Backfiring - The 1947 - Present Chevrolet & GMC Truck Message Board Network

red71cheyenne · 09-26-2010, 09:52 PM

Took the Blazer out for a trip around the block today, just to see how things were going and its backfiring through the carb. Checked the timing, it was off a little, so reset that. Still backfiring. Only does it when I step on the gas or it starts to lug hard. Idling is fine.

This is a 350 with HEI and an Edelbrock carb. Checked the wires, nothing touching, seem to be all in the right place. I also had rebuilt the carb not long ago, so don't think its that. I had thought it was my fuel pump and then it went out, so I just replaced it, so think the fuel pump is ok.

You guys have any other idea's? The haynes manual says maybe improperly adjusted valves, but not sure how I would check that. They aren't clattering.

vectorit · 09-26-2010, 09:56 PM

What did you do to the truck to possibly cause this?
engine rebuild?
Distributor swap?
new wires?

My first thought to come to my mind not knowing the full situation, is that the back fire out the carb is either bad timing or you have your 5 and 7 wires swapped with each other.

VA72C10 · 09-27-2010, 09:51 AM

Quote:

Originally Posted by vectorit

What did you do to the truck to possibly cause this?
engine rebuild?
Distributor swap?
new wires?

My first thought to come to my mind not knowing the full situation, is that the back fire out the carb is either bad timing or you have your 5 and 7 wires swapped with each other.

a little more info on what's been done recently (since problem started) will help us diagnose it...

here's a diagram I use to make sure it's all lined up correctly

red71cheyenne · 09-27-2010, 11:07 AM

New rebuild, only been ran long enough to break in the cam and the usual little messing around. When I took it to the glass man to install the windshield, it was backfiring, but when I got it home the fuel pump had S@#t itself, so I replaced it.

I checked the wires yesterday along with the timing and I thought I had the wires right, I'll check the 5 and 7 again just to make sure. The HEI is a used one, but it fires right up so I think its good. The spark plugs are new as are the wires and all the guts in the HEI are new. Except for the module, but like I say, it fires right up.

The only other thing I could think of was a vacuum leak, but my vacuum is staying pretty good. I'm at about 5 at idle and if I rev it up, it increases to 15-20. I'm only having a problem when its under load. Could the accelerator pump be bad? Like I said, I rebuilt the carb, I bought it used. Don't have a spare to try to see if thats it though.

LONGHAIR · 09-27-2010, 08:01 PM

Quote:

Originally Posted by red71cheyenne

The only other thing I could think of was a vacuum leak, but my vacuum is staying pretty good. I'm at about 5 at idle and if I rev it up, it increases to 15-20.

5 at idle is definately a problem. Your vacuum should be the highest when the throttle is completely closed. (idle)

red71cheyenne · 09-27-2010, 11:02 PM

Quote:

Originally Posted by LONGHAIR

5 at idle is definately a problem. Your vacuum should be the highest when the throttle is completely closed. (idle)

Hmm. I'll put my mityvac on it again and see if my guage in the truck is just off. As it is right now, its low at idle and increases as I increase throttle. I've wondered if my vacuum guage is goofy tho. I'll check that.

ubtripn · 09-27-2010, 08:30 AM

I have the exact same set-up and the exact same problem. I set the timing then adjusted the accelerator linkage (three holes where the linkage attaches with a tiny clip.) and it got better but still does it till I am up to temp. I am looking forward to hearing everybody's ideas. The only thing for me lefty to do seems to be setting the mixture but my 650 is right out of the box so I hate to mess with it. (Meant to say accellerator pump linkage)

VA72C10 · 09-27-2010, 12:09 PM

are you sure the distributor isn't a tooth off. It would run and idle fine, but under load would backfire. Re-check TDC on the compression stroke and make sure that the rotor points at the #1 plug wire when it's bolted down.

