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Author Topic: 302 jetting  (Read 8866 times)
sd1968z28
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« on: May 24, 2007, 12:52:24 PM »

i had my 68z out last night and it seems to be a little off on jetting.  starts and idles fine but has a bobble at cruise speed and gives out one pop every once in a while high rpm shifts.  car has a stock replacement carb from heartbeat city, 140 cam, headers and 4.10 gears.  before i rejet i thought i would get some opinions.  weather was 70 degrees nice weather for a cruise.
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rich69rs
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« Reply #1 on: May 24, 2007, 01:23:07 PM »

and the altitude was sea level, or ...... Huh
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Richard Thomas
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jdv69z
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« Reply #2 on: May 24, 2007, 02:35:29 PM »

If it's the original style replacement 780 cfm Holley, jet size would be 68/76. I found this to be lean on my Z, and currently am jetted 71/79 which works well on warm days, 80 deg F.
Even with this jetting, it will hesitate if the temperature is colder , say 30-40 degrees F. Note that I am running stock 30/30 cam. Hope this helps.

Jimmy V.
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Jimmy V.
sd1968z28
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« Reply #3 on: May 24, 2007, 03:10:08 PM »

south dakota about 1500ft
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GaryL
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« Reply #4 on: May 24, 2007, 03:37:34 PM »

72/76 jets are the normal DZ carb tune from the stock 68/76.
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Gary

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dz2869
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« Reply #5 on: October 09, 2007, 09:52:54 AM »

Can you clarify for me the 68/76 designation?  Is this the stock jet size Primary/Secondary?
 I am having a to rich condition with my Very stock 69Z
Car is in CT so I am close to Sea level

Thanks
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JohnZ
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« Reply #6 on: October 09, 2007, 10:16:10 AM »

Can you clarify for me the 68/76 designation?  Is this the stock jet size Primary/Secondary?
 I am having a to rich condition with my Very stock 69Z
Car is in CT so I am close to Sea level

Thanks

Stock 4053 jetting is #68 (primary) and #76 (secondary); if yours is set up this way, you shouldn't have any rich condition unless you have a leaking or failed power valve. Are you rich at idle, or cruise, or?

Jerry re-jetted my 4053 to 72/76 when he restored it, and it runs fine; stock, with the 30-30 cam.
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JohnZ
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« Reply #7 on: October 09, 2007, 10:19:18 AM »

i had my 68z out last night and it seems to be a little off on jetting.  starts and idles fine but has a bobble at cruise speed and gives out one pop every once in a while high rpm shifts.  car has a stock replacement carb from heartbeat city, 140 cam, headers and 4.10 gears.  before i rejet i thought i would get some opinions.  weather was 70 degrees nice weather for a cruise.

Adding headers normally requires re-jetting slightly richer, especially with the -140 cam; if the carb is jetted stock (68/76), try going up to #72's on the primary side.
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'69 Z/28
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dz2869
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« Reply #8 on: October 09, 2007, 10:38:23 AM »

The engine blows black smoke when I tach up the car hard from Idle.  Also the exhaust smells quite rich when at idle.  No headers on the car, stock exhaust manifolds with stock original dual resonator exhaust system.
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jdv69z
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« Reply #9 on: October 09, 2007, 03:14:13 PM »

Possible fuel leaking into the system somehow, possibly the fuel bowl metering block gaskets?

Jimmy V.
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Jimmy V.
jdv69z
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« Reply #10 on: October 09, 2007, 03:21:53 PM »

Can you clarify for me the 68/76 designation?  Is this the stock jet size Primary/Secondary?
 I am having a to rich condition with my Very stock 69Z
Car is in CT so I am close to Sea level

Thanks

Stock 4053 jetting is #68 (primary) and #76 (secondary); if yours is set up this way, you shouldn't have any rich condition unless you have a leaking or failed power valve. Are you rich at idle, or cruise, or?

Jerry re-jetted my 4053 to 72/76 when he restored it, and it runs fine; stock, with the 30-30 cam.

Thanks for the info on the jetting, especially on the secondary side. I am going to have to back mine down to 76's and see how it reacts; I currently have 79's as I moved up the primarys and secondary together, as that is what I had been led to believe was the correct approach. But since it's already been figured out by others, I can jet mine accordingly.

Jimmy V.
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Jimmy V.
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« Reply #11 on: October 14, 2007, 04:23:33 PM »

If you want a good base line, 72 and 76s are in the ballpark.  That's what we use in all of our restorations.  Wakes them up quite a bit.  Stock was 68 and 71s which is way too lean but good for GM emissions.  If you're performance minded, then you have to stagger jet the carb to adjust for runner length in the intake manifold.  We have spent a lot of time on the dyno to break the code on this.  Works well if your car is set up.

Jerry
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Dave69x33
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« Reply #12 on: December 04, 2007, 08:56:58 PM »

Hello Jerry,

I am in the process of rebuilding my List #4053 Holley for my 69Z, and wanted to consider a few changes from stock (tricks of the trade) that have proven successful.

1.) Have you had success with staggering jets in a 302?  I’ve read about doing this in big blocks but never in a small block.  Or, would recommend staying with the std. 72 primary/76 secondary jets?

2.) With a 302 in stock configuration, what staggered jet configuration would you recommend, or that you have experimented with?

3.) What about the power valve sizes?  Stay with the stock 105 on primary and 85 on the secondary?

4.) What secondary vacuum spring color do you recommend, the std. plain color spring?

5.) Do you use the check ball in the vacuum passage?
 
My 302 Spec's (completed by previous owner – “wanted more torque”):
>DZ block bored +60
>327 crank and rods
>flat top pistons
>stock 69Z heads
>stock Z intake and exhaust
>Pertonix Ignitor II points conversion kit & Flame-Thower II coil
>AC Delco R45S plus set at the std 0.035" gap.

Has any one else with a 302 have any experiences with the questions I have listed?  Your input would be appreciated!
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Jerry@CHP
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« Reply #13 on: December 04, 2007, 09:41:35 PM »

Stagger jetting does work.  I do this for the guys who want to be fast on the street.  Have also done carbs for a few guys in Stock Eliminator.  You compensate for intake runner length by using a larger jet.  I have spent countless of hours on the dyno and measuring EGT temps getting it right, along with the right power valves and or not using power valves.  Don't really like to share all of the technical tricks that's taken me years to prove out on a public site.  It's like letting out all of your speed secrets to the world.  And in the world of street 302s and NHRA's Stock Eliminator, I like to keep some of this close to the vest.

Jerry 
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fireZ
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« Reply #14 on: December 04, 2007, 10:10:09 PM »

I followed what Jerry said with 72 and 76s wow it was like a kick in the backside for my 68 Z. Thanks Jerry
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1968 Z28 LA Built
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