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Author Topic: 302 jetting  (Read 9001 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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« 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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« 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.
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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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« Reply #15 on: December 05, 2007, 11:35:04 AM »

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

If you run a 105 (10.5" Hg.) power valve on the primary, you'll be in it almost all the time with a 30-30 cam, which only pulls 9"-10" Hg. vacuum at idle; the stock primary power valve is a #65 (6.5" Hg.), and the secondary is a #85.
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Dave69x33
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« Reply #16 on: December 05, 2007, 09:16:57 PM »

Jerry,

I understand your position.  Proven knowledge does not come cheep - in time or money.

JohnZ,

You’re probably right on the power valving.  While restoring my Z back in Sept. '01, I experimented with jets and power valves with sequence listed below base on spark plug readings, vacuum readings, etc.  In short, less seamed to be better.  With my engine set up, I get about 12" - 14" Hg, so experimenting with 105 appeared reasonable.

Primary - Jets: 68 to 72 back to 68, Power valves: 85 to 105 back to 85.
Seconday - Jets: 72 to 76 back to 72, Power valves: 65 to 85 back to 65.

In the book, Holley Carburetor Manual by Haynes, Ch. 9 Reference and Specification, for the Model 4150 780, it list 72 jets for primary, 76’s for secondary, and power valves 105 primary, 85 secondary.   Yet the 1969 Chevrolet Chassis Service Manual, p. 11 in specifications for the Holley carb (for 302's) list jets 68 primary, 76 secondary.

All spec's are in the same ball park but must assume they are set more for emission compliance rather than optimum performance?

As other have noted, the 72P/76S jet combination appears to be the ticket but again, I wonder what power valve combinations, etc. are being used?

I suppose that I try the 72P/76S jets with the 85P/65S power valve combonation next.   

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AFONEFE
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« Reply #17 on: December 05, 2007, 10:07:54 PM »

72 and 76s are the ticket for sea level or close to it.  6.5 power valves are what works best in the 302, just like John pointed out.  I will say that we also sometimes block the rear power valve. 

Gary Sommers 69 Z28 has been a test bed car for us and it runs in the mid 13s in 90 degree weather, granny shifting with street tires and through the mufflers.  I did a Stock Eliminator carb set up on this car.  With a good driver and traction this car would run in the mid 12s.  He was on the chassis dyno this summer and it made 350 hp at the rear wheels.  A good Stock Eliminator 302 engine will make 460-470hp at the flywheel.       

Jerry
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GaryL
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« Reply #18 on: December 05, 2007, 11:03:01 PM »

I have mine tuned with the 72/76 jets and it runs very nice. Off idle on up. I run Thorley headers and have the full vacuum advance modification JohnZ has recommended. However, my dist curve is a little different in that I use 18deg initial and 18 deg mechanical. Yes it flat works with the 30-30. Not a hint of pinging with 93 octane.

Hey AFONEFE, what cam, SCR is the Gary Sommers engine running?
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Gary

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Jerry@CHP
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« Reply #19 on: December 07, 2007, 07:33:08 PM »

Stock Eliminator profile camshaft, .485 lift.

I was at AFONEFE's place when writing the post.

Jerry
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GaryL
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« Reply #20 on: December 07, 2007, 09:08:37 PM »

Stock Eliminator profile camshaft, .485 lift.

I was at AFONEFE's place when writing the post.

Jerry

OH. Thought he was Jerry also. Embarrassed
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Gary

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Dave69x33
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« Reply #21 on: December 08, 2007, 01:45:03 PM »

AFONEFE  & GaryL,

Thanks for the input.

Sounds like 72 primary & 76 secondary jets are a good baseline for a stock 302, with stock exhaust manifolds and the 30-30 cam, yielding 9" - 10" Hg  manifold vacuum.

The power valve tuning tip that I read recommends picking a PV 1" - 1.5" below manifold vacuum at idle.  Thus, for a typical stock 302 yielding idle vacuum of 9" - 10" Hg, then a #65 (opens at 6.5" Hg) or #75 would be appropriate.

Again, my 302 block was bored to .060" over, and running with a 327 crank, flat top pistons, and the GM P/N-3972178, 70-71 Z28 mechanical cam (and 3:55 rear gears).  My engine combination yields about 12" Hg. of manifold vacuum.

Staggered Power Valves:

For reader clarification, was purpose to stagger the original PV #’s (with original PV #’s noted in this discussion on the original 69Z 302 with the 780 carb), to optimize fuel delivery during the transition between primary and secondary 4-barrel opening, yet provide good power balanced with good emissions and economy?

Given that I get 12" Hg manifold vacuum, is it recommend that I select #85 power valves in both the primary and secondary metering blocks?

AFONEFE.....you mentioned that #65 PV's work best in a 302. I assume you mean in both the primary and secondary metering blocks?  Or do stagger the PV's with a #65 primary and #85 secondary as JohnZ recommends?

Or, is it wise to match the PV's in both primary and secondary metering circuits so that both PV's are pulling open together and enrich fuel supply to all four barrels simultaneously?  Then when power demand increases, further dropping manifold vacuum and pulling the secondary throttle plates open (via the vacuum secondary diaphragm spring), the carb is set for optimal performance rather than optimal emissions and economy?







 
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GaryL
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« Reply #22 on: December 08, 2007, 05:34:28 PM »

I guess I missed the 8.5 PV on the secondaries. I thought the 4053 carb had 6.5 on both sides.  Huh
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Gary

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Jerry@CHP
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« Reply #23 on: December 09, 2007, 09:57:49 AM »

Use 6.5s in both ends.  You can also block the rear power valve and richen the rear jets four steps to comenpsate.  Works well too.

Jerry
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