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Looking for '70 Anti-Backfire Valve Detail


Marty Rogan

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Attention all you 70 and early 71 owners! And the rest of you that enjoy getting into the nitty gritty details.

I have a '70, blt 6/70 and the A-B valve quit doing its thing. This is a show car, so I trying to replace it with as correct of a part as possible. I got a hold of an NOS valve, P/N 14835-25601, which is supposed to be good up to 6/72. However, it doesn't look the same as the one that came off the car. My early 71 has a valve that is identical to the one on the 70 as well.

If you look closely at the picture below, you can see the differences. The one on the left is the one that came off the car. As you can see the input/output nipples are reversed on these 2 versions. The center nipple is also offset on a 45 degree angle on the new one I got.

So, which one do you have on your car? Plese specify the yeatr also.

Thanks,

Marty

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Are you SURE that the one on the LEFT is the correct part for a 1970 240Z?

Well, that's what I am trying to figure out. The one on the left came off my 6/70 car. I also have a car blt 1/71 that has the identical part. I looked at a picture of Vicky's yellow '72, which has the one on the right. The parts CD says that the later version was for cars built 7/72 and later. Unfortunately, you don't get a detailed enough picture.

I am thinking that if the input/output matches the hoses, it should function properly. The only difference should be which side the center nipple points to; toward the engine, or away from it. If I have a choice, I would prefer to get the correct one.

Marty

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Marty,

The one on the left which was removed is exactly the same as on my 77 260.

(Australian car) My new one I got is nearly the same as your new one, but has a different part #. 14835-N3301.

The 3 photos are my new one. I can't find the old one at the moment to take a picture but when the new one turned up I remember comparing them. Mine is not a show car so it didn't realy worry me.

Alan.

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I'm starting to think that maybe Nissan sourced these units from more than one vendor, and perhaps that accounts for the external differences.

On the other hand, I wouldn't be too sure that "........ if the input/output matches the hoses, it should function properly." The internal parts of units (with different part numbers) could be very different even if the exteruior is identical. Another thing, I didn't see "different" hoses listed for this part, at least in the 240Z years (North American market).

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Carl, you are quite right about the part # thing with internals or some other small thing being different.

The parts CD lists 2 AB Valves for L24-26.

Marty's part # for L24 up to 07-72 and mine for L24-26 07-72 onwards.

This is a stab in the dark but does anyone know the build date when the 240 changed to flat tops. This I believe would have required a change to the AB Valve externally.

Also going along this path of thought, if the new valve did the same thing as the old, did Nissan just decide to make the newer one and box it against both part #.

Alan.

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Carl, you are quite right about the part # thing with internals or some other small thing being different.

The parts CD lists 2 AB Valves for L24-26.

Marty's part # for L24 up to 07-72 and mine for L24-26 07-72 onwards.

This is a stab in the dark but does anyone know the build date when the 240 changed to flat tops. This I believe would have required a change to the AB Valve externally................

Alan.

Hi Alan:

According to www.ZHome.com they list the following equipment changes for N.A. market cars.........." For the 73 Model Year - Carb.'s changed, different E-88 Head Used For Federal Emissions Std.'s in 73. Dash layout changed, with backlight heater/vent controls, new hazzard switch design. Headlight buckets changed from fiberglass to steel. Flame retardent materials were required for interior use. Intermitten windshield wipers added as standard equipment. Two and a half mile per hour bumpers added."

ZHome lists production of these cars to have begun in 09/72

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The parts CD agrees with what the parts supplier is telling me. Part number wise, anyway.

P/N 14835-25601 fits cars up to 6/72

P/N 14835-N3301 fits car from 7/72

The supplier compared my pictures to the stock on hand. They say that the part on the left in my picture corresponds with P/N 14835-N3301. That totally contradicts with the part Allan received.

So, we have seen Nissan ship the same LOOKING part for BOTH Part numbers!

Interesting, but it doesn't get me any closer to answering the original question. Which is CORRECT for a 1970 model?

Secondly, will what I received actually WORK on my '70??? Unfortunately, I won't be able to try it out for another couple of months till the snow melts.

Marty

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Well, I'm kind of late responding to this thread, but why not :).

My '70 had no air injection system when I got it, so I've been collecting stuff to rebuild it. In the attached pic, are three valves that I now have. The one on the left, supposedly part 14835-U6700, I paid $10 on e-bay for. It is actually stamped as a Nissan part, but I can't yet say for what :rolleyes: . It is the cheapest looking part, with a one-piece cast body.

The one in the middle I got from Bob Zola. He indicated it was from/off of a series I 240Z. Not necessarily a '70, though. The one on the right was a freebie I got when I bought a set of 3-screw SU's on the cheap (just wanted to see what was different from my 4-screws). With the carbs, I got the entire manifold, linkage, injection parts, some (ratty) braided hoses, etc. Since they were 3-screw carbs, I assume they are from a later240Z. But, the middle and right valves are essentially identical.

That's all I know so far...

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wait...

what exactly does this thing do again???

It prevents backfires?? How does it do that?

280Z's don't, of course have one... :).

From an old manual I have: Service Manual, Emmision Control System 1970-1971, Nissan

This valve is controlled by intake manifold vacuum and is required to prevent exhaust system backfirings at the initial duration of deceleration. At that time, the mixture in the intake manifold is too rich to burn and ignites when combined with the injected air in the exhaust manifold. The anti-backfire valve is used to provide a supply of air into the intake manifold, thereby making the air mixture leaner to prevent backfiring. If the valve does not work properly, the fuel mixture will go through the combination chambers without being ignited, meet the fresh air, and, at high temperature, backfiring will result.

Also, from another manual: Service Manual, Emission Control System and Safety Device, Nissan (no date)

The anti-backfire valve is used to provide a gulp of air into the intake manifold thereby making the air mixture leaner to prevent backfire. .... If the valve does not work properly, the fuel mixture will back up through it, meet the fresh air, and, at high temperature, backfire will result.

So this is why you sometimes hear it called a "gulp valve" too.

Hope this info is useful to someone. I've been learning stuff... :geek:

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Hi Marty, Everyone,

I found my old valve today and I think I (we) have solved the AB Valve mystery.

On the two attached photos of my AB Valves I have shown the same side of both valves. The valves themselves are the same valve only the vacuum take off tube changes. Both valves have the same bosses both sides, it's just a mater of which side is drilled for the take off tube.

The scan of the parts CD clearly shows both valves, the early car has the angled take off and the later car has the straight take off on the opposite side.

Marty, I believe this means you have the correct part and mine is the one that is incorrect. Which means the original one you removed is for the later model car.

I also found all my old vacuum hoses and checked them against the CD and they are exactly right for the late model fittment. As mine is a 77 model year car this all makes sense.

Alan.

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