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Old 12-25-2012, 04:32 PM
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850 hp dp question

I was wanting to see if anyone had a similar set-up to see where your carb adjustments were or if anyone had some ball park ideas. The problem is when nailing the gas from a dead stop or rolling and nailing it dips off for about a second and then she's gone. Just a 1 sec of drop off hesitation. It is a 69 Chevelle 496 bbc turbo 400 with 4.10 gears. The motor was dynoed before it was put in the car at 648 hp and all adjustments were made. there was an issue with the carb after being dynoed and an 850 hp double pumper was installed although I was told from the shop all adjustments are good I went back through everything and have it pretty straight other than the carb. Timing is spot on. Only prob is the hesitation when first stabbing it also feels like the motor has a little more grunt in it, but its being held back just a little. Carb set up pink accelerator pump cams set on #2 was on #1 spot on carb. 78/78 on primary jets 86/86 on secondary jets 4.5 PV in primary and plugged PV in secondary (vac is 6") squirt nozzles are 28 primary and 29 secondary. In thinking slightly bigger nozzles? Any ideas? Really just trying to fix the 1sec hesitation, but trying to give as much info as possible. Thanks!
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Old 12-25-2012, 10:05 PM
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Ignition spot on eh!!
What's your timing set up?
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Old 12-25-2012, 10:31 PM
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20 initial 38 final at 3,000 rpm plugs have color on the ground strap in the center of the bend. Runs like a scalded dog after the split sec hesitation! Any suggestions on that timing?
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Old 12-25-2012, 10:55 PM
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You're right!
Timing is spot on!
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Old 12-26-2012, 07:54 AM
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Originally Posted by RWENUTS View Post
You're right!
Timing is spot on!
Once I get it worked out it should run perfect. It runs and drives great, but that slight hesitation when stomping the gas is a little aggravating. It's almost like it starved for gas for a split second which made me think the squirters might be to small.
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Old 12-26-2012, 08:08 AM
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Maybe your 4.5 power valve is a little close to the open point with your low (6") vacuum. TRy a 3.5.
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Old 12-26-2012, 11:03 AM
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Quote:
Originally Posted by 396chevelle View Post
Once I get it worked out it should run perfect. It runs and drives great, but that slight hesitation when stomping the gas is a little aggravating. It's almost like it starved for gas for a split second which made me think the squirters might be to small.
It doesn't sound like the carb is idling on the transfer slot. Are you using a vacuum advance hooked to manifold vacuum? If the engine is depending on the increased timing from the vacuum advance, in some cases the drop in vacuum and subsequent loss of advance can cause a slight hesitation. This shouldn't be the case if the torque converter stall is sufficient and the vacuum advance amount isn't excessive (limited to about 10 degrees of vacuum advance max), but stranger things have happened.

Be sure there is absolutely NO clearance between the pump arm and the adjuster nut/bolt at idle. There needs to be a little clearance at WOT so the diaphragm isn't damaged.

If that's adjusted correctly and there's no hesitation of fuel from the squirter at the tip of the throttle (both primary AND secondary has to have fuel from the squirter the instant their throttle shaft moves), go 2 or 3 sizes bigger squirter. 28 primary/29 secondary squirters isn't very much for an engine that size, and doubly so if you have an open plenum intake.

I would first disconnect the secondary throttle linkage to see if the problem is the primary or secondary. It might need more shooter size on BOTH sides, for that matter. But if you can clean it up by going a couple or three sizes bigger on the secondary side, that'll save you some fuel when cruising around.

Be careful using positions other than #1 on the pump cams. Position #2 is usually for when the throttle blades have to be tipped open farther than normal. When that's the case, position #2 will still allow a full pump shot even though the throttle shaft has been rotated open, which would 'use up' some of the cam, if you follow me. Don't use position #3 unless you have the 50cc pump set-up.

If you have the time, go over this post on accelerator pumps: excessive backfire and flames! also rocker arm knock

Good luck, sounds like a fun ride.
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Old 12-26-2012, 12:57 PM
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Quote:
Originally Posted by cobalt327 View Post
It doesn't sound like the carb is idling on the transfer slot. Are you using a vacuum advance hooked to manifold vacuum? If the engine is depending on the increased timing from the vacuum advance, in some cases the drop in vacuum and subsequent loss of advance can cause a slight hesitation. This shouldn't be the case if the torque converter stall is sufficient and the vacuum advance amount isn't excessive (limited to about 10 degrees of vacuum advance max), but stranger things have happened.

Be sure there is absolutely NO clearance between the pump arm and the adjuster nut/bolt at idle. There needs to be a little clearance at WOT so the diaphragm isn't damaged.

If that's adjusted correctly and there's no hesitation of fuel from the squirter at the tip of the throttle (both primary AND secondary has to have fuel from the squirter the instant their throttle shaft moves), go 2 or 3 sizes bigger squirter. 28 primary/29 secondary squirters isn't very much for an engine that size, and doubly so if you have an open plenum intake.

I would first disconnect the secondary throttle linkage to see if the problem is the primary or secondary. It might need more shooter size on BOTH sides, for that matter. But if you can clean it up by going a couple or three sizes bigger on the secondary side, that'll save you some fuel when cruising around.

Be careful using positions other than #1 on the pump cams. Position #2 is usually for when the throttle blades have to be tipped open farther than normal. When that's the case, position #2 will still allow a full pump shot even though the throttle shaft has been rotated open, which would 'use up' some of the cam, if you follow me. Don't use position #3 unless you have the 50cc pump set-up.

