Topic: 1959 Johnson 35 hp Super Quiet Carburetor Main Mixture Screw Threads

This topic has 6 replies, 6 voices, and was last updated 5 years, 11 months ago by chris-p.

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May 10, 2018 at 10:24 am #9877
mercuryspecialist
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In trying to help with a reported issue, a 1959 Johnson 35 horsepower Super Quiet (RDS-20?) carburetor main mixture screw threads are seemingly loose. If an insert is present, how is it retained, or does the float bowl need to be replaced?
The mixture screw is not held firm enough it would seem to prevent engine vibrations from altering the mixture adjustment. After correctly retaining the threaded insert, what would (should) "firm" the mixture screw fit?

Thank You In Advance,
Joe
May 10, 2018 at 11:53 am #75670
frankr
Participant
US Member
I don’t believe there is any threaded insert on the main (high speed) needle. It simply screws into the float bowl. Having said that, I don’t think I have ever paid any attention to how loose it is—or not. The packings and nut should secure it. Unless, of course yours is damaged.
May 10, 2018 at 9:29 pm #75715
garry-in-michigan
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Lifetime Member
If it is loose, add packing behind the packing nut. If the threads are striped there may not be enough material for a HeliCoil insert. That would leave a new float bowl your only option . . . 😎
May 10, 2018 at 10:28 pm #75721
outboardnut
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US Member
wouldn’t a 1959 be a RDS-21?
May 10, 2018 at 10:46 pm #75724
Mumbles
Participant
Here’s an ’59 RD-21 carb. Not sure what the thread pitch is but I can find out.

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May 10, 2018 at 11:14 pm #75730
frankr
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US Member
Probably 1/4-40 but don’t try to take that to the bank.
May 10, 2018 at 11:16 pm #75731
chris-p
Participant
As mentioned, every time I have been told about someone having this problem they were not aware of how the packing nut worked. Not saying that is the case here, but add new packing and tighten down.
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