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Discussion Starter #1
I think my Hollys running a bit rich so am going to strip it down this weekend and re-jet. Car was used purely for strip use before it came to me but I want to use it on the street. Question is what jet sizes are you guys using in your Holley 850 Double Pumpers those who are running them?
 

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How do you know that your rich? Are your eyes burning at idle?

To find the correct jet sizes, you need to put a fresh set of plugs in take the car for a spin and cruise a couple miles at 35, or 45 mph and turn the car while driving and pull over, and check your plugs so you can get a true reading on your plugs. Because at that speed your jets are doing most of the work.

There are 5 circuits on a holley carb and they each do something different and need to be tuned accordingly. But my guess is if it was a race car it would be rich for the street, but cars run there fastest times when they are dangerously lean.

Check your power valve also. Put a vac gauge on there and see what your at with a vacuum gauge and divide by 2 which will put you in the ball park.
 

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Holley shows the 4781,,4781-1 and -2 have 80's square.
The 4781-3,,-4,,-5 have 80's front and 78's rear. 6.5 PV all.

As mentioned, stock is a good place to start on the street.

Best,:devil:
 

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Discussion Starter #6
Holley Jet Sizes (850) D/P

Guys thanks for all the useful replies.
64IIGolf lover your spot on, my eyes burn when I stand anywhere near the engine at idle. Plugs are also wet and black when pulled with only tips being correct colour. Carb model is 4781-6 and vacuum reading is down at 5 which is a bit low from what I read for a Edelbrock performer cam and Edelbrock RPM manifold set up. I'll probably do as suggested, insert stock jets and go from there. It may be that once I strip it down I'll find it is over jetted for the street even though it is a big carb for that use.
 

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You may wish to sell that carb and get a smaller vac secondary carb, easier to tune and more street friendly.....:2cents:
 

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Discussion Starter #8
Holley Jet Sizes

Balljoint , think IM going to go just that route. Been on the Holley interactive site and that is the conclusion it comes to as well as it appears car is overcarbed for street use ,Thanks for all the advice ,new 870cfm with vac secondaries on order!
 

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Discussion Starter #12 (Edited)
Holley Jet Sizes

Interesting recommendations guys............ confused now as Holley interactive carb selector suggests the 870 cfm!
Engine is a built 454 Big Block with Edelbrock Performer cam, RPM manifold and 2 speed power glide. That change your suggestions toward what carb to use? Thanks.
 

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Discussion Starter #13
Holley Jet Sizes

Sevenzeronova or hbanger, if I could have your comments re carb size after my last posting it would be appreciated as appreciate your advices. They say big blocks need air so hence why I went for the 870cfm. Thanks Guys.
 
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