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| Carburation Question https://slantsix.org/forum/viewtopic.php?t=8288 |
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| Author: | RG [ Mon Feb 02, 2004 12:36 pm ] |
| Post subject: | Carburation Question |
The choke pull-off on a Carter BBD has a stem that works independant of the diarhram. Does any one know why that is? What would be the purpose? Thanx. |
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| Author: | Doc [ Mon Feb 02, 2004 3:59 pm ] |
| Post subject: | |
I have seen an outer sleeve on some of the pull-offs, I think it is there to keep the arm straight as it retracts into the diapharm. DD |
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| Author: | SlantSixDan [ Mon Feb 02, 2004 6:29 pm ] |
| Post subject: | |
OK, choke pull-off history: Through the 1963 model year, the pull-off was integral to the carburetor air horn. It consisted of a metal piston sliding in a cylinder cored into the side of the air horn. The piston head faced outward, and the outer end of the air horn cylinder had a passage to manifold vacuum. The connecting rod extended from the back of the piston through a slot in the air horn's inner wall and connected to an arm on the choke plate. When the engine was started, manifold vacuum would pull the piston, which would yank the connecting rod, which would yank the choke plate partway open. The degree of pull-off was controlled by bending the arm on the choke plate where the conrod attached. This system's only problem was that gasoline of the day tended to form gummy, sticky deposits that would stick the piston in the cylinder so the choke wouldn't close, or wouldn't open, or would stay partway in the middle. For 1964, the external, vacuum-diaphragm type pull off was introduced on all Carter and Holley carburetors used on Chrysler products. It had a rigid arm made of stamped steel protruding from the diaphragm housing on the opposite side of the outer can from the vacuum hose attachment. The rigid arm was attached to the choke lever via a link with a U-bend in it. The amount of pull-off was adjusted by opening or closing the legs of the "U". This solved the stuck-piston problem nicely, but adjustment was *really* sensitive. Every link and every spring had to be adjusted EXACTLY right or else the car wouldn't start or run right when cold. The main problem was if the "U" was closed too much, the choke was unable to close completely because the pull-off link would catch the end of the slot in the choke lever before the choke plate reached its fully closed position. But the correct adjustment was just a hair away from this "won't close" position. So, for 1965, the pull-off design was revised. Instead of a rigid arm, a round-cross-section outer plunger was directly attached to the diaphragm, and this outer plunger surrounded an inner pin, which protruded through the hole in the end of the plunger and was spring loaded to snap back into the plunger if pulled out. The U-link connects to this inner pin. With this arrangement, the pull-off will only act to open the choke once the spring-loaded inner pin has been pulled such that it bottoms within the outer plunger. This way, there's leeway for the choke to close all the way even if the U-link catches the end of the choke lever slot, because this will merely pull the inner pin out of the plunger slightly. It effectively makes the adjustment of the pull-off a lot less finicky. Once the engine starts and vacuum is applied to the pull-off, the diaphragm moves the outer plunger outward, which bottoms the inner pin and then yanks on the choke lever via the U-link. There were also two variants of the choke lever itself. One had an inner and outer lever. The outer lever was actually pushed by the choke thermostat rod, and rode on a "bushing" area cast into the air horn. It was attached via a small coil spring to the inner choke lever, which was on the end of the choke shaft itself. This "Spring staged" choke allowed the choke to open a bit due to airflow past the choke plate (remember, the plate is not mounted on center, but off center, so airflow tends to push it open), even if the choke thermostat is pushing HARD to close it, the staging spring allows it to open a little. This helps avoid annoying false starts. So, counting up, that's the thermostat spring (1) acting against the pulloff (2) and its plunger spring (3) and the staging spring (4) and the airflow effect on the choke plate...that's FIVE forces all in opposition to one another. Obviously a delicate balance, which is why everything has to be adjusted properly or it doesn't work right. |
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| Author: | RG [ Tue Feb 03, 2004 4:08 am ] |
| Post subject: | |
Thank you very much S.S. Dan. I never knew there was a difference between the 64 and 65 choke diaphrams. So, from reading your article, the choke lever is adjusted when both the pin and diaphram are retracted? I am attempting to tweak a BBD but was unsure as to wheather the both pieces (pin/diaphram) need to be retracted or just the diaphram. Thanks Again. Rich |
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| Author: | SlantSixDan [ Tue Feb 03, 2004 8:10 am ] |
| Post subject: | |
The choke thermostat adjustment is separate from the choke pull-off adjustment. First, with the choke thermostat rod disconnected, verify that the choke plate is able to close fully. If not, loosen the "U" bend in the choke pull-off link or fix whatever other interference exists so that the choke plate can close completely. Then, adjust the choke thermostat. The adjustment is at the thermostatic spring on pre-'73 style chokes; later choke thermostats do not have any means by which to adjust the tension of the thermostatic spring, so all you can do if you've changed the spacing of the carb to the manifold or anything else is make a new choke rod out of appropriately-sized brass rod, carefully bending it so it's slightly longer or slightly shorter than the original, whatever you need. Then, adjust the choke pull-off. Baseline adjustment is achieved by applying steady vacuum to the choke pull-off (if it will not hold vacuum, replace it) and appying finger pressure to the choke lever to close the choke plate until it stops, then measuring the gap between the top edge of the choke plate and the inner wall of the air horn. The spec is given in the service manual or in the literature that comes with the carb kit. If it is too small a gap, close the legs of the "U" bend in the pull-off link. If it is too large a gap, open the "U". This will allow the engine to start and run more or less correctly, and you can fine-tune from there. Most problems seem to come from excessive choke thermostat tension combined with excessive choke pull-off action. |
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