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Old Today, 02:27 AM   #1
Dustodust OP
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Location: Hillsboro Oregon
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FCR Chokes Part 3

.......In this endeavor to adventurize the downdraft pumper carbs by retrofitting the undrilled carb bodies with cold start enrichment, I came to the settle on a theory that the FCR flat racing downdrafts are adapted race track versions not designed with cold start capability because in downdraft a different system is required for various reasons .
I came to this conclusion after study of the stock CV downdraft carbs and compared them with the intended Horizontal design using applied theory.
Noiw I am building an external emulation of the CVRD downdraft cold start enrichment .
Theory is : Venturi siphon/ plunger in vacuum air stream opens allowing metered and air mixed fuel to be drawn UP from float bowl and into intake in front of slide
Horizontal uses a small .5 mm passage above float level
Downdraft uses a similar small passage that meets a larger 3mm mass airflow passage transporting fuel down
I believe there are valid reasons for this difference in design and probably why the FCR downdrafts did not have the "chokes" added
so far I have tapped into the float bowl
DSC02918
secured the tapped in brass line so that it cannot contact the float or vibrate around
here is where it ends up
DSC02932
Next step is building the emulator plungers
vacuum venturi fuel enters one end , plunger opens fuel into the second 3 mm downdraft tube from filtered air at top and then directly into carb spigot in front of slide
The stock CVRD plungers and cable fit in the tube,
DSC02923
fuel is sealed the same as stock , plunger seal contacts machined brass boss under spring pressure, pull choke lever fuel passage opens into intake port. A compression fitting is tapped into the end to pick up supply from float bowl
here are some of the parts I machined so far :
DSC02930

here it is partly assembled almost ready for soldering. I plan to encase this in aluminum later Next step is tapping the two downdraft tubes to the valve body
It seems to work as planned cable fits and pulls just like OEM CV
I did all the fit up in the frame to be sure the cables all line up
DSC02938
I figure I am about half way there
Next step will be thread tapping the downdraft tubes into the bushing and then soldering the fuel inlet valve and downdraft tube bushing to the valve body
then next step will be tapping the downdraft tube into the fuel spigot
building aluminum mount housing
then I can build the identical second on for the other carb
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Old Today, 03:14 AM   #2
Pete640
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