WTRoadrunner · 09-27-2010, 04:42 PM

If possible run a compression test. Sounds like a valve train issue. I had a broken valve spring that drove me nuts till I finally found it. Basically it Fuel, Fire, and Compression with timing mixed in. Any time you have a backfire its due to either ignition at the wrong time or a faulty intake valve. If it's popping out the exhaust it will be an exhaust valve. Hope this helps and Good Luck.

red71cheyenne · 09-27-2010, 06:35 PM

Thanks guys, I'll check TDC again and the rotor. I'll have to look up the specs for the compression check. Can't remember what I should see on that.

powerdriver 1958 · 09-27-2010, 08:24 PM

valve's too tight
broken valve spring
bent push rod
plugs wires crossed or laying against one another and cross firing...
Just a few to think about.
.
Don't run it too long, A valve too tight could affect cam break in.

CrashBob · 09-27-2010, 08:33 PM

Flat cam? Did you use a zinc laden break-in oil or additive? Today's oils (with the exception of certain premium racing oils) have no zinc which prevents micro-welding between the cam and tappet which will wear the lobes unbelievably fast.

Speaking from experience......

CrashBob

red71cheyenne · 09-27-2010, 11:08 PM

Quote:

Originally Posted by CrashBob

Flat cam? Did you use a zinc laden break-in oil or additive? Today's oils (with the exception of certain premium racing oils) have no zinc which prevents micro-welding between the cam and tappet which will wear the lobes unbelievably fast.

Speaking from experience......

CrashBob

Well, I hope not!

Here's where I'm at now. Checked TDC again, it was right on. Rotor was pointing close to NR 1, but I pulled it and got it as close as I could again and checked timing again, still at 2 Before TDC. Took it out for a spin around the block, still backfiring. Pulled back in and advanced the timing and tried it again, it backfired less. Advanced it some more and now it is only backfiring every once in a while and acts a lot better under load. Did have to back the idle back down each time, but it still is running much better.

Am I still off? Did I install the cam wrong? I'm not understanding why if I advance the timing its getting better, unless I built it wrong.

rollinhard · 09-28-2010, 07:14 AM

Did you istall your cam strait up. Because if you didnt you can't go by regular timing settings.your timing could be way off or maybe your chain has jump a tooth just afew other things to think about
Posted via Mobile Device

red71cheyenne · 09-28-2010, 10:16 AM

Quote:

Originally Posted by rollinhard

Did you istall your cam strait up. Because if you didnt you can't go by regular timing settings.your timing could be way off or maybe your chain has jump a tooth just afew other things to think about
Posted via Mobile Device

Not real sure but I think I installed it right. I put it at TDC and installed the cam then the lifters and push rods/valve train. Pre-oiled and like I said, it fired right up. I would think if I installed it wrong I never would have gotten it started without being way out of time. I had it set at 2 degrees before TDC like the book said, but thats when its backfiring under load. Advancing the timing seems to make it better, so maybe my dizzy is a little crazy? I noticed yesterday when I pulled it the vacuum advance is pretty much frozen in place, but it hasn't prevented it from running.

VA72C10 · 09-28-2010, 11:01 AM

Quote:

Originally Posted by red71cheyenne

Not real sure but I think I installed it right. I put it at TDC and installed the cam then the lifters and push rods/valve train. Pre-oiled and like I said, it fired right up. I would think if I installed it wrong I never would have gotten it started without being way out of time. I had it set at 2 degrees before TDC like the book said, but thats when its backfiring under load. Advancing the timing seems to make it better, so maybe my dizzy is a little crazy? I noticed yesterday when I pulled it the vacuum advance is pretty much frozen in place, but it hasn't prevented it from running.

the vacuum advance is needed to bring the total timing to 36-40 degrees total at part throttle. Maybe that is part of the issue. Also some run as much as 12-16BTDC for initial timing so you might want to try that as well. I ran my last SBC for three years as a daily driver at 12 initial and 38 total. As the throttle increases, vacuum advance decreases and mechanical advance is kicking in.

