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Discussion Starter · #1 ·
Howdy folks!

I have a '72 with a L28 block in it, and you know what? It after-fires like crazy while using compression braking. It does it about 2 times per second, depending on what gear I'm in and what speed I'm going at, of course. It drives me nuts!

The exaust is 2.5 with a header and one of those high-flow mufflers. (loud).

Is after-firing normal for this kind of configuration? I realize that this kind of exaust will amplify the sound, but it drives me so crazy I'd gladly accept a performace reduction if it meant saving my ears. :)

Thanks!!!
-rr
.
 

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Re: after-firing - Ooops!

> Howdy folks!

> I have a '72 with a L28 block in it, and you
> know what? It after-fires like crazy while
> using compression braking. It does it about
> 2 times per second, depending on what gear
> I'm in and what speed I'm going at, of
> course. It drives me nuts!

> The exaust is 2.5 with a header and
> one of those high-flow mufflers.
> (loud).

> Is after-firing normal for this kind of
> configuration? I realize that this kind of
> exaust will amplify the sound, but it drives
> me so crazy I'd gladly accept a performace
> reduction if it meant saving my ears. :)

> Thanks!!!
> -rr
> .
I don't know if it is normal but it definitely is not good. How old are your timing chain and drive sprockets? Do you have a shaved head? Has the crankshaft mount been line bored? The shaved head or an improperly line-bored crankshaft could change your timing slightly. What you may have here is an exhaust valve problem where they are opening before the power stroke is fully complete. This would be would be timing chain/sprocket related vs. ignition timing related which would result in backfiring vs afterfiring.
Anyone else?
Phantom
 

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Re: after-firing - Ooops!

I have the same situation with my 240z and it hasnt caused any damage, i have a header and the 2.5 exhaust with no muffler. It seems to quiet it down quite a bit when i richen up the mixture with the choke slightly.

Good luck- evan
 

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Discussion Starter · #4 ·
Re: after-firing - Ooops!

> I don't know if it is normal but it
> definitely is not good. How old are your
> timing chain and drive sprockets?

The timing chain and sprockets are about 20k miles old. (As is the rest of the engine.)

>Do you
> have a shaved head?

I don't really know what a shaved head is. Is that just a head that's modified to boost compression? If so, no, I don't have one. Or at least, I don't *think* I do. :)

>Has the crankshaft mount
> been line bored?

I don't know what that is! :) I don't know very much about L-series engines, or engines in general for that matter.

> The shaved head or an
> improperly line-bored crankshaft could
> change your timing slightly. What you may
> have here is an exhaust valve problem where
> they are opening before the power stroke is
> fully complete.

The valves are only 20k miles old, but I guess they could still be having a problem ehe?

The car only does it on compression braking, and it wouldn't be such a big deal excpet that I live in the mountains and driving down long steep hills gets *very* loud! Pow-Pow-Pow! :)

I kind of thought that lots of cars did that and there wasn't much I could do about it. Every car I've ever had did it! Then again, they all stopped after I did a valve job and put on quieter exaust. Hmmm...

Thanks for the suggestions!!! You're the best!
-rr
.
 

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Re: after-firing - Ooops!

> The timing chain and sprockets are about 20k
> miles old. (As is the rest of the engine.)

> I don't really know what a shaved head is.
> Is that just a head that's modified to boost
> compression? If so, no, I don't have one. Or
> at least, I don't *think* I do. :)

> I don't know what that is! :) I don't know
> very much about L-series engines, or engines
> in general for that matter.

> The valves are only 20k miles old, but I
> guess they could still be having a problem
> ehe?

> The car only does it on compression braking,
> and it wouldn't be such a big deal excpet
> that I live in the mountains and driving
> down long steep hills gets *very* loud!
> Pow-Pow-Pow! :)

> I kind of thought that lots of cars did that
> and there wasn't much I could do about it.
> Every car I've ever had did it! Then again,
> they all stopped after I did a valve job and
> put on quieter exaust. Hmmm...

> Thanks for the suggestions!!! You're the
> best!
> -rr
> .

Do you still have the smog pump attached? If so, remove the line going to the exhaust and cap the hole in the exhaust.
 

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Discussion Starter · #6 ·
Re: after-firing - Ooops!

> Do you still have the smog pump attached? If
> so, remove the line going to the exhaust and
> cap the hole in the exhaust.

Actually, I just removed all the smog equipment recently. (pre '73 or so cars in California don't need it anymore.)

I should have payed more attention to how it acted before when it had it's smog stuff... but I can't remember if it made any impact on after-firing or not when I removed it.

