shredjesse

Hey everyone!

Chasing tuning my 68 383 Corvette. Learned a lot about carbs along the way, and have had two shops not setup carbs for damn, along with the most recent QFT being way off from the factory... so I'm getting decent at this!

Anyways, the car runs pretty amazing around town right now. 12 degrees initial, 34 total. Vacuum advance with full manifold vacuum makes this car cruise and idle like a dream machine. Car will hold whatever temperature thermostat I put in there, right now 180 degrees.

Carb is a QFT SS-735-VS (735cfm, Vacuum secondary). Went with it as by all equations out there 750 is large for a 383, but everyone even Blueprint seems to go with them, so a 735 should be a solid compromise to increase low end signal while still being able to rip up top.

Problem as I experience them

PLEASE NOTE: These are all wide open throttle from the RPM mentioned. I can ease into the throttle anywhere and it's smooth and crisp.

In 1st gear, the car rips everywhere, no issues.
In 2nd gear, the car does pretty good from 2k or so up, but a bit of a hesitation.
In 3rd gear from 2k the car gets a felt issue. Flattens out in acceleration then catches up, bit of a gurgle. Rips from 2.5k up.
in 4th gear from 2k the car has a noticeable felt issue. The car acceleration flattens out, notable not smooth firing is heard, and it gurgles a bit.

Things I've done to chase the issue

• Adjust accelerator pump. With the throttle all the way closed, I can just slip a 15 thousandth feeling gauge between the pump arm and the throttle mechanism. If I go any tighter, it constantly engages.
• Comes with a 31 accelerator pump nozzle. Went with 35 then 37. Didn't fix, can't tell if better or worse honestly?
• Adjust the basics. Max vacuum at idle (without vacuum advance), establish timing (12 initial, 34 total), float bowl levels set even.

Any ideas? Perhaps what I'm chasing just can't be solved since the car won't accelerate as fast in 3rd or 4th gear from 2k as in 1st or second, and if I fix the issue up there will I be just introducing an issue down low, etc etc.

claysmoker

What rear gear?

shredjesse

Oh yeah, good question! Made me think of some extra details...

-3.7 posi rear gears!
-275/60R15 rear tires (28.1 inches tall).
-I'm 171 feet above sea level

TimAT

Looks like you've about covered everything. Time to try a stiffer spring in the vacuum secondary. Slow the secondary opening a bit.

shredjesse

Oh okay, slow it down eh? I've got an adjustable vacuum secondary which is normal on the QFT. I was going softer (out) to open sooner. Should I be going stiffer (in) to open later then?

TimAT

Your choice- if it didn't help one way, go the other. Another forum guy went thru the same exercise a few years ago. He fought the vacuum secondary thing for months. I loaned him a mechanical secondary carb I had. Cured all his problems. I don't know what all he tried to get the bog out. I'm pretty sure it can be tuned to get most of the bog out, Just a matter of trying one thing at a time until you find the combination.

claysmoker

You could play with pump cams since the carb probably has pink ones on it. A black will give you more initial shot and about the same as pink after 40% throttle. Or maybe a red orange or white. They are easy enough to experiment with.

I have a little more engine than you but essentially the same drive train. I don't get much in 3rd or 4th until I get to 3K rpm either, and I never push those gears at anything under 3K. You didn't mention what cam either.

Another thing which could help is more initial advance and cut back mechanical and vacuum to keep your total the same. I run 24 initial and it's happy there.

claysmoker

Oh, and Tim is right about tightening up the secondary opening, try about 1/2 to 3/4 CCW from bottomed out.

shredjesse

Cool I'll go with the secondary adjustments. I had it all going the entirely wrong direction it would seem.



Oh and as for the post about mechanical secondaries, I had a 650 double pumper on this at one point, and it had the worst bog ever from 1/3 to 2/3 throttle. I chased that thing round and around... I could just never get it to behave at anything other than ripping off the line or just idleing around.

donaldb1

The testing you have posted suggest the timing curve is not optimum for your engine.

If you cannot access a dyno, try plotting the vacuum @ 250 rpm intervals under load, 3&4 gear. You need an assistant to do the recording as you drive. Vacuum gauge connected to tee in hose from manifold to vacuum canister. Once you have the vac plotted, plot the timing on the same graph. record the timing with vac blocked so that you have the wfo curve. I would massage the curve to obtain16-20 degrees at idle speed 650-800rpm with manifold vac advance , 36 degrees mechanical total. Manifold vacuum all in at idle speed. If you have Votec style heads, you may need only 32 degrees total mechanical timing. If the cam is a 350HP cam from the mid 60's, be aware that these cams love open exhaust and cannot get good flow below 3000rpm ant still blow past 7000rpm or so.

