Elusive Backfire Issue

[RJ80]

First, sorry this is so long!

I've had a backfire on my '79 between shifts since I've owned the car. Every time I let off the gas to shift gears, I get a POP! The carb is the stock boat anchor and all emissions equipment is in place. Originally, Mark had pointed out to me that I had a small crack in my catalytic converter and a weld coming apart on my old Ansa exhaust and he figured the backfire was from the leaky exhaust. Since then, I've replaced the exhaust completely. When I first fired up the car after replacement, the backfire was still there, but seemed to be much quieter compared to before. Within an hour of driving it was just as bad as before. Fast forward a few months, and I still haven't gotten around to solving the issue.

I suppose it could be that I didn't seal the slip joint between the new Stebro rear unit and the pipe that it connects to well enough (I didn't use any exhaust paste or sealant) so that's one area to check. Maybe it still has an exhaust leak due to that.

Here's the strange thing though... I've noticed the backfiring seems to be much worse when the car is cold, even popping and spitting at idle until it is warmed up. After driving enough to get the car warmed up... say 20 minutes or so... it seems to be significantly better. At this point, I can accelerate very lightly and not get the backfire. If I put my foot in it, it will backfire very loudly. Another thing that seems to help is if I hold the throttle open just a little between shifts... that can eliminate the backfire under moderate (but not heavy) acceleration. I'm guessing these two conditions happen because 1) the car runs richer when cold and 2) holding the throttle open between shifts keeps exhaust flow moving and doesn't allow air to be "sucked" in though the leak?

The big queston: do those conditions point to anything other than an exhaust leak, or do they still fit into the leak scenario? I've got to get this problem solved. I'm getting tired of having to pussy-foot my car around to avoid the backfiring. I'm also about to finally upgrade to an 1800 manifold and 34ADF carb, but would like to tackle this first so I can enjoy the new performance without blowing my muffler (and my ears) apart!

[mbouse]

honestly? exhaust leak was gonna be my first suggestion.

[Foster48x]

Not the same vehicle but my motorcycle was doing the same thing. It ran fine under throttle load but backfired on decel and warm up.

It needed a valve job.

Rick

[So Cal Mark]

an overly rich mixture will cause that, but your car should have an air pump system and a defective gulp valve will cause that also

[RJ80]

Hmmm... valve job. Hadn't even considered that, but the valvetrain doesn't sound excessively noisy to me. May be something to look at anyway. I've also got to confirm there's no lingering exhaust leak.

Mark, is the air pump part of the emissions system on a '79? Where is the gulp valve located?

[mbouse]

center of the head... connected to one end of that 1" black hose

[So Cal Mark]

no, no, no, the check valve is in the center of the head. The gulp valve is on the right fender. The hose from the air pump attaches to it. Now, the caveat; there were two different air injection systems in 79 so I'll need to know which system you have. One uses a single check valve in the head, the other uses two check valves

[RJ80]

Hmmm... I'll have to look when I get home to see which one I have. I have no idea off the top of my head.

Incidentally, if I were to install the 1800 manifold and 34adf carb, removing the emissions equipment related to the old manifold in the process (until it's time to pass smog again, anyway) would this also remove the gulp valve?

In any case, I'll check tonight regarding the system.

[So Cal Mark]

swapping the intake would have no effect on the air injection system, other than it has vacuum hose to activate the gulp valve attached to your intake now.

[majicwrench]

To have a back-fire in the exhaust you need two things: Fuel and Oxygen

The fuel can get into the exhaust from a poorly running engine. OR
Anytime you let off the gas suddenly (like shifting) some unburned fuel ends up in exhaust
Air gets in from an exhaust leak, OR from some sort of air injection system

Most 79 carbed cars ( not sure of Fiats) had some sort of device to hold rpms up a bit while shifting.
"Gulp valve" was what we called one of those devices "back in the day", it takes a "gulp" of air and lets it into intake when it sees the vacuum suddenly rise ( like when letting off the gas to shift) this keeps RPMs from rapidly dropping.
We called em BCDD's at the dealerships, Boost Controlled Decelleration Devices
MY 72 has an electronice device to keep RPMs from dropping rapidly, and a little push-button to test it.

Air injections systems on all the cars i have seen ( this does NOT include Fiat, but for the most part, physics is the same over there as here) divert the Air Injection when decellerating, so it does NOT pump into the exhaust. The device to do that is called, a Diverter Valve on most cars.

Bottom line, idle cannot drop quickly when letting off the gas to shift. If it does, some device somewhere not correct.
Air should not pump into exhaut on decelleration. If it does, something not correct.
Upstream exhaust leaks need to fixed.
Car needs to run correctly.

Back in the 70's when I had to deal with this stuff all the time, would occasionly fight a car that was backfiring, everything has to work properly together to eliminate backfire.
Good luck

[RJ80]

Wow, that's a thorough response and I have a much better understanding of what's involved at this point. I still need to go down and look at specifically what system I have, but it seems that an inoperable gulp valve may well be the root of the problem. How would I check if the valve is functional when I head down to look at the car?

To have a back-fire in the exhaust you need two things: Fuel and Oxygen

The fuel can get into the exhaust from a poorly running engine. OR
Anytime you let off the gas suddenly (like shifting) some unburned fuel ends up in exhaust
Air gets in from an exhaust leak, OR from some sort of air injection system

Most 79 carbed cars ( not sure of Fiats) had some sort of device to hold rpms up a bit while shifting.
"Gulp valve" was what we called one of those devices "back in the day", it takes a "gulp" of air and lets it into intake when it sees the vacuum suddenly rise ( like when letting off the gas to shift) this keeps RPMs from rapidly dropping.
We called em BCDD's at the dealerships, Boost Controlled Decelleration Devices
MY 72 has an electronice device to keep RPMs from dropping rapidly, and a little push-button to test it.

Air injections systems on all the cars i have seen ( this does NOT include Fiat, but for the most part, physics is the same over there as here) divert the Air Injection when decellerating, so it does NOT pump into the exhaust. The device to do that is called, a Diverter Valve on most cars.

Bottom line, idle cannot drop quickly when letting off the gas to shift. If it does, some device somewhere not correct.
Air should not pump into exhaut on decelleration. If it does, something not correct.
Upstream exhaust leaks need to fixed.
Car needs to run correctly.

Back in the 70's when I had to deal with this stuff all the time, would occasionly fight a car that was backfiring, everything has to work properly together to eliminate backfire.
Good luck

[majicwrench]

I dislike assuming anything, could be many things other than "gulp" valve.
And like I said, I am not familiar with 79 Fiat controls.

But a typical gulp valve will have a large hose bring clean air to the valve, a large hose from the valve to the intake manifold, and a small hose to sense manifold vacuum. When valve sees vacuum rapidly rise (like decell) will open diapram and allow clean air to flow to intake. This will raise idle and provide oxygen to burn the excess fuel in the cylinders.
Hence, when you rev engine and then let off gas, valve will open, and you will hear/feel the valve take in air thru the large clean air hose ( usaully attached to air cleaner on most cars, again, the caveat about 79 fiats)

Again, this is a generic description, you really need a good fiat manual that would describe devices and show proper hose routing etc.

Once upon a time, all 79 cars had a vacuum diagram under hood, is yours still there?? I love those diagrams, is always a good place to start. Everything works together, and everything is there for a reason.

Good luck,
Keith

[So Cal Mark]

diverter valve was the term I hand in mind when I wrote gulp valve. One of those late night brain lock events