89fast5oh

I have a 85 z28 with a 355 that has vortec iron heads, comp 274 flat tappet cam, performer rpm air gap intake, vacuum advance distributor and an edelbrock 750 cfm elec choke carb. Rest of the car has exhaust, T5 trans, and 3.42 gears.

The car starts, idles and has good power, but taking off from a stop it stumbles and bogs. Once it gets going its smooth until I shift into second and lightly give it some gas. When that happens I get the stumble again. The rpms this happens at is between 1200-1900.

When I take off from a stop and I hold the gas at the spot it stumbles at, it does it for 1-2 seconds (which is an eternity when trying to drive a car).

I have the calibration kit. I've gone 1 stage richer in both cruise and power mode, I have also tried 1 stage richer in power mode, and 3 stages richer in cruise/part throttle mode. I've adjusted the accelerator pump linkage to deliver a larger and smaller shot, fiddled with the idle air, and adjusted the timing 50x.

Nothing has gotten rid of the stumble/hesitation.

Apeiron

New carb or used?

89fast5oh

Used carb. Came with the car. Was sitting on a bench for 9 years until I got it. It was clear of gas when taken off then.

Apeiron

Get a kit and rebuild it, be sure to flush out all the passages and air bleeds.

89fast5oh

I will disassemble, clean, and put it back together today and see if that has any impact.

I have been doing lots of reading and it seems that many people have the same tip in throttle stumble with this same carb. There has to be a way to tune this carb to run properly.

Aside from the carb itself, is there anything else I should be looking at?

89fast5oh

I took the carb apart and cleaned it out then put it back together. It was spotless inside and out.

Readjusted everything and the off idle stumble is still there.

sofakingdom

iTrader: (1)

Yeah... I guess so.

Apeiron

What did you use to flush the passages?

89fast5oh

Pressurized carb cleaner and compressed air.

89fast5oh

While it was off the car i did notice that the idle transition slots were not even exposed by the throttle blades. When it was on the car I had to turn the idle screw that far out to get the idle down to 800rpm. I am going to set that distance and try and tune it again.

Savage388

im having the exact same problem in my T/A, itll really bog down before about 1500 rpm and although i believe its been on there a while it looks very clean and otherwise runs good.

89fast5oh

Adjusting the idle transition slots helped a little bit. Still stumbles on the ever so slight application of the throttle. Most people seem to think that the stumble is when giving it throttle while accelerating. That is not the case. The problem is there when the blades are ever so slightly opened, i'm talking millimeters of movement.

Apeiron

Is your vacuum advance on ported or manifold vacuum?

Ash1969

I went with a Holly H.P. 650 double pumper with a 4 corner idle and have the old school 202 ported and polished heads . Before I put this carb on I has similar issues. I too have comp flattappet cam whichis similar to yours. I had to adjust the floats when I first drove the car and n oticed on very humid days the car runs a hair rough at times, but is not noticable to the rider, but is to me. Normal days it runs like a dream. I am currently using a Weiand Team G intake.

midias

With that cam, head combo I would expect problems with a 750 unless the heads are worked over.

I think you would be happier with a 600 or a 650.

89fast5oh

Currently manifold. Have also tried ported.

BrownBullet83Z

A 750 cfm is alot for that setup. A 600 - 650 is PLENTY for what you've got however if it was good at one point and you like it, dont knock it. I had an Edlebrock for YEARS. I swore by them but I kept running into problems like yours. I eventually got a Demon. HOLY &%$# !!!!!! Even with the Edlebrock at its best, it never felt the way the Demon did right out of the box with no tuning what so ever. The car felt like it went injected and gained 100 HP! Since then I have built approx. 85 motors ( for sale) and I have used either Holley or Demon (thank god they are back!) and I will NEVER use another 'brock again. I wouldnt use one if you gave it to me. Thats my opinion though. P.s can show you Dyno results from Edlebrock to Holley. straight up carb change only made tremendous difference.

Edwardgp

Isn't there a power valve on that carb that can be adjusted?

Savage388

interesting. hows the fuel economy difference though. ive always heard that edlebrocks werent great for power but were decent on gas.

BrownBullet83Z

No. Edlebrock does not use any kind of a power valve like Holley does.

BrownBullet83Z

The economy is extremly comperable. It is a HAIR less but you really have to be paying attention. This car passes everything but the gas pump anyway so fuel ecenomy was never my priority but I havent noticed any differences to write home about.

Savage388

well then a demon 750 may be in my future if i cant get this to stop stumbling after a good tune up and going through. Are demon's as tricky to get right as holleys for tuning because ill be the first to admit I'm dont have too much experience with carb tuning but i want it to run right.

BrownBullet83Z

If you are used to the Edlebrocks then they are a little more "tricky". The Demmon is almost the same setup as a Holley. One of the main reasons why I used the Edlebrocks on my engine builds was because of the "ease of use" and quick tuning. My guys could set them up no problem However now that I've been using the Holley / Demon setup, it is just as quick and easy. THey have 4 mixture screws rather than 2. Demons are wet tested after assembly and they dial them in pretty good so there is minimal adjustments needed after installation. Could I make a suggestion for you though? If you are going to do a Demon or Holley DO NOT GO 750! A nice 600 - 650 double pumper will already be better and snappier than that Edlebrock 750. A Demon 750 (or even Holley) will probabily be too much. Just speaking from experience. TRUST ME!

Savage388

I was thinking of dropping down but, would a 750 really be too much for my 383? Im not too sure of what heads or cam but Ive always been told they need a lot of cfm.