On some Rochester 2-bbl's and Q-Jets, the gasket between the throttle body and the float bowl sometimes has what's called a 'vapor vent passage'. Basically, it's a slot in the gasket that leads from the throttle bore (primaries only on the Q-jet) directly to the outside air. A rebuild kit typically contains gaskets with and without this vapor-vent. When is this vapor-vent needed? What's it for?
Different vent. Good book, though I've read through the whole thing and haven't found an explanation. The Float Bowl is vented to the air-horn within the air-cleaner. In some carbs there's a second "idle vent" that opens to the outside air when the throttle is closed. What I'm trying to figure out is the "Vapor Vent Passage", a cutout in the throttle-body gasket. It exposes the throttle bores to outside air just above the throttle blades. Page 36 of that book shows a 2-bbl exploded diagram. The throttle-body gasket (#50) has the vapor vent passages. Actually, here's a better picture from an overhaul kit. The "vapor vent passages" don't line up with any casting. They extend beyond the edges of the carb body. They seem clearly designed to provide passage to outside air - a built-in leak, if you will. What baffles me is why would you want a leak in that part of the carburetor? What purpose does it serve?
The Carter AFB's had holes drilled into the bores above the throttle plates(in '65 anyway). The Service Manual says they are for venting vapor to help hot starting. I don't see them doing much good though. I don't think the Rochester 4GC had them. Maybe that's what it's for. o No: It seems to me to be a source for dirty air to get in.
Now that makes sense to me - thanks for the post! I agree that it appears to be a good source for dirt. Granted, the vacuum is significantly higher below the throttle plates and these vapor vents are above, but I would imagine there is some intake through these passages. I opted to use the solid (non-vented) gasket when I rebuilt my 2GV. I also plugged the idle vent into the float bowl (the main vent still exists). The idle-vent is to prevent hot-start problems. It also was a source for gasoline odors when parked in the garage. Perhaps these two changes will cause hot-starting troubles - I'll wait and see.
It does not seem reasonable that those gasket cut-outs would lead to outside air. It would allow UNFILTERED air into the engine. If they lined up with a passage in the carb, and therefore to a source of filtered air, that would be a different story--but you've said they vent to the outside, not to a carb passage. First guess: On some carbs, there IS a passage on the carb body that those gasket cut-outs utilize. Those gaskets are included in the carb kit to fit only the carbs that would actually need 'em, they are not for general use on "whatever" carb you happen to have.
This should explain some things. The vent is intended to allow the vapors to escape to the air cleaner, mixing with the air, and being again, pulled into the carb, as di;luted vapor. The problem here is due to the temperature of the fuel in the bowl. If the temperature in the float bowl rises sufficiently, the vapor will be pulled directly into the carb venturis as excessive fuel. The resulting rich mixture will load the carb up, until the engine stalls. If the vapor is allowed to expand into the incoming air stream, the vapor concentration is diluted, lowering the effect on the mixture. One other thing to remember as well is that if the vapor were released into the engine compartment, the possibility of a leaking spark would create a fire hazard. The air cleaner venting allows the vapor to expand, and remix with the incoming air, diluting its effect on the mixture. The set up vents the vapor into a passage sending it into the air cleaner. Ray
Some Rochester 2-bbls have an external idle vent. It is opened by a tang on the throttle arm whenever the gas pedal is released (ie - at idle). The float bowl is already vented internally to the air cleaner. The reason for this is not so much cleanliness, but consistent performance. A carb works by creating a difference in air pressure between the venturi and the float bowl. The low pressure in the venturi sucks the fuel in. Put another way, the atmospheric (higher) pressure in the float bowl pushes the fuel out. If the float bowl were vented to the outside, then a dirty air filter would create more suction and the mixture would run progressively rich. By venting the float bowl to the air cleaner, 'downstream' of the air filter, both the float bowl and the venturi see the same standard pressure so the relative difference stays the same. Thus, a dirty filter will not affect the fuel mixture - performance will still go down some, but the mixture ratio will stay the same. Venting to the air cleaner presents a problem, however. With enough jostling and vibration