Thread: alittle guidance appreciated on 850 holley

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Hello all. I have a question. I have been running a holley 750 dp on my 505 with no power valves, thats the way i bought it. now i have purchased the new holley 850 ultra hp. It came with a 4.5 power valve. I do not see any vacuum ports on this carb nor do I need any i do not run power brakes or vacuum advanced dist. Now I was wandering how to get a vacuum reading to find out if that will be the correct power valve. Also on my old carb I was running 79 jets in front and 82 in back. I have read that if I run a power valve my jets should be bigger. I noticed the 850 came with 84 in front and rear. thanks for any info, tips or tricks with this new carb.

your intake as no vacuum ports .what did the engine have for vacuum in gear at idle with old carb ? then that would be were you could start i cut the reading in 1/2 so lets say you had 8 inchs of vacuum in gear i used a 4. or4.5 power valve if less then 3 inchs at idle i plug them then jump up on jets size so if you plug the power valves in the new carb any were from 90 to 94 i would start on the smaller size jets.if race stuff i just plug the power valves

Thanks pat. I have never checked vacuum with old carb. But I will before I remove it. This motor is just for mud bogging and ridding trails in the woods. I am running a manual trans. I try to do as much research as possible le before asking questions and more times than not it will redirect me to a post on this forum so I figured it has to be a good spot to ask questions. Will the power valve plug out of my older 750 holley double pumper work with this new 850 ultra hp?

Originally Posted by 505onrocks

Thanks pat. I have never checked vacuum with old carb. But I will before I remove it. This motor is just for mud bogging and ridding trails in the woods. I am running a manual trans. So I would check my vacuum in neutral correct. I try to do as much research as possible le before asking questions and more times than not it will redirect me to a post on this forum so I figured it has to be a good spot to ask questions. Will the power valve plug out of my older 750 holley double pumper work with this new 850 ultra hp?

yep your power plugs will work in the new carb how big is the cam at 050 . if you had power plugs in it before then you may have low vacuum ? what ever you have at idle i have aways drop down under max vacuum at idle on power valve size by 1/2

Thanks again pat. I do not know the specs on this cam. I destoyed my first 505 that I knew all the specs on and had dynoed. I bought this engine already assembled from a friend. Im not sure if i know what you mean by how big is my cam at 050. ? If that means duration@ .050 I think i am around 262/273.

if that big @050 on dur i would go to just power valve plugs

I mostly run new Holleys right out of the box without major adjustment..I'm not sure how they tune them at the factory..but they do run exceptionally well right from the box.
I feel most guys get less performance the more they play with the jets and valves.
I would seriously live with your new carb for a while before modding it.

hard to get them to run great out of the box if you have a 250+ @ 050 cam they do run ok out of the box on mild stuff He is far from mild