Backfiring

[red71cheyenne]

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]

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.

[ubtripn]

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]

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]

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.

[VA72C10]

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]

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]

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.

[LONGHAIR]

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)

[powerdriver 1958]

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]

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]

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.

[red71cheyenne]

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]

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
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[red71cheyenne]

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
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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]

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.

[red71cheyenne]

Originally Posted by VA72C10

the vacuum advance is needed to bring the total timing to 36-40 degrees total under load. 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.

Ahh, I thought vacuum advance was only used at startup! Maybe thats the whole issue then. Guess I'll be rebuilding the dizzy and go from there. Thanks guys for all the help!

[VA72C10]

Originally Posted by red71cheyenne

Ahh, I thought vacuum advance was only used at startup! Maybe thats the whole issue then. Guess I'll be rebuilding the dizzy and go from there. Thanks guys for all the help!

actually don't know what I was thinking vacuum advance is from idle up to part throttle...at full throttle it goes away and the mechanical advance should have kicked in. Maybe the mechanical advance is the issue...I am going to edit my post so no one reads it and is confused

[MrBlue]

Greetings,
I had the same problem. Checked everything mentioned above. As I am also running a Weber, I decided to see if this was the issue. In the Weber documentation, it has a troubleshooting chart. Jet and metering jet change later, no backfire!
Just a thought,
MrBlue

[stevencvn72]

Yeah...I'm going to go with bent push rods, or pulled rocker studs. It'll only take a couple minutes to pull the valve covers and see what's going on in there. Be sure the rocker arms are seated right on the valve stems...esp on the intake valves. Good luck!

[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

[red71cheyenne]

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]

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.

[red71cheyenne]

Originally Posted by MrBlue

Greetings,
I had the same problem. Checked everything mentioned above. As I am also running a Weber, I decided to see if this was the issue. In the Weber documentation, it has a troubleshooting chart. Jet and metering jet change later, no backfire!
Just a thought,
MrBlue

Thanks, I'll have to pull the book out on my Edelbrock and see if it has some troubleshooting too.

[aintmisbehavinn]

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