Distributer vacuum? or just fresh air?

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Big cap dizzy in a desmogged 85/60 I have the large line to the firewall for fresh air to the distributor. Threads say the other nipple in the cap is a vacuum line but...oem would have it going to the air cleaner...not the intake. There is vacuum in the air cleaner but not really much. I have no vcv in that line, and what does that do anyway in that line...If I go to the base of the carb or intake won't that be a big vacuum leak "just apullin that air thru there"? Waiting for the brain trust to help me out! and thank you..
 

OSS

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The VCV snapped into the bottom of the air cleaner housing controls when the valve opens to the air cleaner. When vacuum from the manifold pulls on the vcv, a passageway inside it opens and air can flow.
That VCV is there to shut off the passageway from the air cleaner housing to the distributor cap.
When the engine shuts off, gasoline vapors evaporating out of the blazing hot float bowl migrate up and out into the air cleaner housing ... and then migrate down into any attached hose - including an open vent hose to the distributor.
Without that VCV blocking off the flow when the engine is off, gasoline vapors HAVE migrated down into the distributor cap and HAVE caused it to explode the next time the engine is cranked over.

Venting the distributor is important. An unventilated unit will corrode away relatively quickly due to the nitric acid created inside it from the ozone reacting with water vapor.

The FZJ80 VCV is still (or was recently) available. It's the same thing as the FJ60 2F unit but with reversed pipes. Rig one up if you can get one.

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OK, I knew someone had the low down. I was planning on venting it just did not know how much vacuum was needed. So essentially there is a fairly passive negative airflow from the distributor but only when it is present opening the one way valve. Don't know for sure what filter housing I am going with but I'll make it work. Thx for the info.
 

OSS

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I looked up that description you gave and that's a check valve. It wouldn't work right in the distributor venting.
When my 2F was idling and revving, I placed my finger over the pipe on the air cleaner housing that normally connects to the distributor vent hose. There's zero vacuum that I could feel coming from that pipe. I suspect that the distributor vents to the air cleaner because it generates ozone as its sparking and the ozone wafts up the tube into the air cleaner to get sucked into the carb.
 

rhah

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I looked up that description you gave and that's a check valve. It wouldn't work right in the distributor venting.
When my 2F was idling and revving, I placed my finger over the pipe on the air cleaner housing that normally connects to the distributor vent hose. There's zero vacuum that I could feel coming from that pipe. I suspect that the distributor vents to the air cleaner because it generates ozone as its sparking and the ozone wafts up the tube into the air cleaner to get sucked into the carb.
Both my “vents” are open and have been for the last 60k or so. Can’t say I have noticed any problems with natural venting but yes like OSS said just vents no vacuum

@Sandhiller
 

OSS

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It's a vacuum actuated valve.
When vacuum is applied to one pipe, the passageway between two other pipes opens up and air can flow. When the vacuum is removed, that passageway closes.
A check valve only prevents air from flowing the other direction. It's a one way valve. There isn't enough (or any) air pressure in the distributor venting tube to make a check valve do anything.

For what it's worth, millions upon millions of engines (and distributors) have been built using rudimentary distributor venting. Maybe see what the GM and Ford and Chrysler and Honda and Nissan and Porsche and Mercedes etc guys are doing. Old time distributors just had a breather hole in the cap. Older ones had nothing.
 
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It's a vacuum actuated valve.
When vacuum is applied to one pipe, the passageway between two other pipes opens up and air can flow. When the vacuum is removed, that passageway closes.
A check valve only prevents air from flowing the other direction. It's a one way valve. There isn't enough (or any) air pressure in the distributor venting tube to make a check valve do anything
Got it. Thx
 

Spike Strip

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The Toyota VCV is more of a mechanical switch that is ON/OFF depending on various engine parameters.

A check valve is obviously just that: a one-way gate.

If you're desmogged, the check-valve is better than nothing. There are threads with pictures of exploded dissy caps from gas fumes finding their way back, then igniting when the truck is started.

You might find a good used one in the Classifieds for cheap.
 

1911

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There are threads with pictures of exploded dissy caps from gas fumes finding their way back, then igniting when the truck is started.

The dissy by its nature generates ozone gas, which besides being explosive is also highly oxidizing (i.e. conducive to rust). For every dissy that explodes, there are hundreds that have non-functioning advance mechanisms because of rust. It's a Good Thing to have the vacuum evacuation system in place.
 

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