some fuel could splash up the vent pipe and get sucked into the airstream. This unmetered fuel would drastically affect the mixture (and performance). A 'solid' airhorn gasket and splash shield around the fuel inlet helps to prevent this. Another problem is hot-soak. With the car at idle and little airflow through the engine compartment, the engine begins to warm up the carb. The fuel in the float bowl can percolate and vaporize. These vapors will travel up the vent and into the air cleaner. At idle the engine is just sipping fuel, so a few extra vapors can easily make it run too rich, causing it to stumble and stall. The mechanical idle vent opens a second vent into the float bowl, allowing these vapors a chance to escape directly into the atmosphere (well, engine compartment). Above idle speeds there is sufficient airflow through the engine bay and enough cool fuel flowing from the gas tank to keep things comfy. When the car is shut off, all the airflow stops so the heat really builds up. If all of the vapors collected in the air cleaner then it would take several seconds of cranking to expel this overly rich mixture before the car would start - aka, a 'hot-starting' problem. An external idle vent 'cures' this problem by venting the vapors overboard. This idle vent is also one reason why the garage gets very aromatic after pulling in. The idle vent is open whenever the car is stopped. With the engine off there is zero airflow, so all the heat really warms up the carb. The float bowl vents a lot of gasoline vapor out the idle vent and into the garage. Not an explosive amount, but certainly noticeable. This is an emissions no-no, so later cars have the idle vent plugged and/or a charcoal canister to capture these fumes. Back to my original question - the vapor vent passage in the throttle-body gasket. Based upon Wildcat2's post, I believe it's also a 'hot-start' fix by venting vapors overboard. Yes, hard to believe that a carb would have a built-in hole in the side, but follow this logic: The 'venturi effect' that makes a carburetor work is based upon a fast-moving stream of air, not a vacuum. A caburetor will meter fuel whether the air is being sucked from the bottom (the engine) or blown in from the top (a turbocharger). Airflow through the venturi generates the low pressure. Once past the venturi, pressure returns almost to normal. The 18"-22" of vacuum one measures with a gauge occurs below the throttle plates due to their restriction, not from the venturi. Indeed, the 'timed vacuum port' on a carb depends upon this. This port is located just above the throttle plates. At idle, there is no vacuum present at this port. As the edge of the throttle plate moves past this port, just above idle, vacuum appears for the distributor advance. This allows no vacuum advance at idle for better emissions, yet normal vacuum advance at all other times for reasonable performance. Back to the 'vapor vent passage' in the throttle body gasket. This vent is on the opposite side of the bore from this timed port. It, too, sees no vacuum at idle. However, being on the opposite side means it's half of the throttle plate is moving down as you press on the pedal, not up. Above idle, there is no plate edge moving past the vapor vent to generate a vacuum. That vent only sees increasing local airflow. At WOT, the pressure drop across a 2-bbl carburetor is only 3". That would mean some suction through this vent, but the opening is tiny so it wouldn't affect metering that much. This vent would do nicely to prevent 'hot-soak' starting problems. Imagine this - you park your Buick and run an errand for a few minutes. The float-bowl heats up and vapors are pushed out the float-bowl vent into the air cleaner. The vapor, being heavier than air, will settle into the venturis and collect above the closed throttle plates. Except now they'll find an escape route and exit the carb. You come back from your errand, turn the key and the car starts immediately. Without the vent, the vapors would make the mixture too rich, and you'd have to crank and crank before the rich mixture is expelled and the engine will fire. Now, not that I've seen very Rochester 2-bbl out there, but I've yet to see one that has a casting or fitting to match the 'vapor vent passage' in the throttle body gasket. Strange as it may seem at first glance, it's truly an open passage to outside air. However, that doesn't mean that a vented gasket can be used on just any 2-bbl. Some engine/carb/body combinations may have a hot-soak issue that this cures, others may not. In theory, the carb # will denote exactly which gasket you need as determine by GM engineers. In practice, carb kits say to match the old gaskets. Unfortunately, after several decades of rebuilds by persons unknown, the old gaskets may not be the originals so errors in matching may occur!
