Silverado LWB 4x4 350 SM465 NP208 77 Chevrolet Blazer Chalet 400 TH350 NP203

02-28-2003, 06:28 PM	#2
chargedstang89 Registered User Join Date: Oct 2002 Posts: 90	I should also add that it misses bad too during this time. Also, after the engine is warm, it runs perfectly. No problems at all. It seems to be a cold problem...any suggestions???? josh

02-28-2003, 11:05 PM	#4
chargedstang89 Registered User Join Date: Oct 2002 Posts: 90	power valve????

03-04-2003, 11:01 PM	#6
chargedstang89 Registered User Join Date: Oct 2002 Posts: 90	Ok...here is a new twist. It seems that if I just jump in the truck and go as soon as it is started, it will drive okay. However, if I let it run for a few minutes before I take off and it's not fully warmed up to operating temperature, it will spit, sputter, backfire and die. The carbureator is an Edelbrock 1904 from Summit. I had to return one carbureator as the well plugs were leaking into the engine after it was shut off. Is it possible I have yet ANOTHER defective carb? The guy at the shop I took it to said his father had to get 3 different Holley carbs from Summit b/c he kept getting defective ones. Anyone got any guesses?

03-05-2003, 04:07 PM	#8
chargedstang89 Registered User Join Date: Oct 2002 Posts: 90	hmmm....I have a chilton's manual for this truck. maybe i'll take it to the shop and have them look into what you suggested. They owe me some free work since i spent $200 there and the damn thing still didn't run right. I just haven't been back to them yet since i've had the truck back for about a week. I've been too busy. Ill have them check what you suggested and I'll keep updated. josh



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