Blanking vs plugging off bypass valves?

I have a S3 OTS that has been de-smogged except for the 4 bypass valves. My idle climbs a bit and will fluctuate some with a hot engine. I suspect these valves are the cause. I know there is a lot of information about the function and necessity of bypass valves on the forum but my specific question is this: can these valves just be plugged off with caps rather than blanked off with a gasket and achieve the same result. I realize the internal gaskets may be leaking but does that really make a difference if they are merely capped off and the carbs are balanced and mixtures set properly at idle? Hope this makes sense.

Robert P

well I dont see why not. but when you say capped off how are you planning to do that?
It is still simpler to make your own gasket without holes and achieve the same result.
if you have already done that and you still suspect air is bypassing someplace I would look elsewhere, double check all of the carb to manifold gaskets, if you have gasket/insulator/gasket setup there is a good chance its really getting around the carb there and not the bypass.

I guess you could make a flat plate to cover the bypass, but you still need a gasket, so why not make new thicker or rubber gaskets that you know cant pass any air and try that first

just an idea

Bob

Each bypass valve has an orifice to which a thin vacuum line is attached. The idea is to plug that off with a vacuum cap. I could replace/rebuild/block off the units but that requires removal of the carbs because of lack of in-situ access. Much easier to cap off if the result is the same.

OK, that makes sense, admittedly I am a 6 cylinder guy, so I dont know the details on the 12.

that will work if you suspect the extra air is coming from there. but as far as I can tell the bypass on the strombergs is meant to open and bypass thru an orifice inside the carb and around the throttle plate, so if its still a leaking bypass valve you may not see an improvement, but it wont hurt to try as long as you plug the hose you pull off too.

I am not familiar with a bypass that vents to an outside line, but that doesnt mean there isnt one.

looking at the haynes book on strom’s here is a pic of the bypass, if that “tube” is a nipple for a hose it would still seem to me thats on the atmospheric side of the bypass valve, meaning if the valve is compromised, or not sealing the air you still have air getting around as designed inside the carb and not from that hose. if the hose nipple is plugged it would still pull from its intended path inside.

do you know if the bypass valves were ever replaced? there is a screw adjusment to increase spring tension to change the vacuum point where they open, but again you have to be certain the valve isnt torn or leaking or its a waste of time.

Bob

On my 72 S3, there is a small vacuum hose on each bank which runs to both bypass valves on that bank’s 2 carbs. These 2 hoses were both originally connected to the same vacuum port at the right rear intake manifold. I capped off both of these 2 hoses at the middle of each bank and my idle was significantly slowed and improved in overall quality. I plan to leave it this way.