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Carburetor enigma

hcccs

hcccs

Well Known Member
I took my carb apart for a thorough cleaning and check and found a hole in the throttle throat I haven't seen before. I think I read about it in the Marvel Schebler handbook and they call it the BACK-SUCTION TYPE ENRICHMENT SYSTEM. The hole (fig 1) goes through the carb to the bottom (fig 3) and is exposed to atmospheric pressure. It also has a vent to the fuel chamber (fig 2). The hole should create some under-pressure at various throttle settings which should be fed to the fuel chanber. On the other hand the bottom hole is always exposed to atmospheric pressure so it doesn't add up. Does anyone have better knowledge on this?




Fig 1



Fig 2




Fig 3

 
I can't quite remember, but doesn't the gasket block that hole isolating the top and bottom?
This would allow the back suction economizer feature to function anytime the throttle isn't at the full throttle position.

Please correct me if I'm wrong, and I have a gasket I'll go have a look at.
 
Where does the shaft terminate in the top half of the carb? The top of the flloat bowl must be fed with ambient / static pressure. This is sometimes done with a hole in the top of the chamber. Some times with a shaft that goes to a port in the top of the throat. It looks like in your case they are feeding the static from the body downstream of the venturi. Pressure should still be near static there (upstream of throttle plate) and am guessing their is an Engineering reason why they now want it from a position downstream of the venturi. I know the old carbs with the source near the entrance would have turbulence issues that would mess with the statc pressure source and therefore mixture control (I moved the source on my old carb to the radius of the throat). This is probably what they are addressing by putting it deeper into the throat.

Do NOT block that hole with a gasket, if that is the only source of static pressure in the float chamber. Pic doesn't show the whole chamber top.

Larry
 
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sstocker31 said:
I can't quite remember, but doesn't the gasket block that hole isolating the top and bottom?
This would allow the back suction economizer feature to function anytime the throttle isn't at the full throttle position.

Please correct me if I'm wrong, and I have a gasket I'll go have a look at.
Click to expand...


That sounds logical to me. Could it be that the wrong gasket is fitted to my carb. Waiting for your info.
 
lr172 said:
Where does the shaft terminate in the top half of the carb? The top of the flloat bowl must be fed with ambient / static pressure. This is sometimes done with a hole in the top of the chamber. Some times with a shaft that goes to a port in the top of the throat. It looks like in your case they are feeding the static from the body downstream of the venturi. Pressure should still be near static there (upstream of throttle plate) and am guessing their is an Engineering reason why they now want it from a position downstream of the venturi. I know the old carbs with the source near the entrance would have turbulence issues that would mess with the statc pressure source and therefore mixture control (I moved the source on my old carb to the radius of the throat). This is probably what they are addressing by putting it deeper into the throat.

Do NOT block that hole with a gasket, if that is the only source of static pressure in the float chamber. Pic doesn't show the whole chamber top.

Larry
Click to expand...


The strange thing is that the vent in the float bowl is fed from two different sources, the hole in the venturi which is also connected to a hole at the bottom of the carb at the filter box. When the hole in venturi generates some suction the hole at the bottom of the carb overrides this with atmospheric pressure so where then is the desired suction? In flight there would even be some pressure slightly over atmopheric due to the ram effect in the filter box and that would counteract the suction even further. Do I make any sense?
 
hcccs said:
The strange thing is that the vent in the float bowl is fed from two different sources, the hole in the venturi which is also connected to a hole at the bottom of the carb at the filter box. When the hole in venturi generates some suction the hole at the bottom of the carb overrides this with atmospheric pressure so where then is the desired suction? In flight there would even be some pressure slightly over atmopheric due to the ram effect in the filter box and that would counteract the suction even further. Do I make any sense?
Click to expand...

suction implies a vacuum, which is less than ambient pressure. The differential between static and venturi pressures (less than static) is how the carb jets meter fuel. A float chamber must be fed a static pressure, typically found near the entry opening of the throat. It should be seeing the same pressure that the engine is exposed to, so ram air is no issue, as that is ambient as far as the engine is concerned. Other functions may also reuire ambient and it wouldn't be surprising for the designer to pull that from the float bowl chamber.

Larry
 
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I checked both of my gaskets and they both have the hole that would line up with the passages you indicate with arrows.
Maybe give M/S carbs a call, possibly they could help you out....they've been very good to me in the past.
 
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