A little vacuum and smog help (1982 720 Z24).

[datzenmike]

Yes it should have one. 

 

At the start of any deceleration the air mixture goes too rich to burn properly but it will light and burn in the exhaust manifold because of the AIS air injection system into the exhaust manifold. To prevent this popping of exploding gas, the AB valve senses a high intake vacuum above a preset limit through the smaller of the three hoses and opens and allows filtered air into the intake to lean out the air fuel mixture. When the intake vacuum returns to a normal level it closes.

 

Your Z24 will run fine without the AB valve but you may notice popping in the exhaust.

[GeneralDipDIp]

I can't seem to find any online, are there other compatible AB valves that are easier to get? Also, how is the AB valve mounted?

[datzenmike]

Try the Z22 engine for the 720. Probably the two larger stiff hoses just position and hold it in place. On the L20B there is a C shaped clip it sits in attached to the side of the air filter?

 

 

[GeneralDipDIp]

In the diagram 720_daniel showed, there is a canister purge line going into the AB valve. Where should this line go while I don't have the AB valve? You mentioned it would run fine without it but I don't want open vac lines. Thanks again Mike.

[datzenmike]

 The canister purge line should connect to the vacuum advance. There would be NO purge of the canister at idle or full throttle

 

And there's the AB valve. It's the round thing at the bottom of the yellow crayon mark. That fuel bowl vent line is not connected to it. Looks like it just hangs in position by the hoses. The small sense line is out of sight behind it and connects to the intake.

 

Post a similar picture of your set up or lack of. 

 

[Sephakrid]

On 8/25/2025 at 8:44 AM, GeneralDipDIp said:

I can't seem to find any online, are there other compatible AB valves that are easier to get? Also, how is the AB valve mounted?

 I found one for a Stanza a while ago. Not totally sure on compatibility but it's the only one I could find anywhere

 

https://www.rockauto.com/en/parts/standard+motor+products,DV25,air+diverter+/+bypass+valve,10192

 

[GeneralDipDIp]

https://imgur.com/a/ilK92YJ

 

Here are a few photos of my setup. Everything is connected as 720_daniel had it, besides the canister purge line since there is no AB valve. In that diagram, the canister purge is not connected to the vacuum advance?

 

It seems on other air cleaners there is a large L connection for the AB valve to connect to, however my air cleaner doesn't have this. I also followed fastboatman's tutorial on Nicoclub for some of the air cleaner connections. 

 

Thanks for the link Sephakrid.

[datzenmike]

 

The top hose of the three on the TVV thermal vacuum valve should connect to the air filter and TEE into the front of the three hoses going to the altiture compensator. 

 

 

 

Your heater hoses should never be connected together like that unless you have some how blocked flow through them.

 

 

The rest just looks like a basket of snakes. Vacuum advance should come from the carburetor base and go downs the front of the engine to the distributor.

 

[GeneralDipDIp]

The heater hoses are blocked with a wooden dowel, the heater core was leaking under the dash. I have the top hose of the TVV routed correctly. Any idea on where the AB valve would even connect? 

[datzenmike]

 

Right side to an elbow fitting on the underside of the air filter? The left hose to the intake in the area indicated connecting to the base of the carburetor? The small sense hose to a fitting shown below with loop of hose on it...