Hesitation when you go WOT, anyone not have it? - Chevelle Tech
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post #1 of 85 (permalink) Old May 18th, 19, 8:18 PM Thread Starter
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Hesitation when you go WOT, anyone not have it?

I am running a Quick Fuel 780. Had it to one of the areas best carb guys and there is still a hesitation if I go all out WOT. He could not get rid of it. If you feed it has gradually it is fine, but if you go ar it hard it stumbles then goes, will do it every gear when rowing through the gears. Maybe it is just a typical issue, but it frustrates me.Need to hit up some people to see if they have a carb I could put on to try.
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post #2 of 85 (permalink) Old May 18th, 19, 8:33 PM
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Re: Hesitation when you go WOT, anyone not have it?

More timing.
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post #3 of 85 (permalink) Old May 18th, 19, 8:38 PM
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Re: Hesitation when you go WOT, anyone not have it?

Wish I was closer I have a bunch you could try out.
Timing is a big bog maker.
As you probably know that already.
My timing curves in all my carbed vehicles have been Without vacuum advance.

Quick curves all in by cruise rpm or much sooner.
18 initial at idle has been a minimum for all my vehicles.. from inline 6 mustangs to 302 Ford powered Vans with 6000 lb weight and another 6000 in tow.
Then you find total timing usually start with 35 and work up.

Most of my curves are all in by 2000 or 2200 rpm.
I had to dial one all in 1500 rpm as I had 2.29 gears and 29" tires.. that was fun to do.


If you run vacuum advance and have 24 or 18 initial with it hooked up what is timing cruising down the road ??
What is timing when you punch it from a dead stop.
Not as much when the vacuum drops out.. You could have 10 initial timing and heavy springs and at 2500 may still only have 24 total.

I have had cars come in with 22 total with no vacuum advance old school SBC low compression with 240+ Duration at .050"
That just does not work well at all.


Carb.
What is your idle vacuum ?
What is cruise vacuum?

I have got rid of the slow to respond by changing power piston springs to stiffer ones.
AFB and Q jet.
Holley carbs usually have a 6.5.
I have found for my cars pulling as little as 11" vacuum at idle really respond to something in the 8.5 or even the 9.5 power valve.

I do not have any bog out of the hole or from a roll or any speed.

Here is a video of what some may call over carbed.
350". Vehicle weight with passengers today would be 3900lbs.
280H comp magnum flat tappet TH 350 4500stall(Cheating I know).
Ported 601 heads 1.84-1.50 valves.

Tunnel-ram and 2 1405 edelbrocks 600 cfm. Stock autozone single point distributor with the point set that came with it and quick timing curve.
Point bounce sets in at a tick past 7000 rpm.
Even with 2000 stall and 3.00 gears this thing did not bog.. it just went.


But really high velocity heads smallish 170cc runners and tall ram is cheating.
It now has an HEI an RPM intake and revs just the same but no issues all the way to 7300rpm.
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post #4 of 85 (permalink) Old May 18th, 19, 8:53 PM Thread Starter
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I wish mine responded like that Jeff! Not even sure what my timing is set at, would have to look at the dyno sheet. It is really annoying!! Lol.
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post #5 of 85 (permalink) Old May 18th, 19, 8:57 PM
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Re: Hesitation when you go WOT, anyone not have it?

Yea that is a big need to know.
Are you running vacuum advance??
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post #6 of 85 (permalink) Old May 18th, 19, 10:50 PM Thread Starter
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Yes running vac advance. If I remember right initial was around 15 and total was around 34. The shop that did the work wa supposed to send me the dyno sheet but has not yet.
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post #7 of 85 (permalink) Old May 18th, 19, 11:05 PM
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Re: Hesitation when you go WOT, anyone not have it?

I have had hesitations with all brands of carbs from time to time. The least amount are with the Annular Discharge models. Best way to figure it out is disconnect the secondaries and work on one side of the carb at a time. Had the worst luck with a 600 Eddy carb, found it was the secondary side, got it fixed and it was lighting quick after that.
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post #8 of 85 (permalink) Old May 18th, 19, 11:12 PM
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Re: Hesitation when you go WOT, anyone not have it?

Bump your all in timing to 36 degrees and check where your initial timing sits. Your initial should be about 18 with the vac advance disconnected. Then test start it. If it still turns over and starts easily give it a test drive and make sure you don't have detonation issues. If it is too difficult to turn over or pings, then back it off 1 degree at a time until you find you engine's "Happy zone" You may even be able to push in more advance. It's trial and error.

Since you have a 4 speed you have 2 other things that are going to make tuning a challenge. You have vacuum secondaries and a single accelerator pump. Mechanical double pumper usually works better and is easier to tune at the cost of some economy.

You probably need to increase your accelerator pump charge and slow the opening of your secondaries to get rid of the bog.
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post #9 of 85 (permalink) Old May 19th, 19, 12:05 AM
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Re: Hesitation when you go WOT, anyone not have it?

Last sentence above. If the secondaries have too weak of a spring you will get too much air too fast. The transition is not coming in fast enough. One small trick is to set the carb for best idle then add 1/4 to 1/2 turn OUT on the idle mixture screws.

Check for a vacuum leak in the secondary diaphragm or assembly. That will cause your symptom as well.

If you go full throttle from 3000 RPM or higher does it do it then?
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post #10 of 85 (permalink) Old May 19th, 19, 8:00 AM Thread Starter
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I will see if I can get out later and video what it is doing and post it..
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post #11 of 85 (permalink) Old May 19th, 19, 8:42 AM
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Re: Hesitation when you go WOT, anyone not have it?

I suggest cutting the problem in half to see what the issue is....carb or timing.

Simply crank in about 10* more timing and with a couple of quick test hits you'll know right away if you are on the right track. If it improves, which I think it will, you will have to recurve the distrib.

IMO, 15* really isnt enough initial timing for these engines.

*69 SS 502 EFI /224* HR/9.8:1/T56/3.90 11.5/124mph
*11 C6, 6.2, 6spd
*96 Stang 6.0 turbo 10.5/135mph
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post #12 of 85 (permalink) Old May 19th, 19, 12:58 PM Thread Starter
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Hey Vince, you ever in the Harrisburg area? I will buy you dinner if you help me dial it in
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post #13 of 85 (permalink) Old May 19th, 19, 1:59 PM
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Re: Hesitation when you go WOT, anyone not have it?

"Lean Stumble" -- Typical for a Quick Fuel Carb. Check your Accellerator Pump. Try adjusing that first. If that don't work replace the carb with a Holley! You'll be glad you did.

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post #14 of 85 (permalink) Old May 19th, 19, 2:22 PM Thread Starter
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Re: Hesitation when you go WOT, anyone not have it?

Here are a couple videos it was actually6 better behaved today than usual,,but still there

Gradual WOT


3,000 RPM WOT


Slow roll WOT
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post #15 of 85 (permalink) Old May 19th, 19, 3:35 PM
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Re: Hesitation when you go WOT, anyone not have it?

I'm running a near stock L78 396 (stock cam + .025 valve lift) with headers, 800 Holley DP (#72 & #78 jets, 6.5 PV with 50 CC secondary pump) and a MSB Pro Billet Distributor with no vacuum advance. Timing is set to 12* initial and 37* total, all timing in by 2,500 RPM. Absolutely no bog or hesitation, the car takes off like a rocket when I punch the throttle at say 3,000 RPM in 1st gear.
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