56 Buick

What gasket to use on power valve in 1956 Rochester 4GC rebuild

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I am putting a kit through the Rochester 4 barrel after cleaning same. There are a number of small gaskets in the kit and I am trying to choose a gasket for the power valve. There are 2 likely canidate gaskets I could use. One being thinner and the other a fibre gasket about twice as thick. The height of the power valve likely has impact on how it operates and the amount of fuel released given the power piston that works against it is in a set position. My query is whether the height of the gasket will make any difference? And irrespective of this should I be using a particular gasket for some other reason, for example don't use the fibre gasket? Thanks

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I would try and match the gasket to what you removed originally. That's a hard call without knowing which one is correct but I would go with the taller one personally given how these carbs are known to have a lean bog off idle. Having that power valve come in earlier might help alleviate that issue. 

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Hi Ben, the taller gasket would mean more fuel earlier but I am not sure exactly how much fuel or whether it would be noticeable.  I failed to take good notice when I pulled the power valve out but I think it was a thinner gasket.  The only other issue I am not sure about is whether a fibre gasket is okay in constant contact with fuel - not 100% certain but it seemed to me the inside of the gasket would be exposed to fuel at all times.  Thanks

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The "power valve" should not operate at normal intake manifold vacuum levels.  Only when the manifold vacuum level drops to about 5" Hg, or there-abouts.  With more vacuum, it should be closed.  Off-idle response would NOT be affected by the power valve, I suspect, as it is related to the main system rather than the idle or idle transition systems.

 

IF there are any hard deposits in the idle feed tubes, which can also have a "sized" orifice near their bottom tip, that can restrict the idle fuel a small bit, but if the sized orifice is similarly caked-up, that WILL affect the amount of fuel that enters the idle tubes and idle feed holes in the bottom of the carb body.  To the extent that the carb will not idle on the idle system (once it gets hot enough for the automatic choke to come off).  The bottom orifice can be re-sized with some twist drill action, but just enough removal to get to "brass".  These deposits will NOT be removed by any carb rebuild "soak" fluid, by observation.  Which means only "mechanical" removal works in that case.

 

Just some thoughts,

NTX5467

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The gasket under the power valve should be a fairly thin fiber washer.

 

Changing the thickness will not effect the total amount of fuel provided by the power valve, but might effect the timing of the activation of the power valve. Maybe by 1 or 2 micro-seconds. Too thick a gasket might cause the valve to always be open, which would be much less than desirable.

 

Rochester themselves used a shorter plunger on many of the power valves to SLOW the timing of the valve, not advance it.

 

I would use the thinner washer.

 

Jon.

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I just noticed the comment "more fuel earlier" in an earlier post on this thread.

 

The Rochester power valves are basically either open or closed; there really is no partial opening.

 

Rochester offered at least 19 different power valves for the 2G and 4G carburetors. There are two different physical valves, 2 different plunger heights, and several different calibrations. The most common calibration (and the valve commercially available) is one with a long plunger and (4) four #54 fuel orifices.

 

The different height plungers were to change the timing of the operation of the valve.

 

Several years ago, we developed a spring kit for the power valve actuator valve, to assist in tuning when the owner had changed the camshaft to a camshaft providing significantly less vacuum.

 

So the valves are adjustable in two dimensions: (1) timing, and (2) fuel orifice size.

 

Jon.

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Thanks for the replies. I would have used the thinner gasket but I have now already put it back together.. i ran the car the ofher day.and it seemed to run okay. The idle was rolling a bit but I am yet to fine tune it. If the power valve was activating at ixdle and low spesds I am curious to know ahat signs I may see? 

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The 2 idle mixture screws in the rebuild kit were different to those I pulled out the carb. Those removed had a long gradual cone shape but the replacements had a long thin cylinder type nose before a short steep graduating cone shape. They look very different but I am not sure what this means practically.

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As long as the idle mixture control screws allow the required volume of fuel (these control volume, NOT the mixture), the profile of the screw is somewhat unimportant.

 

Basically, there are two different profiles: (1) the oldest, which is a large angle, short taper, and (2) a newer design, which is small angle, long taper.

 

Rochester started playing with the small angle, long taper screws on some carburetors in the 1950's. The long taper gives the tuner the ability to more finely tune the idle circuit, as changing the volume of fuel requires more rotation of the screw(s).

 

Fast forward to 1968, when Federal Smog Emission became the law of the land, and virtually all O.E. carburetors started using the long taper for more precise adjustment. Rochester was just ahead of their time on this one.

 

Jon.

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