Carbed LS swap Backfire - The 1947 - Present Chevrolet & GMC Truck Message Board Network

RRC · 10-23-2016, 11:54 AM

I have a stock 5.3 with a Holley carb. I have tested and researched about all I can find. I have talked to MSD and have also tried a Dayton Sensors smart spark with no luck. Everything I have done to this point has made a difference. But i still have a backfire through the exhaust at start up. Good grounds and 12v, new sensors... i have got rid of the misfire and have a good idle. I have tried different timing curves with the laptop. Im finally out of ideas so here i am looking for any help... I'm sure it's something simple..... also have good vacuum.

Andy4639 · 10-23-2016, 12:09 PM

Does it only happen when cold? If so check the bolts for the exhuast manifolds are headers which ever you are running. They may have a slight gap in them at startup but once warmed up they close up.
Just a thought.

RRC · 10-23-2016, 12:19 PM

I have a stock 5.3 with a Holley carb. I have tested and researched about all I can find. I have talked to MSD and have also tried a Dayton Sensors smart spark with no luck. Everything I have done to this point has made a difference. But i still have a backfire through the exhaust at start up. Good grounds and 12v, new sensors... i have got rid of the misfire and have a good idle. I have tried different timing curves with the laptop. Im finally out of ideas so here i am looking for any help... I'm sure it's something simple..... also have good vacuum.

davepl · 10-23-2016, 01:47 PM

Check your cam sensor connector to make sure it's solid, though I doubt it would actually run otherwise.

Have you changed anything about the ignition tables? If your cranking spark were way off, it'd do that.

RRC · 10-23-2016, 02:16 PM

Quote:

Originally Posted by davepl

Check your cam sensor connector to make sure it's solid, though I doubt it would actually run otherwise.

Have you changed anything about the ignition tables? If your cranking spark were way off, it'd do that.

I'm going to try a different Cam sensor today.... I have tried different degrees of timing at 0 rpm

davepl · 10-23-2016, 07:29 PM

I don't know that ECM very well but there might well be cranking spark tables separate from the RPM based table.

10-23-2016, 11:54 AM	#1
RRC Registered User Join Date: May 2012 Location: Moose Jaw Saskatchewan Canada Posts: 1,099	Carbed LS swap Backfire I have a stock 5.3 with a Holley carb. I have tested and researched about all I can find. I have talked to MSD and have also tried a Dayton Sensors smart spark with no luck. Everything I have done to this point has made a difference. But i still have a backfire through the exhaust at start up. Good grounds and 12v, new sensors... i have got rid of the misfire and have a good idle. I have tried different timing curves with the laptop. Im finally out of ideas so here i am looking for any help... I'm sure it's something simple..... also have good vacuum. __________________ My build thread http://67-72chevytrucks.com/vboard/s...d.php?t=563712 Instagram: http://instagram.com/rockylakekustoms Rocky Lake Kustoms Russ

10-23-2016, 12:09 PM	#2
Andy4639 Old member Join Date: Jan 2002 Location: Liberty, & Garden City S.C. , U.S. Posts: 19,945	Re: Carbed LS swap Backfire Does it only happen when cold? If so check the bolts for the exhuast manifolds are headers which ever you are running. They may have a slight gap in them at startup but once warmed up they close up. Just a thought. __________________ 1971 LWB Custom, 6.0LS & 4L80E, Speedhut.com GPS speedometer & gauges with A/C. 20" Boss 338's Grey wheels 4 wheel disc brakes. My Driver Seeing the USA in a 71 Upstate SC GM Truck Club 2013,14 and 2016 Hot Rod Pour Tour http://upstategmtrucks.com/ Get out and drive the truck this summer and have some fun! It sucks not being able to hear! LWB trucks rule, if you don't think so measure your SWB! After talking to tech support at Air Lift I have found out that the kit I need is 60811. Per the measurements I gave them. Ride height of truck inside spring and inside diameter of springs.

10-23-2016, 12:19 PM	#3
RRC Registered User Join Date: May 2012 Location: Moose Jaw Saskatchewan Canada Posts: 1,099	Carbed LS swap Backfire I have a stock 5.3 with a Holley carb. I have tested and researched about all I can find. I have talked to MSD and have also tried a Dayton Sensors smart spark with no luck. Everything I have done to this point has made a difference. But i still have a backfire through the exhaust at start up. Good grounds and 12v, new sensors... i have got rid of the misfire and have a good idle. I have tried different timing curves with the laptop. Im finally out of ideas so here i am looking for any help... I'm sure it's something simple..... also have good vacuum. __________________ My build thread http://67-72chevytrucks.com/vboard/s...d.php?t=563712 Instagram: http://instagram.com/rockylakekustoms Rocky Lake Kustoms Russ

10-23-2016, 01:47 PM	#4
davepl Registered User Join Date: Aug 2006 Location: Redmond, WA Posts: 6,332	Re: Carbed LS swap Backfire Check your cam sensor connector to make sure it's solid, though I doubt it would actually run otherwise. Have you changed anything about the ignition tables? If your cranking spark were way off, it'd do that. __________________ 1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible

10-23-2016, 07:29 PM	#6
davepl Registered User Join Date: Aug 2006 Location: Redmond, WA Posts: 6,332	Re: Carbed LS swap Backfire I don't know that ECM very well but there might well be cranking spark tables separate from the RPM based table. __________________ 1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible

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