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 Post subject: primary jet size
 Post Posted: Tue May 29, 2012 5:25 pm 
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Can the primary jet size affect the secondaries or the overall performance of WOT. Last night I raised my primaries by three. (74 pr, 87 Sec on a 800 Holley spreadbore mechanical) and it seems to run better overall, the best yet, but the problem is not quite solved yet. A couple of days ago I went lean, 71 p and 77s and it defiantly was no liking it. I know I am going in the right direction, but should I stop and focus my attention on fuel pressure, maybe an electronic fuel pump. The increase in jet size might just be adding more fuel so it takes longer to run dry if that is whats going on.

Just primer


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 Post subject: Re: primary jet size
 Post Posted: Tue May 29, 2012 7:07 pm 
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If the jetting on an engine is too lean it will seem like its running out of fuel at the top end. If the driver lifts off the throttle a little bit at high RPM and it goes harder then its generally too lean. Actually running out of fuel is similar but you have to lift off a lot more to bring the mixture back into line and make power again.


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 Post subject: Re: primary jet size
 Post Posted: Tue May 29, 2012 7:17 pm 
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Keep jetting until it slows down (MPH) then go back 2. The wagon likes 82's square in a 850.

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 Post subject: Re: primary jet size
 Post Posted: Wed May 30, 2012 9:20 am 
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Yes, I agree I have to keep jetting until I see where it peaks. I finally know that I am in the right direction. I have always thought that I should jump three at a time. Is this right, should I jump the front by three until it starts to react negative. I am not to worried about the back, because I went all the way up to 90 and it seemed to slow it down. I have tried the 70's and it was defiantly running out of fuel. The upper 80's seems to be perfect.

So am I correct? Jump the front three at a time, or will I miss something.

Yesterday I took my old plugs to the Hotrod shop and the owner who I have trusted for many years, said I should leave a factory gap of 40. I have been tweaking my own to 60 and I probably have them all crazy. I used to get a factory gap of 60 by ordering them for a 76 Olds delta, (I run an Olds 455) but with the aluminum heads I have to have a thin tapered plug. Some of my techniques have maybe just confused my tuning and were backwards. I have an Accel coil and factory HEI but my spark might be stronger with less gap.

Just primer


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 Post subject: Re: primary jet size
 Post Posted: Wed May 30, 2012 9:36 am 
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General rule of thumb is 2 steps on the jet, not 3. Not saying your method won't work, just that I've been taught to only do 2 and then as Steve says, once you cross over and the car slows down you back up to the previous numbers. I'd start with the factory calibration though and work up from there. Each time you change jets, do both primary and the secondaries at the same time so you keep things "close" as the factory setting isn't usually very far off.

Plug gaps? On a pump gas engine I'd say that something close to the factory setting should be ok. Big compression ratios are what usually dictate smaller gaps.

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 Post subject: Re: primary jet size
 Post Posted: Wed May 30, 2012 11:35 am 
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Yes, but the factory settings were 62p and 85s which is a little strange to me. I am kind of in a zone right now where I am seeing results of were I am at. The power changed drastically when I went from 71p to 74. I understand what you mean by starting from factory settings, but how can they calibrate a carb from the factory that is not engine specific. I know Olds had 800 Quadrajets, but to me that is a whole different animal. I have never seen a secondary bore and boosters so big on a street carb as with the 800 Holley spreadbore. I will go by two, and change my plugs to a new set at 40. Still thinking I should build a new Isolation box for the Ram air. i saw my ram air even change the reaction at cruise speed. That was a different engine, cam and carb.

Just primer


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 Post subject: Re: primary jet size
 Post Posted: Wed May 30, 2012 12:35 pm 
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just primer wrote:
Yes, but the factory settings were 62p and 85s which is a little strange to me.

That's cause it's a spreadbore. The big venturis need a big jet and the little ones need a little jet.

