Backfire on deceleration.

[grayw0lf]

I just put a 400sbc in my '70 & a Holley 4175 (650cfm spreadbore) I had sitting around. I am running a HEI ignition system w/ Summit 50k coil. I get a little over 20" of vacuum at warm idle. Cheap headers to dual exhaust.

My problem is that when i let off the gas, is sometimes gives a popping exhaust backfire. This happens more if I'm getting on it & then let off.

Any ideas what causes this?

[Bowtie001]

OK, depending on the noise, this may be normal. For a relatively open exhaust, on decel you'll hear "poppity pop pop pop" in a sedued sort of way. If it goes "BANG!" then it's not normal. I am not exactly sure why, but it's a result of scavenging happening backwards or something like that.

Mine does it because the exhaust leaks more than a kitchen strainer.

-Brian

[OlChev67]

It could be runnin a little rich, mine was backfiring on decel, and we put smaller jets in it and it stopped.

With a leaky exhaust, oxygen gets sucked in and mixes with unburnt fuel, and ignites because the headers are really hot. At least thats how it was explained to me.

[grayw0lf]

It is more of a "poppity pop pop" in a subdued sort of way.

I wouldn't be suprised if I had an exhaust leak. It sounds slightly different on the passenser side.

It could be running rich, but the 9923 model Holley that I have was supposed to be a replacement for a 77 K-10 w/ 350 & I'm running it on a 400sbc. Maybe I'll play w/ the jets when I take it apart to adjust the floats. You gotta love stalling out when you stop too fast.

[72 cheyenne pdx]

Mine does the poppity pop pop on decel. I was told that I was running a little rich or that the timing may be a little off.

[grayw0lf]

I think I already know the answer to this, but does the poppity pop pop on decel hurt anything?

[nvrforgotten52]

nope doesnt hurt a thing. and even turbo mufflers with headers will pop... you cant get around it... a balace pipe or h pipe might help a little

[Slammed70]

What kind of exhaust are you running? Might be a back pressure problem. Had that for happening for awhile on the V8 S-10 with straight pipes.

[WorkinLonghorn]

Cheyenne says;

Mine does the poppity pop pop on decel.

Mine does the bangity bang BOOM! on decel. I've got a cracked rams-horn which I need to replace,but for some reason I LIKE the noise it makes. Guess I'm weird.In this day of heated and cooled leather seats etc. it just seems somehow kinda cool for some guy at a stop hearing a bunch of explosions and looking up at his rear-view mirror,he sees a '69 grill bearing down on him.(I'm in the big city where this stuff doesn't happen).

[EAST SIDE LOW LIFE]

I would be inclined to check the timing. It may need to be advanced a hair. When you shut your truck off does it just stop or does it run on? If it does not run on then you can advance it a little. Also how's the timing belt? Can you turn the engine by hand? It might have jumped a cog. If the battery/tach wires to the dist. are not at the 3 oclock position and appear to be more at the 2 oclock position and you can turn engine by hand the timing chain could have jumped a cog. This would cause the timing to be out. If this is the case retime the engine.

esll.

* Hope this helps at least it's free and does not take long.

[grayw0lf]

Originally Posted by EAST SIDE LOW LIFE

I would be inclined to check the timing. It may need to be advanced a hair. When you shut your truck off does it just stop or does it run on? If it does not run on then you can advance it a little. Also how's the timing belt? Can you turn the engine by hand? It might have jumped a cog. If the battery/tach wires to the dist. are not at the 3 oclock position and appear to be more at the 2 oclock position and you can turn engine by hand the timing chain could have jumped a cog. This would cause the timing to be out. If this is the case timeing the engine could be out.

esll.

* Hope this helps at least it's free and does not take long.

It doesn't diesel or run on when its shut off. The timing chain is also new (fresh rebuild). The battery/tach wires are in about the 3:00 position.

Its very possible that I may need to advance a little timing, but as it sits, I can't advance the timing any further b/c the vacuum advance canister is running into the intake.

The exhaust is cheap headers going into about 2.25" dual pipes w/ glass packs (no glass left in them).

[72BLUZ]

I Myself Would Play With The Timing. Thats Where I Have Heard The Most Popping Or Backfiring In Any System.

[bad72c10]

I'd call that one muffler noise. Glasspacks pop and rasp and all that stuff. I think that's prolly where its coming from. Justin

[blacktruck]

i've got the same basic problem. but i have a nice little exhaust leak. i was wondering if the large explosion in the tailpipe would somehow hurt my valves. i guess not. mine sounds like a shotgun going off under the truck. a friend of mine was following me one night when it happened. he said i had a flame shoot out the tailpipe. if wonder if i could be running a little rich? i'm kinda sure my timing has retarded itself again.

[Blazer1970]

Leaky exhaust causes this. It would be foolish to try anything else before you fix the exhaust leaks.

[WorkinLonghorn]

Leaky exhaust causes this. It would be foolish to try anything else before you fix the exhaust leaks.

100% good advice.

[Eichorst69]

Advance the timing!
After I rebuilt my 350 it would backfire pretty bad on deceleration just like you have described.
The timing was set at stock specs but it now had a mild cam.
It ran great but had the backfire.
After about 300 miles I got tired of hearing the backfire, although it did sound pretty cool!
So I advanced the timing like 8 degrees and the backfire is 99% gone and it has much more power.

Rob

[grayw0lf]

Although I still need to look at the timing, I found the problem.

I used the cheapo exhaust gaskets from the cheapo MrGasket engine kit. #6 is definately leaking. It got worse today, to the point that when it backfires, I can see the spark (looking right at it, of course).

Don't you love when problems identify themselves, in a non-desctuctive manner!.

[eedgell]

On deceleration, pop and bang can be caused from exhaust valve not sealing or being adjusted too tight. Have you tried running the rack? Or maybe a compression check?

[Wreckless_71]

I've also got the full length headers and 2.5" glassies... deffinitly will pop on decel. Check the timing (mines at 8 degrees), also wouldn't hurt to check the plugs and wires. Mine still pops mildly but it runs like a champ!

[EAST SIDE LOW LIFE]

I've always had good results using Percy's Life time gaskets. They cost a little more but well worth it.

esll.

[Nachos82]

mine has the same sound with cheap headers and 2 1/4 dual exhaust. I know my problem is the fact I used the cheap collector gaskets that dont seal for anything. I will be using copper ones from now on. Dont waste your money on the crappy cardboard style ones.

[grayw0lf]

Originally Posted by Nacho82

mine has the same sound with cheap headers and 2 1/4 dual exhaust. I know my problem is the fact I used the cheap collector gaskets that dont seal for anything. I will be using copper ones from now on. Dont waste your money on the crappy cardboard style ones.

Yeah, I think you are on track here. I replaced the header gasket w/ better cheap crappy paper gaskets & it still has a slight leak. I guess I'll have to to multilayer after all.

& I also have collector gaskets, which I've never used before. I always just went without.

[10.5stang]

my buddys truck did the same thing if you floored it and then let off it would pop a few times. he only had one gasket under the carb. he then put a new gasket with a plate between and a secound gasket. did doesnt pop anymore.
his dad said that sometimes the intake manifolds dont have a flat surface and it seals better like the way he did it.