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Rebuilding an Anti-Backfire Valve


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So I have been struggling since I bought my 1984 720 pickup with the mostly-just-annoying snap, crackle, and pop in my exhaust manifold any time I lay of the accelerator at high RPMs. It is sometimes bad enough if I am engine braking down a steep hill to turn the heads of unwitting pedestrians... I always figured that it was something to do with the previous owner having deleted all of the emissions equipment in a less-than-professional finished product, and only recently learned just what is responsible for it.

 

Now I know that there are a lot of "just rip it all out" emissions types here, and I was on the same train, but I figured I would give it a try when I saw one on a D21 in the pick-n-pull. Naturally, the valve that I pulled didn't work when tested. I figured that it wasn't worth the trip back (20 miles) to return it, so I decided to just pull it apart. Here is what I found:

 

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I was able to pretty easily pull back the crimped edge of the valve canister (chamber?) and in the process realized I might even be able to crimp it back together if I was able to service the moving parts.

 

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Ahhhh... so that's why sucking on the little doohickey didn't seem to do anything. At least I looked cool while I was "testing" it. I was downright pleased to see that the diaphragm is reinforced with fabric, which was in good shape. At this stage, I was able to check that simply pushing down on the plunger assembly would allow the valve to open so that manifold vacuum could be decreased during deceleration.

 

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In this photo, you can see the orifice that is connected to the vacuum line. As long as the chamber above it is sealed, the valve should be able to operate.

 

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I cleaned up the crusty, flaky bits of rubber off of the diaphragm to make the best seal possible. Here you can also see that one of the three holes in the center is different. This is a tiny passage which I presume will slowly allow the diaphragm to move back into place as the vacuum eases off. I ran a very thin piece of wire through it to be sure that it was clear.

 

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I happened to have some RTV on hand, so here's hoping that this will hold up over time. But hey, it's got to better better than what was in there, right? I allowed it to cure on the diaphragm after smoothing it out rather than clamping down the top of the valve body right away. I figured that would give me better odds of fixing it if I buggered something up while putting it all back together.

 

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Here is the reassembled valve. Not exactly pretty looking, but neither is anything else in my engine bay 😞 

 

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And here is what I now have lurking on the passenger side of my intake manifold. Getting the damn bolt out that the previous owner had plugged the port on the manifold with was the worst part. I could have sworn he had used some unknown-to-me military r&d grade loctite on that bastard, but by the time I had drilled the thing out, it looked like it was just tightened by a very, very strong arm.

 

And now I can happily say that I experience not a single backfire when I am decelerating, shifting under heavy load, or any other instance in which I would have before!  Moral of the story: don't let the internet tell you what is and isn't repairable. Or do whatever the hell you want.

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The '84 didn't use an AB valve but it had something just as good to control the intake vacuum level.  L20Bs had them and I'm surprised that the D21 did also, yet the Z24 did it differently with a 'boost control' unit on the inner passenger side fender and a 'by pass air control' unit built into the carburetor.  The boost control sensed too high intake vacuun and told the by pass control on the carburetor to open and bleed air into the intake to control at a pre-set amount. I see you have a Weber so the original system was interoperable, one way or the other.

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