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kawagumby · 03-01-2021, 07:33 PM

Hi all,
I have some questions regarding the fuel adjustment needle, orifice (jet), and housing. I have a 1931 roadster with the Zenith carb. The fuel adjustment jet is just an orifice in the casting, which is relatively round and seems flow-worthy.
When I got this car it was running very rich...the fuel adjusting needle was not coming close to being full-in due to corrosion in the brass-housing thread area. I cleaned that up and the car runs OK, but is still producing the gas-smell and sooty spark plugs common to that circuit being rich (around town driving).
I measured the needle length as it was fully threaded into the brass housing (while in the carb casting) and got a measurement. I then took the needle out and placed it in the casting orifice without the housing and took a measurement. The difference was 3/16", meaning the needle while fully seated in the brass threaded housing was out a full 3/16" from the orifice. Now, I've read where some needle tips are gouged by owners turning the adjustment full in too much, but that seems impossible with the setup I have. The brass needle housing has a stepped section held together with a small snap ring that prevents the wider part of the needle from going further in. I ordered another needle and it is mostly the same idea (some dimensions are inconsistent but I rack that up to typical model A replacement part variations), so I don't think the needle is the issue. As an experiment I machined the needle a bit so it would go another 3/32" closer to the orifice. This helped, and the gas smell is gone, and the car now starts almost instantly! But, I digress, as I am curious as to whether any of you know if the needle is supposed to insert completely into the casting orifice....as having to machine the needle to make it work better seems kinda mickey-mouse. Could the brass housing itself be a culprit?
Thanks for any input...
Tom

03-01-2021, 07:33 PM	#1
kawagumby Senior Member Join Date: Nov 2020 Location: Hollister, CA Posts: 133	Fuel adjustment system question... Hi all, I have some questions regarding the fuel adjustment needle, orifice (jet), and housing. I have a 1931 roadster with the Zenith carb. The fuel adjustment jet is just an orifice in the casting, which is relatively round and seems flow-worthy. When I got this car it was running very rich...the fuel adjusting needle was not coming close to being full-in due to corrosion in the brass-housing thread area. I cleaned that up and the car runs OK, but is still producing the gas-smell and sooty spark plugs common to that circuit being rich (around town driving). I measured the needle length as it was fully threaded into the brass housing (while in the carb casting) and got a measurement. I then took the needle out and placed it in the casting orifice without the housing and took a measurement. The difference was 3/16", meaning the needle while fully seated in the brass threaded housing was out a full 3/16" from the orifice. Now, I've read where some needle tips are gouged by owners turning the adjustment full in too much, but that seems impossible with the setup I have. The brass needle housing has a stepped section held together with a small snap ring that prevents the wider part of the needle from going further in. I ordered another needle and it is mostly the same idea (some dimensions are inconsistent but I rack that up to typical model A replacement part variations), so I don't think the needle is the issue. As an experiment I machined the needle a bit so it would go another 3/32" closer to the orifice. This helped, and the gas smell is gone, and the car now starts almost instantly! But, I digress, as I am curious as to whether any of you know if the needle is supposed to insert completely into the casting orifice....as having to machine the needle to make it work better seems kinda mickey-mouse. Could the brass housing itself be a culprit? Thanks for any input... Tom Last edited by kawagumby; 03-01-2021 at 07:48 PM.