What does it do, and how can a failure be detected?
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Re: 3461S-Hot Idle Compensator
It's a bimetal strip that opens a bleed orifice to admit more air when the inlet air is very hot in order to prevent an overly rich mixture, which can cause a loss of idle speed and quality.
I've never seen any specs, so about all you can do is inspect it and make sure the bimetal strip is not broken. It should be closed at room temp, but if you put it in a pan of water and slowly bring the water to a boil it should open. My guess is that it will start to open somewhere between 100 and 150 degrees. If you want to run the test, use a meat thermometer in the water so you can keep track of the temperature rise, and let us know when it starts to open.
If your engine doesn't lose idle speed/quality in hot weather stop and go driving, then it is probably okay.
Duke -
Thanks Duke,Jack H *NM*
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Hot Idle Compensator Characteristic
Duke Checked the Hot Idle Compensator operating characteristics and found it begins to open around 130/135, with the gap increasing with temp up to about 200 degrees. New Question- For the 3461S, does the idle/off idle circuit draw fuel thru a small hole in the primary venturi's emulsion tube? The hole is just below the gasket line, close to the off idle intake in the main body. Tnx LesComment
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Re: Hot Idle Compensator Characteristic
I believe the idle/off idle fuel is drawn in through the emulsion tube that extends from the bottom of the primary venturi to the bottom of the fuel well. and the idle/off idle air bleed is the thin brass tube that extends up from the venturi.
The main air bleed is a drilled hole in the primary venturi.
Sounds like your hot idle compensator is okay.
DukeComment
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Idle/Off Idle Circuit
Duke
I think you right about the idle/off idle fuel being drawn into the emulsion tube..I just can't quite figure out the circuit path from the tube to the idle/off idle intake in the main body of the carb. (The intake is on the horizontal surface under the primary venturi mounting gasket, and in line with the idle/off idle air bleed (brass tube) you mentioned . Originally I thought the crossover was in main body of the venturi. But, I could not find a passageway between emulsifier tube and the idle/off idle air bleed chamber(just below brass tube). The only other option is a small hole drilled in the side of the emulsifier tube which is just under the main body idle/off idle intake mentioned above. My thinking is that idle/off idle vacuum at the throttle plate would pull fuel thru the hole in the emulsifier tube. Is this correct? Tnx LesComment
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Re: Idle/Off Idle Circuit
Idle and off idle fuel is metered on the basis of manifold vacuum, not venturi signal due to air flow. At idle and off idle there is not enough venturi vacuum to create a useable metering signal.
From the idle emulsion tube and air bleed that are part of the primary venturi assembly the air/fuel emulsion travels through drilled passages in the main body to the idle and transfer ports in the main bores and the idle passages pass through the adjustable idle mixture needle valves.
It's tough to explain without pictures, but there are a number of drilled passages in the main body that are plugged at the end. If you follow their paths from the front of the carb near the idle mixuture screws, you should be able to determine the flow path for the idle and off idle air/fuel emulsion.
DukeComment
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Last Stab- Idle/Off Idle Circuit
Duke Sorry if Im not explaining myself very well, but some of the terminology is new. Let me try again.. I understand the idle/off idle (i/oi)pathway in the main body. But I don't understand how fuel is emulsified in the primary venturi assy for the i/oi circuit.
In the primary venturi assembly, the air bleed and chamber for the idle/off idle circuit lines up immediately above the main body i/oi intake. Apparently fuel is somehow drawn into the chamber then fed directly to the main body i/oi intake. But,tests show no obvious connection inside the venturi assy between the i/oi chamber and the emulsion tube.
There is however a milled slot on gasket side of the venturi assy, which is cut between the i/oi air bleed chamber hole and the emulsion tube. And that slot leads to a recessed annular area around the fuel pick up part of the emulsion tube(largest dia). It would seem for i/oi operation, vacuum in the main body would draw fuel up the emulsion tube and then thru a small hole close to the annular area, and into the milled slot and into the i/oi air bleed chamber (then into the main body}. Does that seem right? TnxComment
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Re: Last Stab- Idle/Off Idle Circuit
I think you have it correct. .
Fuel flows through the main jet into the fuel well below the booster venturi emulsion tube. Manifold vacuum is transmitted to the emulsion tube, but bleed off to some degree through the i/oi air bleed. The path is probaby as you describe, but without having the pieces in front of me, I'm not sure. It's been awhile since I had an AFB apart. As long as the throttle blade is above the idle hole and some or all of the transfer slot in the throttle bore, fuel air emulsion will be drawn through the i/oi circuit, but decreases with decreasing manifold vacuum.
As the throttle is opened creating more air flow and a higher depression in the venturi, fuel begins to flow in the main circuit and enters at the booster venturi nozzle. Meawhile fuel flow through the idle circuit decreases as manifold vacuum is reduced. This called "transition" (from the i/oi circuit to the main circuit) and is the trickiest aspect of carburetor calibration.
DukeComment
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