my67c20 · 09-28-2010, 06:05 PM

Where is your vacuum advance hooked to? Did you set the timing with it plugged? Here is a good site with some good info.

http://www.nationaltbucketalliance.c...uum/vacuum.asp

red71cheyenne · 09-28-2010, 10:04 PM

Quote:

Originally Posted by my67c20

Where is your vacuum advance hooked to? Did you set the timing with it plugged? Here is a good site with some good info.

http://www.nationaltbucketalliance.c...uum/vacuum.asp

Vacuum is hooked to the metered port, but I forgot to plug it. Since the advance doesn't work, I'm thinking this isn't much of an issue. I just had the valve covers off not too long ago when I had them reversed(duh), and didn't see anything amiss in there. May want to go ahead and pull them anyway just to verify.

red71cheyenne · 09-28-2010, 10:06 PM

A strange thing I did notice though since adjusting the timing, my overflow tank was staying full most of the time and I was thinking maybe it was getting hot, but now, it has sucked all the coolant into the radiator and hasn't offered to spit any back into the overflow. Would timing affect the cooling that much? My temp guage has been hit and miss ever since I got the thing and I still can't get it to work.

aintmisbehavinn · 09-28-2010, 10:15 PM

Distributor off one tooth, pull distributor reset at 0 then point to number 1. You are firing with one valve open. It may be 180 out or your firing order is way off. A lean condition will backfire thru the carb, a rich condition out the pipes. Something is firing at the wrong time, look at your wires again, make sure they are not side by side and in correct order. Look at your plugs, they will tell you everything. JA Suggestion

WTRoadrunner · 09-28-2010, 10:40 PM

Did you check the compression on every cylinder? Should be about the same give or take maybe 5 psi. Feel of the valve springs, I found 2 broken that I missed for a while. # 7 and # 8 exhaust. That will cause a pop out the exhaust. With a compression check you know what the cylinder is doing.

WTRoadrunner · 09-28-2010, 11:13 PM

I don't mean to be a pain...I built heads for 15 years...If the intake valve is closed when it fires...no way will it back fire through the carb and the same for the exhaust. If the timing is on. 8 before tdc is close enough, I prefer 10 with a 10 to 1 comp. I replaced carb, set valves, changed wires, plugs and even called Lunati before I ran the compression. Thats when I discovered 2 cyl that were weak causing problems. Its really simple if you stick to the basics. This is why we play with these old trucks. Good luck!!!

09-26-2010, 09:56 PM	#2
vectorit What? Join Date: Jun 2007 Location: Southern California Posts: 7,617	Re: Backfiring What did you do to the truck to possibly cause this? engine rebuild? Distributor swap? new wires? My first thought to come to my mind not knowing the full situation, is that the back fire out the carb is either bad timing or you have your 5 and 7 wires swapped with each other. __________________ Chris 1968 K20 Suburban 1972 K10 LWB PU

09-27-2010, 11:07 AM	#4
red71cheyenne Back in the sticks Join Date: Jan 2001 Location: Fordland, MO Posts: 3,188	Re: Backfiring New rebuild, only been ran long enough to break in the cam and the usual little messing around. When I took it to the glass man to install the windshield, it was backfiring, but when I got it home the fuel pump had S@#t itself, so I replaced it. I checked the wires yesterday along with the timing and I thought I had the wires right, I'll check the 5 and 7 again just to make sure. The HEI is a used one, but it fires right up so I think its good. The spark plugs are new as are the wires and all the guts in the HEI are new. Except for the module, but like I say, it fires right up. The only other thing I could think of was a vacuum leak, but my vacuum is staying pretty good. I'm at about 5 at idle and if I rev it up, it increases to 15-20. I'm only having a problem when its under load. Could the accelerator pump be bad? Like I said, I rebuilt the carb, I bought it used. Don't have a spare to try to see if thats it though. __________________ 1971 Cheyenne C-10 w/700R4 and Tuned Port Injection 1969 K5 Blazer w/Tuned Port 2010 2SS/RS Flaming Orange Camaro 2011 K1500 Suburban 2014 K1500 Pickup 2008 Nissan Altima? The wifes' hoopty

