Well, I finally swapped my old Performer intake out for a Performer RPM Air-Gap on the 355cid.

My best time with the old intake was 8.36 in the 1/8th and it seemed dialed in pretty good. No bogs, coughs, stumbles, spits or sputters.

However, now that I've installed the RPM Air-Gap, I have a SEVERE off-idle stumble when I go WOT. Nothing was changed except for the intake.

The carb is a 3310 Holley 750cfm vacuum secondary with the following:

Rebuilt 2 years ago by me, still looks fresh.
70/78 jetting (as I have the secondary metering
block kit)

Yellow spring in the Quick Change Secondary Pod
6.5" power valve in front with no PV in rear
30cc accelerator pump with the "extra diaphragm
trick"

.035 squirter

I'm not surprised that I'll need to do some tuning with the new manifold, but I'm just looking for some direction from you guys. I've (hopefully) introduced more air to the combo, so I'm guessing I need to add some fuel. I can't imagine going to a bigger squirter, but I wonder if I should go to a slightly stiffer secondary spring.

How does the jetting sound? Some specs include:

355cid
Canfield 195cc heads with 10.1 CR (no porting)
Edelbrock Performer RPM intake
Hooker 1 3/4" Super Comp. headers (running open
as of right now)

Lunati hyd. roller cam
290/300 adv. dur.
232/242 @ .050
.510/.525 lift w/1.5 Scorpion roller rockers
108 LSA with 104 ICL

3.91 Posi gears
PTC custom 10" stall; flashes to 3500
28x10.5 M/T Sportsman Pro's

With the old manifold it would run out of breath at 5500rpms just as advertised. Hopefully the RPM Air-Gap will extend the powerband. Any comments or critiques are welcome. I'd like to get a 7.99 to achieve my goal. smile.gif

Well, I think I'd start at the begining...
with the vacuum gauge. Get the idle mixture set to achieve the highest vacuum, and then consider the power valve. With that cam, I'm guessing you don't make a ton of vacuum... select the approiate power valve, then maybe move on to the secondary spring??

might take a little more jet too,,,my son had a cruise condition surging with his 750 on his 355,,jetted from 70's to 72's and that seems to have cured it.

Robert, yes I have a little bit of a "buh, buh, buh" sound coming at a low-speed cruise. It was that way with the old manifold as well.

I was at 72/80 for a bit last year. I may go back up and see if it doesn't clean up that surge. I really need to buy a jet kit.

What might cause the stumble? Secondary spring as Dennis mentioned? It's right at idle when I go WOT. I haven't tried it from a cruise to WOT yet.

Thanks guys!

The yellow spring is the lightest spring....
That, and the wrong power valve are most likely the cause of the stumble.
How much Manifold vacuum does the engine make?

Dennis, I THINK there are 2 different yellow springs. Not sure which one I have, so I'll have to check.

I need to get some better (re: current) info for you guys as I've done little to the car except for a test drive or 2 with the new manifold.

I WAS pulling about 12" of vacuum with the old intake in gear at about 1000 rpms (she likes to idle a bit quick).

EDIT: I posted the wrong rear jetting in my original post. I fixed it to read 70/78 which is my correct jetting at the moment.

Also, Dennis, I'm curious as to what times your '69 Nova runs? We have the same cubes, about the same gears (3.91's for me) and about the same HP. I've got a nickel that says your quicker than me. graemlins/thumbsup.gif

Hi Justin,
Times?? No clue. I really wanted to get to the track this summer, but I never made it. I still need a few things before I consider it track ready. She goes like hell on the street though...
I need a Posi unit for starters, line locks, drive shaft loop, Cal-tracs, and some sticky tires. Maybe a good winter project THIS year...
You seem to make a lot more vacuum than I do...
I can only muster about 8". I run an Isky Z-70 solid. 548 lift, 304 adv, 264@.050 on 108
There's a couple pictures here if you're interested:
http://community.webshots.com/album/51619384CDvWPz