PAmotorman

you set the pump arm clearance to .015 at WIDE OPEN THROTTLE

rklessdriver

Camshaft specs? Compression ratio? Cyl heads?

Honestly it sounds like you don't have enough timing advance at lower RPM.

A halfway decent 383 SBC should take the lightest spring possible in a Vac seconday carb... right off idle.
Will

gkull

​​​​​​
Actually you want zero clearance so any throttle movement causes the squirters to flow

I find it hard to believe that you couldn't make a 650 double pumper work

PAmotorman

you need the .015 clearance at WOT so you don't rupture the pump diaphragm

HeadsU.P.

I agree with gkull, a DP will cure a "healthy" engine bog. But will also make a "sick" engine worse. DPs are great for instant front end lifting - tire smokin'- neck snapping
. . . . (but you have to pull a utility trailer with a 50 gal tank behind you)

Most of your issues seem to be after 2nd gear. That's about the time your fuel bowls are starting to empty, especially if your fuel pump can not keep up. By the time you are in 3rd the carb is starving for fuel. Also, what type of fuel filter is incorporated? Is it restrictive or free flowing? Float levels set a pinch higher may ease some issues. Both Holley & QF rear fuel bowls could use jet extentions ($25) and a notched float. Otherwise, under rapid acceleration the rear jets are exposed / dry / starving.
So your concerns are:
Fuel pressure ok at WOT?
Rear jets exposed?
Fuel filter flow rate?

cagotzmann

You are going to having a real problem tuning this without AFR Readings. Nothing but Trial and error and yet you will never know if it peaks.

I have a ZZ383 with a holley 770 street avenger.

I also have AFR O2 sensors that helped me tune the engine. Here is what I did.

1. Tune the primary circuit (Jet Sizes). 2,3,4th gear constant cruise ~ 2000 - 3000 RPM AFR ~ 14 - 14.4.

2. Tune secondary circuit (Jet Sizes). Same testing you are doing but smooth acceleration vs instant WOT, 2k RPM - 5500 RPM. AFR ~ 12.5 - 13.

3. Change vacuum secondary spring until test 2 is a smooth AFR Transition. No spike or as little as possible spike in AFR from little throttle to WOT. This test is done with a smooth acceleration not smashing to WOT

4. Adjust the Accelerator Pump Discharge Nozzle sizes / accelerator pump cam profiles for best quick WOT Response. Try to minimize spikes in AFR.

Then I take it out to the 1/4 track to adjust timing for the best 1/4 mile MPH.

I have a MSD 6530 Programmable ignition so I can use my PC to adjust the timing curve at any point. Looking at the AFR response and adjust based on RICH / LEAN to maximize 1/4 mile MPH.

shredjesse

Hey headsup... just a heads up (see what I did there!) that this is from cruising in a given gear. So cruising at 2k in 3rd, I jump on WOT and the car just bogs. From 3k up it pulls pretty darn good.

I'm going to try the 3/4 turn out secondary adjustment tomorrow and go to 1/2 turn just to see what it does.

Another thing I'm considering is adjusting timing. I've read and heard of a few places lately that do really aggresive initial timing with no vacuum advance of 21 degrees, then 14 degrees mechanical advance to a total 35.

I'm not sure I want to go that advanced, but if it works for them and gets snappy response, doing 16 degrees initial with 18 degrees mechanical advance to a total of 34 should do pretty good. I also talked to some people at the car show and they go with a pretty aggressive ignition curve and have pretty good luck, so I am envisioning the second to most aggressive MSD curve I can get from my distributor. I also do have my vacuum advance hooked up as it's really made cruising idle excellent, hoping that persists with these changes...

One thing at a time though, I'll try each thing individually for a while and document changes.

427Hotrod

I'd definitely use more initial timing and limit centrifugal to get total. 20* wouldn't be out of line.

As described above...accelerator pump should have no play at all....but shouldn't bottom out when at WOT.

If you're having issues flat footing it from 2K in 4th gear....first thing I'd say is don't do that. Select the appropriate gear. You'd have to get into it pretty hard for secondaries to try and activate. But it does sound like they may be opening early for your driving style.


JIM

shredjesse

As it would turn out... this is the lucky winner!

3/4 turns out from bottomed out and this puppy pulls clean from everywhere and the throttle response is much improved when jumping on it!