here's the set up. 85 TA, 383 Stroker, vortec heads, edelbrock 750 elec choke carb and intake. Mallory dizzy and msd wires, t-5 manual trans.

problem:

slight, one time hesitation under light to moderate power in each gear. It happens right after I shift and introduce a little throttle. no other problems through out the power band. it also happens if i'm at a set RPM and slowly introduce a little power. it hesitates very slightly and then operates normally. this only happens when the car has been running for a while but it is not choke related because the choke is fully open as it should be at this time. it almost seems that it does not happen when the choke IS on.

i just had the carb and timing tuned. 36 degrees centrifugal and about 45 with vacuum. No detonation problems running on 93 octane.

could this be a problem with the step up springs not being correct? The manual that came with the carb decribes symptoms similar to this and lists the step up springs as a possible adjustment.

 

Pump shot... either not enough shot, the wrong accelerator pump cam, or improperly adjusted accelerator pump linkage.

 

Does it seem to have a light surge at the same throttle position/speed/rpm you feel the hesitation? If so it is more than pump shot/volume.

What step-up springs are in it? Also put a vacuum gauge on it and see what the vacuum reading is when you get the stumble. If your step up springs are too light it will bog just as you described. Edelbrock runs way too light of a spring, if you have a near stock vacuum camshaft. Their step-up springs are typically fully up at 4 in/hg of vacuum, way too late IMO. They also tend to run a very rich power metering rod/jet combo. They tend to run a lean cruise jetting/metering rod combo. They keep the rod burried in the jet as long as possible for fuel economy. Improvement can be had by taking a look at the Jet/Rod that is in it. This has worked for me in the past. Take the jet and go down like 2#, then go with a smaller rod on the power end, keep the cruise the same, then put in their stiffest step-up spring=8 in/hg. Worked well on my 750 Carbed 355. What that does, is keeps the power mixture or WOT the same, leans out the high vacuum, low load mixture some, and gives a quicker transition to the power mixture. Some improvement can also be had by playing with the idle mixture screws. Open them up like a 1/4 turn and see if the stumble reduces without affecting idle too much. Finally you may have a little too much timing when you feel this. What is the vacuum advance connected to. Ported or Manifold, if it is ported that may be what you are feeling, you tip-in, the advance comes in, and then you have too much advance. I would try to limit the vacuum advance, or reduce the timing slightly. Vortecs tend to like about 32* Total advance and no more than 45 when cruising.

On a final note, on the calculating the Jet/Metering rod size, you need to take the diameter of the jet and rod, determine the area of each, and the area for the gas to flow through. That is the number you will be shooting for. The rods are rated like 32-47 that is .032" @ WOT and .047" @ Cruise, and say you have a 104 jet=.104".

 

thank you for the info. It does happen at common rpm's and throttle position. It's usually just as I add a little throttle after slight deceleration or steady cruise. I have adjusted the accelerator pump shot and the jets and rods with no change at all. Also I have adjusted the idle mix screws with no change as well. This is why I think it may be the step up springs. Also, hard throttle and romping on it does not cause the problem at all. It is only light throttle. I'll be getting a spring kit tomorrow to try. Thanks again and let me know if you think of anything else.

 

Another thing, make sure you are getting pump shot, as soon as you go onto the throttle, look down the throat (engine off), just start to tip the throttle in and watch to make sure you get a stream out of the squirter. If you do, look elsewhere like the step up springs.

Put the stiffest one in there and see what happens.

This is what an edelbrock tech to me for determining the required spring, measure your idle vacuum, divide by two, then put in that spring or the closest to it. For example lets say you have 16 in/hg at idle, put in the 8 in/hg spring. Worked well in every build so far.

You didn't say what your Vacuum advance was connected to. Connect it to manifold and it may go away if it was on ported. You want the vacuum advance to go away as you roll onto it, not increase (ported). Also try pulling about 2-4* of timing out of your combo. I bet it picks up. Every vortec build I have seen seems to like about 32* total timing and no more than 34* (car crafts battle of the titan 350 with a monster 244/254* @ .050 cam) With a more practical cam I bet it would have used 32* too.

 

again, thank you all for your information. it is very helpful. the vac advance is hooked up to ported right now. I have tried it on direct manifold and there is no change in the specific problem. I also have tried adjusting the timing and found that the problem does not change with timing and the engine runs better through all other phases of operation with the current timing set up I have. I'll try the step up springs and report back tomorrow or after christmas. Thank you again for your help and knowledge.

 

the edelbrock 750 with the electric choke is set up for economy the manual 750 is set up two stages richer.i know this because i ran in to the same thing with my built 355 so i dumped the edelbrock for a holley 770 street avenger and have never looked back trust me when i say the holley is a far better carb for performance

 

I'll try the step up springs first since they are only about $7. Then i might start entertaining other solutions.