Author Topic: Finding the best jet/rods combo  (Read 1444 times)

Offline Tyler73Deville

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Finding the best jet/rods combo
« on: December 03, 2022, 06:09:41 AM »
Hey all, new here, signed up after reading cliff's book (great book btw!) So I have some questions and would also like recommendations on my setup in the Gto. It's a 68 Pontiac 406 (16 heads, flat tops and cast iron intake) with a crower 60916 installed at 108 ICL and 1.52 comp roller rockers.
Ramair manifolds with 2.5" X pipe and finally a 3.55 Posi/TH400 stock stall converter.

 Carb was a 7028268 (dropped it on accident oops, circled in red) that ran great, it had 74 jets 43 rods and DA secondaries on a "k" hanger. Power piston spring is from my parts carb (7043234 what's the vacuum rating on these?)

Now I'm using a 7028276 main body but has a 69 airhorn which has POE (68 was a bit warped) I have already switched over the
air bleed, down channel etc.. calibrations I made from that carb onto this carb, only now it has poe added, Idles smooth at 800 rpm pulling 14" vacuum and goes to 600 in gear. Should I alter rod size due to this or no? Forgot to mention .070 main air bleeds, 1/4" float and 1.35 needle/seat as well as solid secondary air valves (still have the notched should I go back to those?)
Last question is what would actually be the best combo to use for my setup. I have BE, DF, CC, DA rods for secondaries to choose from, everything else I probably have lol. Thanks alot everyone!
« Last Edit: December 03, 2022, 06:13:17 AM by Tyler73Deville »

Offline Tyler73Deville

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Re: Finding the best jet/rods combo
« Reply #1 on: December 03, 2022, 06:03:07 PM »
After research, the power piston spring would be an ac Delco #7037851.

Offline Kenth

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Re: Finding the best jet/rods combo
« Reply #2 on: December 04, 2022, 06:57:07 AM »
The vacuum rating for #7037851 PP spring is 6-3 in/hg.

Rating for most 1968-72 Pontiac RA/HO #7037305 PP spring is 10-6 in/hg.

Offline Tyler73Deville

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Re: Finding the best jet/rods combo
« Reply #3 on: December 04, 2022, 02:33:07 PM »
Thanks, didn't think it was THAT light but considering my piston springs are 50 years old I should order new power piston springs (I did replace the air valve spring for proper tension and cam with cliff parts except I used the silver screws that came in a big pack elsewhere)
As far as selecting a good spring I've read that a spring rating about 1/2 the engine vacuum at idle (in drive or neutral?) Should be good.
If so, I'll use the light blue on the Pontiac; 14" vacuum and dark blue on the Cadillac; 20" vacuum.

Offline novadude

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Re: Finding the best jet/rods combo
« Reply #4 on: December 06, 2022, 08:02:44 AM »


Rating for most 1968-72 Pontiac RA/HO #7037305 PP spring is 10-6 in/hg.

Any idea of the free length, wire diameter, and coil count on the 10-6 in-hg spring?  I'm not sure where to find this info for OEM GM PP springs.

Offline Tyler73Deville

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Re: Finding the best jet/rods combo
« Reply #5 on: December 07, 2022, 04:02:01 PM »
This is the one that came in my 268 carb.. about 15 coils and 1 5/32 length diameter measured .19. not 100% Positive its the original one but feels just a tad less stiff as the dark blue in cliff's kit and a bit more stiff than the green.

Offline Kenth

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Re: Finding the best jet/rods combo
« Reply #6 on: December 08, 2022, 03:55:40 AM »
Any idea of the free length, wire diameter, and coil count on the 10-6 in-hg spring?  I'm not sure where to find this info for OEM GM PP springs.

I have a few NOS 7037305 PP springs and they have 15 .014" coils, with a free length of 1.268" and .199" diameter.

My NOS 7037851 springs has 15 .012" coils, 1.156" free length and .196" diameter.

HTH

Offline novadude

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Re: Finding the best jet/rods combo
« Reply #7 on: December 08, 2022, 05:20:28 AM »
Thank you!