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 Post subject: Re: primary jet size
 Post Posted: Mon Jun 04, 2012 12:40 am 
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Location: Adelaide Australia
AFR stands for Air fuel ratio.
You need to maintain a split on the jetting like this due to the venturi sizes. Primary #73 secondary #94 or #95. The required secondary jet is a split between the 2. So what this means is you need to maintain this number difference. For instance if you go to a #71 primary then put in a #92 secondary.If you go #75 primary use a #96 secondary. Just keep the secondary jet 21 sizes bigger than the primary. This is how to keep the same AFR going down the primaries as the secondaries WITHOUT using a PV ANYWHERE.
The original carby had a jet slit of 23 and thats because Holley like to go conservative on the secondary and make it slightly too rich for warranty claim reasons etc and also the primary butterfly size is too small for the venturi so the primary IFR will be increasing the vacuum on the main jet even at WOT relative to the secondary side balance.
Take your pick what jet split to run, either would be close. I dont know what the power of your engine is but if its less than 400hp use the 21 jet split.


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 Post subject: Re: primary jet size
 Post Posted: Mon Jun 04, 2012 8:06 am 
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just primer wrote:
The only thing is that when I plugged the back PV it idle a little rougher...


If you have a PV in the rear you need to ditch it and jet up because there is no way to extend the pick up for the PV. You run jet extensions to keep the fuel at the jet during acceleration but can't do that for the PV.

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 Post subject: Re: primary jet size
 Post Posted: Mon Jun 04, 2012 8:36 am 
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Makes sense. Can you tell me exactly how the PV changes it and how to predict the changes. I thought it was mostly for Idle and off idle. I have a 4.5 p and a 10.5 S. I do get the feeling this would be much easier with a square bore. However I still have a spreadbore manifold and I have very little invested in the 800. I bought it at a swap meet for 15$. Mostly just the front bottom bowl screw were rigged. All of the linkage was still in place. I removed all of the choke, I really don't need it much here, it's hot most of the time.

Just primer


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 Post subject: Re: primary jet size
 Post Posted: Mon Jun 04, 2012 3:37 pm 
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The PV just by-passes the MJ and it depends on how big the PV channel in the block is as to how much fuel it will flow. The common thinking is they're not needed in a race application but it helps to have one in the primary side to make low and part throttle a little leaner and smoother. I don't know if there is any way to predict how much the PV will change things. I think you pick the PV according to how much vacuum you have. I have heard anything from 8-12 jet sizes bigger when you plug the PV.

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 Post subject: Re: primary jet size
 Post Posted: Mon Jun 04, 2012 8:05 pm 
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gearhead1011 wrote:
just primer wrote:
The only thing is that when I plugged the back PV it idle a little rougher...


If you have a PV in the rear you need to ditch it and jet up because there is no way to extend the pick up for the PV. You run jet extensions to keep the fuel at the jet during acceleration but can't do that for the PV.


I will keep that in mind. I did run extensions when I plugged the back, but I was still not in the right range of jet sizes and it still was acting like it was running out. I have considered that my fuel pump was on the way out but it ran fine with the 650 Holley yesterday.

Are you saying that the pv will still uncover during hard acceleration and suck air. I thought that it was closed at the point of any serious launch and would not be a factor?

Just primer


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 Post subject: Re: primary jet size
 Post Posted: Mon Jun 04, 2012 8:43 pm 
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The PV closes at high manifold vacuum like when it's idling or under light load, it's open when accelerating hard or under heavy load. The number on the PV indicates how much vacuum it takes to operate it.

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