Holly mixture screws
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Senior User
Joined: Nov 2000
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From: sunny san diego
Holly mixture screws
After tuning up my '85 460 I checked the idle mixture settings. After adjusting the primary screws I tried the secondaries.I could turn both of them in all the way and the idle did not change, but turning them out more than 2 turns would roughen the idle. I set them both 1 turn out. Is this normal? It runs and idles perfectly. Also, it is a 4180C California emissions and the screws require an allen wrench, they are located under the fuel bowls, not on the metering bodies. The float levels are good. Thanks
Posting Guru
Joined: Mar 2007
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What you are saying, is you have a 4 corner idle mixture screws? But not only do you have the screws on the side of the primaries, and also the secondaries, you also have a set of allen screw under the primary bowl?
Usually I use a vacuum guage to set the mixture right, Keeping both sides even, then smelling the exhaust, as a final measure to see how rich it is, or lean... The primary screws are usually around 2 to 2 1/4 out... If you can screw them in all the way, and the engine doesnt stall, then you have a blown power valve, or to big of carb.
Usually I use a vacuum guage to set the mixture right, Keeping both sides even, then smelling the exhaust, as a final measure to see how rich it is, or lean... The primary screws are usually around 2 to 2 1/4 out... If you can screw them in all the way, and the engine doesnt stall, then you have a blown power valve, or to big of carb.
Thread Starter
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Senior User
Joined: Nov 2000
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From: sunny san diego
There are NO screws on the metering blocks, the power valve can't be blown out because it just passed an emissions test with ease....it's the secondary screws that don't seem to work like they should, but it runs perfect. Hmmmm.......maybe I should just leave it alone. This carb has vacuum secondaries
MSEE
Joined: Apr 2004
Posts: 10,386
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From: Austin, TX
What makes you suspect the secondaries aren't opening? You can't see vacuum secondaries open if you're reving the motor out of gear. There has to be a load on the engine; it happens when you're driving.
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Joined: Dec 2001
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From: Louisville,Ky.
He said that the secondary idle screws are not working like they should.
I don't think that they are designed to be able to choke down the engine like the front ones do. They are to add some additional fuel at idle if needed. The front ones do most of the work.
I think they would only choke down the engine if turned all of the way in if the front ones were adjusted in way too far.
They are located in the base plate, not the metering blocks.
Sounds like it is set up correctly.
I don't think that they are designed to be able to choke down the engine like the front ones do. They are to add some additional fuel at idle if needed. The front ones do most of the work.
I think they would only choke down the engine if turned all of the way in if the front ones were adjusted in way too far.
They are located in the base plate, not the metering blocks.
Sounds like it is set up correctly.