09-27-2010, 08:30 AM	#7
ubtripn Registered User Join Date: Apr 2010 Location: Boise, ID Posts: 1,280	Re: Backfiring I have the exact same set-up and the exact same problem. I set the timing then adjusted the accelerator linkage (three holes where the linkage attaches with a tiny clip.) and it got better but still does it till I am up to temp. I am looking forward to hearing everybody's ideas. The only thing for me lefty to do seems to be setting the mixture but my 650 is right out of the box so I hate to mess with it. (Meant to say accellerator pump linkage) Last edited by ubtripn; 09-27-2010 at 09:37 PM.

09-27-2010, 12:09 PM	#8
VA72C10 VA72C10 Join Date: Mar 2008 Location: Midlothian, VA Posts: 25,267	Re: Backfiring are you sure the distributor isn't a tooth off. It would run and idle fine, but under load would backfire. Re-check TDC on the compression stroke and make sure that the rotor points at the #1 plug wire when it's bolted down. __________________ Looking for a 67-72 swb or blazer project in or around VA.

09-27-2010, 04:42 PM	#9
WTRoadrunner Registered User Join Date: Sep 2010 Location: San Angelo Texas Posts: 11	Re: Backfiring If possible run a compression test. Sounds like a valve train issue. I had a broken valve spring that drove me nuts till I finally found it. Basically it Fuel, Fire, and Compression with timing mixed in. Any time you have a backfire its due to either ignition at the wrong time or a faulty intake valve. If it's popping out the exhaust it will be an exhaust valve. Hope this helps and Good Luck.

09-26-2010, 09:52 PM	#1
red71cheyenne Back in the sticks Join Date: Jan 2001 Location: Fordland, MO Posts: 3,188	Backfiring Took the Blazer out for a trip around the block today, just to see how things were going and its backfiring through the carb. Checked the timing, it was off a little, so reset that. Still backfiring. Only does it when I step on the gas or it starts to lug hard. Idling is fine. This is a 350 with HEI and an Edelbrock carb. Checked the wires, nothing touching, seem to be all in the right place. I also had rebuilt the carb not long ago, so don't think its that. I had thought it was my fuel pump and then it went out, so I just replaced it, so think the fuel pump is ok. You guys have any other idea's? The haynes manual says maybe improperly adjusted valves, but not sure how I would check that. They aren't clattering. __________________ 1971 Cheyenne C-10 w/700R4 and Tuned Port Injection 1969 K5 Blazer w/Tuned Port 2010 2SS/RS Flaming Orange Camaro 2011 K1500 Suburban 2014 K1500 Pickup 2008 Nissan Altima? The wifes' hoopty

09-27-2010, 06:35 PM	#10
red71cheyenne Back in the sticks Join Date: Jan 2001 Location: Fordland, MO Posts: 3,188	Re: Backfiring Thanks guys, I'll check TDC again and the rotor. I'll have to look up the specs for the compression check. Can't remember what I should see on that. __________________ 1971 Cheyenne C-10 w/700R4 and Tuned Port Injection 1969 K5 Blazer w/Tuned Port 2010 2SS/RS Flaming Orange Camaro 2011 K1500 Suburban 2014 K1500 Pickup 2008 Nissan Altima? The wifes' hoopty

09-27-2010, 08:24 PM	#11
powerdriver 1958 Kid's Mechanic Join Date: Nov 2006 Location: Kansas City Posts: 8,241	Re: Backfiring valve's too tight broken valve spring bent push rod plugs wires crossed or laying against one another and cross firing... Just a few to think about. . Don't run it too long, A valve too tight could affect cam break in.

