302 with 750 double pumper

[69Zlemansblue]

Has anyone tried to run a holley 750 mech. secondary double pumper (#4779) on a 302? I need to replace my carburetor and I'm wondering if I should stay with the 750 double pumper or go back to the original style 4053 vacuum secondary. Engine is bored .030 over, 11.1 compression, MSD ignition, headers, and will be installing a 140 cam soon.
Have a slight hesitation with the double pumper, but really comes on strong. I would like to hear some opinions. Does anyone think the double pumper is a better carb?

Thanks, Rich

[tmodel66]

IMO the double pumper is best. If you have a hesitation it just needs adjusting or rejetting. You can tinker with it and get that out of it. I run a 780 on my 350 and it works great.

[69Zlemansblue]

Thanks tmodel66, I just wish you were running a 302. It's a different animal. I sure like the double pumper up high on the RPMs. Maybe JohnZ has an opinion, or someone with a 302. Any opinions are welcome. Rich

[tmodel66]

I'm sorry Rich I might have misled you. I left out a few details.  My old car is running about 490 HP and I come off  the line at about 8200 RPM.  I have run several 302 engines and that's why I chimed in but I'm sorry about that.



Daniel

[JohnZ]

For normal street operation, the 4053 (vacuum-secondary 4150) is a much better solution; the 302 is way over-carbed with the 780 CFM 4053 (it was only that big to make it legal with SCCA for Trans-Am racing), and the vacuum secondaries make it more driveable and easier to tune. The double-pumper is a 1/4-mile carb, and is much more finicky to tune for normal street operation.

The 4053 doesn't slow Jerry MacNeish down any - he just set the new NHRA D/Stock National Record with his '67 (10.45 @ 126.10) last month.

[Stingr69]

I am running a 650 DP 4777 on my 302 and it took a bit of tinkering to get the stumble out. Try different pump cams and keep adjusting the lash every time you make a change in the pump cam or position. The adjustment is critical.

Let me know if you want more detail and I will provide it.

-Mark.

[69Zlemansblue]

Thanks JohnZ, I can't argue Jerry's success. Now my next question. I don't have a vacuum advance on my MSD distributor, it's mechanical. How would or could I make the 4053 carb work for me with this kind of set-up? What would I have to do with the vacuum lines, etc?

Thanks Mark, I would appreciate anything you could tell me. My carb is simply worn-out. I'm just checking my options.

Rich

[JohnZ]

Thanks JohnZ, I can't argue Jerry's success. Now my next question. I don't have a vacuum advance on my MSD distributor, it's mechanical. How would or could I make the 4053 carb work for me with this kind of set-up? What would I have to do with the vacuum lines, etc?

If you primarily race the car, the lack of vacuum advance isn't much of an issue. However, if it's primarily street-driven, you're leaving a lot on the table by not having a vacuum-advance distributor. See reply #3 in this thread:

http://www.camaros.org/forum/index.php?topic=64.0

[RAfbody]

The 4053 doesn't slow Jerry MacNeish down any - he just set the new NHRA D/Stock National Record with his '68 (10.45 @ 126.10) last month.

That is actually a '67 Z Jerry set the record with.

[JohnZ]

That is actually a '67 Z Jerry set the record with.

Yup - revised the typo in my post.

[69Zlemansblue]

Thanks JohnZ, that article you wrote about vacuum advance is an eye-opener for me. I've been running without it for 10 years now.  I never really understood how it worked, now, thanks to you I have a grasp of it. I can't thank you enough for your help and I can't wait to get my Z running the best it can. BTW, this car is used on the street, 99% of the time. Drag race, once every 3 or so years.  Rich

[NorthStarSTS]

For about 15 years, I ran my 69 302 with a 850 double pump, manual choke on a Offy intake and long tube headers with no other mods. Ran like a rocket but couldn't pass a gas station.

[Stingr69]

Thanks Mark, I would appreciate anything you could tell me. My carb is simply worn-out. I'm just checking my options.

Rich

Here is where I am today. Ignition curve is quick and short with a proper vacuum advance.

I run factory manifolds with the repro chambered exhaust and a Crane f-278-2 cam that has similar opening and closing points as the LT-1 cam but more lift. Bee hive springs and roller tip rockers. The rest is pretty much stock. Not too crazy.

The carb is a Holley 4777 650 double pumper with an electric choke kit.

After hooking up my wideband O2 sensor and setting all the baseline/idle mix adjustments, I went for a ride and recorded the air fuel ratios. The wideband told me the primary liked the 66 jets but on colder mornings I could get lean popping at tip-in so I have swapped in 67's. The 67's or 68's are in the sweet spot so 68's are probably fine. The secondary had 76's in there and they work fine. The power valve is a 6.5 so nothing fancy there.

On the accellerator pump cams, the white cam works fine for the secondaries. I could pump in more gas with a more aggressive pump cam but the engine does not "need" it. The primary cam is a red cam in the first hole. Set the linkage so there is no slack and some free travel left at WOT. The importance of this setting can not be stressed enough.

I needed .028" shooters to cure the stumbles.

The engine's preference for what could be considered "conservative" jetting tells me the restrictive exhaust is probably holding it back a bit but the car runs well and is a lot more fun to drive now that it does not smell like gas fumes any more.

Hope this helps,

-Mark.

[69Zlemansblue]

Thanks for the reply Mark. July 2nd, I purchased a Holley 750 dbl pumper with the choke horn milled off. It will flow 790+ cfm. It has high-flow 110" needle and seat assemblies, 30cc pumps, #32 squirters front and rear, and a vent baffle. The jetting is 72 up front with a 6.5 high-flow power valve. 80 in the rear with  the power valve circuit blocked. Has pink cams front and rear. I should also add that I have headers, with chambered exhaust.  I leaned out the idle screws a bit and jumped my initial timing to 18 degrees and 36 degrees total. Idles great at 900 rpms, doesn't run hot or have a gas smell, and is quite responsive. I'm working on a very slight hesitation when I instantly stomp on it, then it's, " hold on tight". A little adjusting and I should have it figured out. It's been raining ( flooding ) here every other day, so it's hard to get out and work on it. Thanks again for your detail and if I would've bought a 650, I would have used your input, for sure.

Be well, Rich

[Stingr69]

You have plenty of jet for the primary.

You might try enabling that power valve circuit. It could help. If you decide to open it back up you will want to drop that secondary jet back down at least 6-8 sizes IMO. This I would definately do even if just for the driveabillty improvement.

It sounds like you may have a lean tip-in issue so the transition slot would be a good thing to check. Without touching anything, remove the carb and turn it over. Look at the transfer slots and see if the butterfly is uncovering too much slot. It should not look like "a slot", it should look like a "square hole" with the butterfly in the idle position. Any more of the slot exposed and you loose the ennrichment effect you would otherwise get when you crack the throttle. Lean bog or dead spot would be the result if this adjustment is off. Adjust the idle screw to make the slot opening look like a square and put the carb back on. Readjust the mixture screws and maybe play with the distributor initial advance setting to get the idle RPM exactly where you want it. Test that and play with the cams/nozzles from there.

The pink cam is pretty aggressive and you have a big shooter to get more gas in sooner. The bigger nozzle will make for a shorter time duration shot but a bigger/quicker stream. That is a lot of accellerator pump there to still have a hole.

Hope this helps,

-Mark.