Rick


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 Post subject: Re: primary jet size
 Post Posted: Wed May 30, 2012 11:44 pm 
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AC Delco's are now made in Mexico. They were supposed to be gaped at 40, they were at 47, some at 45. Not to long ago the pre-gaped were always right on. I used to check them everytime, and before that pre-gap didn't exist. I am running out of plugs that I like. I have tried the multi-tips and wasn't impressed. I used to use Bosch, then I needed a thin tapered plug for Edelbrock heads. I have swore off Autolight. They were too cold to burn off the oil on the plug. I have sworn of Champion. I once had a straight 6 and a plug went bad in one month. Though I had lost a cylinder. I am running out of company's that I feel loyal to.

Do the spark plug gap pliers, like in Summit work really good? Do they keep the strap parallel and straight?

Just primer


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 Post subject: Re: primary jet size
 Post Posted: Thu May 31, 2012 5:33 am 
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And gap your plugs at .028 to .030

Big plug gaps are old school


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 Post subject: Re: primary jet size
 Post Posted: Thu May 31, 2012 8:48 pm 
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I agree with Scott. It more important to achieve AMPS flow rather than high voltage with a large gap.


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 Post subject: Re: primary jet size
 Post Posted: Fri Jun 01, 2012 7:54 am 
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We went from 35 to 25 at 14:1 It seemed to help mainly in the heat. ;-)

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 Post subject: Re: primary jet size
 Post Posted: Fri Jun 01, 2012 1:48 pm 
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So I guess I kind of screwed this all up. I was changing jets in the back until it seemed to slow down. I went as high as 90 which is the highest I have right now. Then I started to mess with the primary just by chance, because I really thought that the problems were secondary only. Now I went from 74p ,84s up to 76p, 84s. It really did not react as good. Slowed down and even seems to pop a little as it tries to take it all in. So of course 75 should be ideal. However I have played with a million different jet sizes on each side one at a time. I didn't know any better. Ken said to change both at the same time and Rick said to keep the wide gap similar to the factory.

So now I may be lost. Should I use the 75p and then put 98 in the secondary side to keep the gap that the factory had (62p,85s)? I have kept my selection of jets set under 90 because I never thought I would need anything higher. Buying single sets at a time is kind of like eating one potato chip.

Just primer


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 Post subject: Re: primary jet size
 Post Posted: Sun Jun 03, 2012 10:48 pm 
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I now know exactly how the car reacts when the jets are too high. I had a bowl screw strip out on the 800. It was already tapped out to 1/4 20 so I transferred everything including cams onto my 650 and I placed 75 in the front and 87 in the back. Stock was 83 and it poped and slowed just like when I stuck a 76 in the front of the 800. After going down to 83 it performed good, but still does not feel the same in the primaries as the 800. So I have a heli coil kit coming,(Amazon is so cheap) and a 90-100 jet set. I believe that 74 is perfect in the front of the 800, and I have had the back as high as 90 without a real over rich reaction. So I need to go up from there until I find it. I have never learned so much about Holley's as in the last month. I would always just guess and then leave well enough alone. I used to run jets in the 50's and 60's but that was with the old engine and w-30 cam.

Just primer


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 Post subject: Re: primary jet size
 Post Posted: Sun Jun 03, 2012 11:07 pm 
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If you block the PV and just run main jets I can work out the jetting to give you even AFR down primary and secondary spreadbore barrels. Tell me the venturi size of prim and sec. Once you tune it with mains only then you can resort back to a PV in the front if you want to and just alter the front jet to get your AFR's back to where they were without the PV.
See its difficult tuning different size venturi but there is this way and it will work.
You need to establish first a reasonable close AFR front to back. I can tell you the jet split but only if there is no PV.


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 Post subject: Re: primary jet size
 Post Posted: Mon Jun 04, 2012 12:12 am 
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What does AFR stand for? Air flow? The only thing is that when I plugged the back PV it idle a little rougher, with a 10.5 and a 4.5 in the front it idles almost like I have fuel injection and I can hear the lope of the cam in perfect harmony. With out it it shakes a little and jumps in and out a little. When I went from 74p to 76 it was rich. I tried 75 and it still poped like it was rich, so 74 seems real strong on the primary. Just need to figure the back half. It is model R6211, 4165,800, primary venturi DI 1-5/32, Secondary venturi DI 1-23/32, Primary throttle bore DI 1-3/8, Secondary throttle bore Di-2. I have heard that a pv was equal to 8 jet sizes but not sure if that is accurate.


Just primer


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