Holley Carb 850 Question
Thread Starter
Pro
Joined: Nov 2008
Posts: 680
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From: Denham Springs LA
Holley Carb 850 Question
I have 4150 4781-2 850cfm carb that came on an engine I bought. The carb hadn't been ran in years. I bought a Holley rebuild kit and am installing. This carb has mechanical secondaries. The metering plate for the secondaries does not have idle mixture screws.
My question - where do I set the secondary throttle plate position? Should the plates be open a little bit, enough to uncover the idle slots, or should the plates be closed?
Thanks
Roger
My question - where do I set the secondary throttle plate position? Should the plates be open a little bit, enough to uncover the idle slots, or should the plates be closed?
Thanks
Roger
Drifting
Joined: Mar 2011
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From: PA
I would start with them totally closed if I was setting the carb up from scratch, but you will end up wanting a little crack of light showing. You won't have the same .020 window showing on the secondaries transfer slot though.
The secondary idle system really just allows a small amount of fuel to flow in order to balance out the carb and keep the fuel fresh in the bowl. An 850 is a good sized carb so I'm guessing you might also need some extra bypass air that cracking the secondaries will allow. Unless you have holes in your throttle blades already.
The secondary idle system really just allows a small amount of fuel to flow in order to balance out the carb and keep the fuel fresh in the bowl. An 850 is a good sized carb so I'm guessing you might also need some extra bypass air that cracking the secondaries will allow. Unless you have holes in your throttle blades already.
Last edited by boltnut; Jan 8, 2012 at 10:21 AM.
Thread Starter
Pro
Joined: Nov 2008
Posts: 680
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From: Denham Springs LA
Thanks guys.
I have the Holley tuning paper from Lars for a Holley with Vacuum Secondaries. I didn't find anything regarding setting the secondary throttle position. I emailed Lars to ask him.
I opened mined slightly, I turned the adjustment screw 1/4 turn after it made contact with the shaft. I found 2 Holley documents with differerent numbers (one said 1/2 turn, the other said 1/4 turn).
I have the Holley tuning paper from Lars for a Holley with Vacuum Secondaries. I didn't find anything regarding setting the secondary throttle position. I emailed Lars to ask him.
I opened mined slightly, I turned the adjustment screw 1/4 turn after it made contact with the shaft. I found 2 Holley documents with differerent numbers (one said 1/2 turn, the other said 1/4 turn).
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
All four corners the same, make the transfer slots square both pri and sec. and when it comes to idle mixture screws they should all be exactly the same amount out. 
Le Mans Master
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From: Gladstone MO
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Old style Holley with idle mixture screws on the primary only. No real difference in adjustment. When you end up, they will still be pretty close to the same on each side.
Newer ones have a 4-corner idle system.
Newer ones have a 4-corner idle system.
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I usually take the sec stop screw out and install it from the top side so you can easily tweek it without pulling carb if necessary. I'd put them pretty close to the same. In fact...I've opened the secondaries a hair more sometimes to get some airflow so I could close the primaries a little more and make idle screws mor repsonsive.
Unless you have a BIG motor...or huge cam...you likely won't need to drill holes. Think about it...a BIG carb allows a lot more air in naturally around all that plate area...so should be plenty of air.
JIM
Unless you have a BIG motor...or huge cam...you likely won't need to drill holes. Think about it...a BIG carb allows a lot more air in naturally around all that plate area...so should be plenty of air.
JIM
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Drifting
Joined: Mar 2011
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From: PA
What you will find is that after your primary transfer slot is set and your curb idle speed is set you can use the secondary screw for fine tuning idle speed. If your idle is a little high after setting the primaries.....just close the seconds down more. You can also open them if you find you are not getting enough bypass air ( big cubes or aggressive cam). You can open the seconds until you see too much transfer slot and then the next step is drilling throttle blades for more. For example my 462ci race motor with and 850 race carb ( 4 corner idle) did not need the plates drilled but did need the secondaries open more....the 670 on my 350ci does not need more than a crack.....
Race Director
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From: Corsicana, Tx
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Here's a couple of pics I found:
http://www.nastyz28.com/~copo/images...stop_screw.jpg
http://www.nastyz28.com/~copo/images/hreb53.JPG
http://www.nastyz28.com/~copo/images/hreb54.JPG
JIM