The lines are still going into the header, but I capped the hose coming from them a foot back or so. I didn't want to risk one of those things twisting off inside the header and needing drilling or anything.

Thanks for the idea!!!

-rr
.
 

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Re: after-firing - Ooops!

> Actually, I just removed all the smog
> equipment recently. (pre '73 or so cars in
> California don't need it anymore.)

> I should have payed more attention to how it
> acted before when it had it's smog stuff...
> but I can't remember if it made any impact
> on after-firing or not when I removed it.

> The lines are still going into the header,
> but I capped the hose coming from them a
> foot back or so. I didn't want to risk one
> of those things twisting off inside the
> header and needing drilling or anything.

> Thanks for the idea!!!

> -rr
> .
If it's only doing it closed throttle, that's somewhat normal. When you engine brake, the mixture is richer because it is effectively choked, and so not all the fuel is burned, it goes on out the exhaust pipe and ignites there. I believe it is more prevalent in larger free flowing exhausts because when the butterfly is closed, the exhaust flows better than the intake, and allows this to happen. I have similar results on my 240, with a rebuilt engine with <10000 miles on it, and it is more prevalent during enginebraking (granted I need to tune the **** thing and I've been lazy about it.) I also am running a 2.25 exhaust with a straight through round muffler (flows like a dream, but it's loud as **** in a good way) I also have removed my emissions junk and plugged it all up properly (no under hood inspection on pre75 in arizona) and it still passes the tailpipe test by quite a decent margin. My car particularly has a habit of backfiring if I rev the engine just prior to enginebraking, obviously because it starts to intake alot of the air fuel mixture only to be suddenly stifled for air, causing an excessively rich mixture. Personally I think it sounds kind of kewl when your exhaust pops a little when you let off the throttle, but if it's sounding like gunfire on a regular basis, that's probably bad. ( I did rather enjoy decelerating around a turn at night and noticing the flashes of fire coming from my exhaust pipe in the side mirror view though, kinda gave me a warm formula 1 kind of feeling in my heart )
 

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Discussion Starter · #8 ·
Re: after-firing - Ooops!

> If it's only doing it closed throttle,
> that's somewhat normal. When you engine
> brake, the mixture is richer because it is
> effectively choked, and so not all the fuel
> is burned, it goes on out the exhaust pipe
> and ignites there. I believe it is more
> prevalent in larger free flowing exhausts
> because when the butterfly is closed, the
> exhaust flows better than the intake, and
> allows this to happen. I have similar
> results on my 240, with a rebuilt engine
> with <10000 miles on it, and it is more
> prevalent during enginebraking (granted I
> need to tune the **** thing and I've been
> lazy about it.) I also am running a 2.25
> exhaust with a straight through round
> muffler (flows like a dream, but it's loud
> as **** in a good way) I also have removed
> my emissions junk and plugged it all up
> properly (no under hood inspection on pre75
> in arizona) and it still passes the tailpipe
> test by quite a decent margin. My car
> particularly has a habit of backfiring if I
> rev the engine just prior to enginebraking,
> obviously because it starts to intake alot
> of the air fuel mixture only to be suddenly
> stifled for air, causing an excessively rich
> mixture. Personally I think it sounds kind
> of kewl when your exhaust pops a little when
> you let off the throttle, but if it's
> sounding like gunfire on a regular basis,
> that's probably bad. ( I did rather enjoy
> decelerating around a turn at night and
> noticing the flashes of fire coming from my
> exhaust pipe in the side mirror view though,
> kinda gave me a warm formula 1 kind of
> feeling in my heart )

When I had my tail-pipe and muffler off on my ZX it did the same thing while compression braking. It stopped when the pipe and muffler were welded on.
 

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Re: after-firing - Ooops!

Leslie and zebra are right on the money.You are getting some fresh air pulled into the exhaust system through a small leak in the pipe or manifold.I had the same problem with my motosport sectional 2.5 exhaust system.I put U clamps around the pipe joints and this virtually eliminated the problem! Later ,norm
 

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Re: after-firing - Ooops!

To Leslie Lord

ran across your name searching forum for fuel mix stuff re my 83 280zxt. I've got a high hc and co problem. changed o2 sensor. no effect. gas mileage is 20 and up and it runs strong.the green light on the computer doesn't come on and cycle like the book says it should. it just comes on unpredictably and not very often and it doesn't blink. when it's on the emission levels drop way below the max tolerances and when it goes out they go way up. Any ideas? Many thanks
 
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