09-27-2010, 08:33 PM	#12
CrashBob Registered User Join Date: Jun 2004 Location: Utica Nebraska Posts: 244	Re: Backfiring Flat cam? Did you use a zinc laden break-in oil or additive? Today's oils (with the exception of certain premium racing oils) have no zinc which prevents micro-welding between the cam and tappet which will wear the lobes unbelievably fast. Speaking from experience...... CrashBob __________________ "Life is all about ass. You're either covering it,laughing it off, kicking it, kissing it, busting it or trying to get a piece of it.

09-28-2010, 07:14 AM	#14
rollinhard Registered User Join Date: Jun 2010 Location: British Columbia Posts: 192	Re: Backfiring Did you istall your cam strait up. Because if you didnt you can't go by regular timing settings.your timing could be way off or maybe your chain has jump a tooth just afew other things to think about Posted via Mobile Device __________________ If you can't impress them with your inteligence, dazzle'm with your stupidity 1969 GMC lwb 2wheel drive can you say big stroker motor to come hehe

09-28-2010, 06:05 PM	#17
my67c20 Registered User Join Date: Jun 2002 Location: Tucson AZ by way of WI & CA Posts: 510	Re: Backfiring Where is your vacuum advance hooked to? Did you set the timing with it plugged? Here is a good site with some good info. http://www.nationaltbucketalliance.c...uum/vacuum.asp

09-28-2010, 10:06 PM	#19
red71cheyenne Back in the sticks Join Date: Jan 2001 Location: Fordland, MO Posts: 3,188	Re: Backfiring A strange thing I did notice though since adjusting the timing, my overflow tank was staying full most of the time and I was thinking maybe it was getting hot, but now, it has sucked all the coolant into the radiator and hasn't offered to spit any back into the overflow. Would timing affect the cooling that much? My temp guage has been hit and miss ever since I got the thing and I still can't get it to work. __________________ 1971 Cheyenne C-10 w/700R4 and Tuned Port Injection 1969 K5 Blazer w/Tuned Port 2010 2SS/RS Flaming Orange Camaro 2011 K1500 Suburban 2014 K1500 Pickup 2008 Nissan Altima? The wifes' hoopty

09-28-2010, 10:15 PM	#20
aintmisbehavinn Registered User Join Date: Dec 2008 Location: Lawrenceville GA Posts: 64	Re: Backfiring Distributor off one tooth, pull distributor reset at 0 then point to number 1. You are firing with one valve open. It may be 180 out or your firing order is way off. A lean condition will backfire thru the carb, a rich condition out the pipes. Something is firing at the wrong time, look at your wires again, make sure they are not side by side and in correct order. Look at your plugs, they will tell you everything. JA Suggestion Last edited by aintmisbehavinn; 09-28-2010 at 10:19 PM.

09-28-2010, 10:40 PM	#21
WTRoadrunner Registered User Join Date: Sep 2010 Location: San Angelo Texas Posts: 11	Re: Backfiring Did you check the compression on every cylinder? Should be about the same give or take maybe 5 psi. Feel of the valve springs, I found 2 broken that I missed for a while. # 7 and # 8 exhaust. That will cause a pop out the exhaust. With a compression check you know what the cylinder is doing.

09-28-2010, 11:13 PM	#22
WTRoadrunner Registered User Join Date: Sep 2010 Location: San Angelo Texas Posts: 11	Re: Backfiring I don't mean to be a pain...I built heads for 15 years...If the intake valve is closed when it fires...no way will it back fire through the carb and the same for the exhaust. If the timing is on. 8 before tdc is close enough, I prefer 10 with a 10 to 1 comp. I replaced carb, set valves, changed wires, plugs and even called Lunati before I ran the compression. Thats when I discovered 2 cyl that were weak causing problems. Its really simple if you stick to the basics. This is why we play with these old trucks. Good luck!!!



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