Fast355

iTrader: (2)

Well I swapped a Holley onto my 305 in my 1980 Vette. Its a pretty healthy engine with flattop pistons, vortec heads, crane 272H10 cam, edelbrock super victor intake, 1 3/4" primary headers and 2.5" dual exhaust. Ran 109-110 MPH in the 1/4 with a stock stalled, stock TH-350C and 3.07 gears. Well I am ready to get this 600 vacuum secondary carb dialed in on it. It has a hesitation on tip-in but runs good otherwise. The secondaries open a little too soon for my liking as well. Where should I start. Jetting seems about right, the 6.5" power valve seems appropriate and it is not blown. I think it needs a touch more initial timing advance, but I am limited to 10* by the 22* centrifical advance in the distributor. I need to either weld up the slots or get a new distributor that can use an advance bushing. I really think this car would like 12-14* initial advance and 18* centrifical for 30-32* total. But I do not feel its the distributor thats causing the issue. Feels lean to me when the throttle is stomped. Where would be the logical place to start. The pump cam or the squirters or both. Do not feel this engine needs a 50cc accelerator pump, but its not out of the question. I should also mention I built a plenum divider for the intake and have a divided plenum. The low and midrange torque feels identical to the GMPP Vortec intake that was on this engine prior to the super victor. The 60' times are practically identical to the dual plane, but mph is higher with the single plane across the board with nearly 2 mph gain in the 1/8 and 2-3 in the 1/4. The tach in this video is JUNK, but was on the car when I got it from my brother. Its reving every bit of 5,500 maybe even 6,000.

JaBoT

The cam is not that big that it should not need a ton of timing down low, but the single plane intake would need more.
I would try putting a blue pump cam in the #2 position and see if that cures the bog. If the bog is still there you may need to get a more adjustable distributor or make some kind of advance stop in the one you have to get more timing at idle.
You should never need a 50cc accelerator pump on an engine and cam that small. If it does there is something else wrong.

Fast355

iTrader: (2)

I have never tuned a Vortec head engine that did not want or like a huge pump shot on tip-in. I am not sure why they are so much different than typical small block heads, but I have tuned EFI engines and had to increase the Acceleration Enrichment tables up to 50% over stock. When I tuned my Express it just ate up the AE and loved the low rpm timing increases I made. On the dyno I saw up to a 40 ft/lbs gain at 1,000 rpm at the wheels by bringing the advance from negative numbers up to 12* with no detonation issues on 87 octane. Made over 20 pulls on the Mustang dyno, locked in 3rd gear, with the TCC force locked to dial in the timing curve. IWith the 4L80Es horrible 1st gear it made a HUGE difference in throttle response from a stop. The Express has a milder cam than the 305 in the Corvette.

The cam does have a tight 110* LSA and likes a 700 rpm idle speed. Below that the overlap is very noticeable in the 305.

The converter is definately too tight, but it is expensive and difficult to find high stall TH-350C converters and I feel more stall would just cause me traction issues.

JaBoT

yea your are right on with the vortec heads liking a big pump shot for some reason (I am sure someone here knows the technical reason for it needing a lot of ae). I usually wind up with a blue cam and anywhere from a 32 to 37 squirter on a 350, but 50cc is a lot!
And same with the timing as you were saying. The faster you can bring it in with out detonation the more low end torque it's going to make.
If it's a hei distributor, pick up a Moroso 72300 advance kit and use the light springs. You'll have all the timing in by 2500. Should help the low end a lot.

FrigginA

Sometimes carb problems are manifested elsewhere. Check a couple things before you go changing a bunch of stuff.

1. Have you looked at ALL your plugs lately? Make sure they are attached sucurely and check the resistance of the wires.
You simply can't tune a carb if that stuff isn't up to snuff.
Good Plug reading info http://forum.grumpysperformance.com/...php?f=70&t=202

2. Check how much voltage your distributor is getting when the engine is running. If it's an HEI they have to have at least 12v and I've read that they love 14v.

Always get your ignition/distributor/timing issues sorted before you go messing with the carb. It would definitely like a little more initial timing.

Part of the problem might be with the 600vs carb it's self. Most of those were designed to be ran on a totaly stock engine with high vacuum at an idle. Their idle/part throttle/cruise circuits are designed to be leaner than the other carbs. It also might be that particular carb with that intake. I can't see that as a match made in heaven.

With the engine fully warmed up put a vacuume gauge on there and see what your pulling at an idle in gear with your foot on the brake.
Take that number divide it in two and use that number power valve or a slightly sooner opening one.
Lets say you are pulling 10 in gear at a stop. In most cases you would need a PV 1.5 smaller, meaning an 8.5 up to a 5.5 PV.
The 5.5 will delay full main circuit enrichment longer keeping your plugs cleaner.

Fast355

iTrader: (2)

Idle vacuum is not a problem. 17-18 in/hg @ 750 rpm.

As for the ignition, it does not miss a lick and it has a MSD 6a box triggering the coil directly from the HEI pickup coil. Cap and rotor are new as are the wires. The issue I have is strictly fueling during the transients. Huge lean spot on the wideband. It runs well idle/part throttle and cruising, just hesitates on the primaries at WOT.

Engine ran great with the edelbrock 500 cfm that was on it, but even it needed huge changes to the accelerator pump circuit.

Carb might not be a perfect match with that intake but I plan to make it work well or rip it off and run a Q-Jet.

FrigginA

I like that idea.
I've never liked the 600vs carb and the QJ will blow it away performance wise IF you can get it tuned correctly.
The poor old QJ has been left behind by the manifold manufactureres these days. Once Edelbrock started making Carter type carbs the writing was on the wall.

JaBoT

Put the blue pump cam in and let me know what the wideband is doing. Should make a huge difference.

FrigginA

Some good stuff in here
http://www.aedperformance.com/tuning-tips.html