Need your collective knowledge guys; I Just reinstalled my original DZ engine back in my 69 z-28. Fresh steel 3inch stroke crank, good used lt-1 rods with sps screws, new 60 over hypereutectic pistons- only ones with wide 5/64 rings and 11-1 domes available, double molly rings, 1970 '178 cam-wanted some low end, factory style intake and 4053 carb. have trans ign, and headers. Distributor set to specs-4 degrees before and performance curve-vacuum conected. Currently have 70 jets in front, 72s in the back, factory setting is 68s in front 72 in back-this with all smog and castiron manifolds in place-both items removed from my car at this point. Carb mixture screws adjusted with vacuum gauge and tach dwell with goal of highest lean idle speed. Car idles good, Sharp off idle response. Great acceleration but suffer from part throttle surge, all carb gaskets appear good, carb overhauled by very reputable shop last year. What did I do wrong-too lean primarys,? Motion Performance recommends 72 jets front and back with headers. Please note, surge appears to partially disapate at speeds over 70 mph.

Too lean!

even with the LT1 cam.. you need bigger jets...

especially if you have headers on the car...

i run 74-78 in my 302 with the 30-30 cam and manifolds and it runs great.



with the LT1 cam and headers... i'd start with 73 and 78- or even 80 depending on how hard you want to drive it.

finally... do you have a powervalve in the secondary block or a a powervalve plug...

some of these carb rebuilders like to put plugs back there an that requires that you jump the secondary jets up about 6 sizes to compensate.

that could also be your problem

good luck
A

Does timing have to be -4°?

My front jets are #72 and the rears #76 (stock). I have headers and the 30-30 Crane.

I run mine (stock 302 with manifolds) with 72/76 jets, 8*-10* initial, 36* total, all in at 2900, VC-1810 vacuum advance connected to full manifold vacuum source, zero issues, runs like gangbusters on pump 93 octane. :)

Gentilemen; After studying all the submissions, I went out and increased my primary jets from 70s to 73s, secondarys from 72 to 78s. I also found the secondary power valve gone and a plug in its place. I removed the plug and put an 8.5 p/v in its place-factory also used 8.5 in original production. Also, replaced bowl and metering block gaskets with aftermarket anti-stick blue design-don't remember name. Surge is gone, car accelerates smoother. THANK YOU ALL!! Next item-timing-factory calls for 4 degrees before. What should I set, car is solid 11-1. I have to have very short tip plugs or the piston domes hit the electrodes. Please note I have access to limited amounts of AVgas (110 low lead)-5 gallons at a time- I always tell them its for my lawn tractor.

If you had the 30-30 cam I would tell you 18* initial and 18* mechanical @ 2800 Rpm. I am not real sure what you can dial in with the 178 cam. RamAirDave has an LT-1 he can only get to maybe 8* as I remember. I think the 8.5 rear PV may be too high.

1

what about the total? do you have a chart that tells how much advance the distributor gives? if it's the original distributor...
4 degrees initial is not good for anything, you could easily go like 10 degrees higher. But watch out, that high CR might give you starting problems if you go too high on initial.
Now that you have really made your carb pour gasoline you should do your self the favour of assuring that the timing is dead on:)
Low CR SBC's can handle a lot of initial. 20 degrees no problem, but 11-1 wont handle 20 initial.
Anyways, whats more important is your total.