If you have the time, go over this post on accelerator pumps: excessive backfire and flames! also rocker arm knock

Good luck, sounds like a fun ride.
Thanks for the good info! I have the nut on the pump arm adjusted like u have stated. Also I do have the vacuum advanced hooked into full manifold vacuum at 10 degrees vacuum advanced. I also have a 3,500 stall converter. As far the the plenum I have the eldy air gap under the carb. I believe I will move the cam position back to #1 spot because it really made no change anyway. I will check the pump arms one more time though just to make sure. I have ordered a pump cam kit and jet kit along with several different squirter sizes to play with and have on hand if need be. I believe it's the primary that's causing it because I grabbed the linkage and hit it one time while I watched the streams and I noticed the hesitation right before the secondary kicked in. I can hit it to the floor and it does it or if I nail it, but not wide open just a quick stab and it will do it. Again though other than that the car runs great. I can even tell in a video that my wife took at the local track that it dropped off for a split second when I hit it off the line. I also double checked float level which seems to be good. Trial and error I guess! Thanks again for all advice!
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Old 12-26-2012, 02:26 PM
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Quote:
Originally Posted by 396chevelle View Post
Thanks for the good info! I have the nut on the pump arm adjusted like u have stated. Also I do have the vacuum advanced hooked into full manifold vacuum at 10 degrees vacuum advanced. I also have a 3,500 stall converter. As far the the plenum I have the eldy air gap under the carb. I believe I will move the cam position back to #1 spot because it really made no change anyway. I will check the pump arms one more time though just to make sure. I have ordered a pump cam kit and jet kit along with several different squirter sizes to play with and have on hand if need be. I believe it's the primary that's causing it because I grabbed the linkage and hit it one time while I watched the streams and I noticed the hesitation right before the secondary kicked in. I can hit it to the floor and it does it or if I nail it, but not wide open just a quick stab and it will do it. Again though other than that the car runs great. I can even tell in a video that my wife took at the local track that it dropped off for a split second when I hit it off the line. I also double checked float level which seems to be good. Trial and error I guess! Thanks again for all advice!
You are on top of everything that I would have suggested.

While I was away I started a page on the Holley accelerator pump, so thank you for the motivation!
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Old 12-26-2012, 07:21 PM
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Originally Posted by cobalt327 View Post
You are on top of everything that I would have suggested.

While I was away I started a page on the Holley accelerator pump, so thank you for the motivation!
Thanks for sharing. That is some useful info
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Old 12-26-2012, 07:49 PM
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You are running almost the exact same setup as I am. Except mine is in an 81 GMC truck, and you have a Chevelle.
I have an 850 Mighty Demon carb, and I had all kinds of trouble with it, but it's sorted out now.
I have the same vacuum as you, and I also have a 4.5 PV in it. That's exactly what you need.
My timing is 18 init and 36 max.
Our jets are the same too. I'm running same intake as you. ( awesome choice )
Your hesitation is most likely caused by a lean hit when you dump it hard.
I would just make sure you have no play in your accelerator pump linkages, and then start going bigger on your squirters.
Also make sure you have your fuel bowl levels set good.
I'm sure you got your idle screws dialed in.
Sometimes them squirters just don't get that shot in that big hole soon enough.
I never had a hesitation issue, I was always on the other side of it.
I was constantly pig-rich until I got it all dialed back.

BTW... what do you run in the 1/4 mile? I'm just curious what that setup can do, since I have not been able to get mine on the track yet.

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Old 12-27-2012, 07:21 AM
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when I ran an 850 on my 67 Camaro 355,I was at S.I.R. and I had #78s in the primary side.It was horrible,,,, I had also forgot my jets at home.
The engine didnt stumble but did not make any power and would not rev out. 80s are minimum for me,I would throw a set of 81s in to start.
Thats a small carb for a 496,there should be no hesitation issues at all
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Old 12-27-2012, 12:15 PM
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Quote:
Originally Posted by 632Mantis View Post
You are running almost the exact same setup as I am. Except mine is in an 81 GMC truck, and you have a Chevelle.
I have an 850 Mighty Demon carb, and I had all kinds of trouble with it, but it's sorted out now.
I have the same vacuum as you, and I also have a 4.5 PV in it. That's exactly what you need.
My timing is 18 init and 36 max.
Our jets are the same too. I'm running same intake as you. ( awesome choice )
Your hesitation is most likely caused by a lean hit when you dump it hard.
I would just make sure you have no play in your accelerator pump linkages, and then start going bigger on your squirters.
Also make sure you have your fuel bowl levels set good.
I'm sure you got your idle screws dialed in.
Sometimes them squirters just don't get that shot in that big hole soon enough.
I never had a hesitation issue, I was always on the other side of it.
I was constantly pig-rich until I got it all dialed back.

BTW... what do you run in the 1/4 mile? I'm just curious what that setup can do, since I have not been able to get mine on the track yet.

I don't have a 1/4 mile close by, so I drove it up to our 1/8 mile track and ran it. Fully trimmed out chevelle it ran a 7.2 which if you convert is around 11.2's in the quarter the 60ft was horrible though I really feel like I can squeeze a flat 7 or a 6.9 out of it which in a quarter should put me at high 10's. at least that's my goal I ran the 7.2 on street tires. She is really strong motor is 95% there except for that quick dip off the line then she haulin grass!! Let me know what you run when you run it! I would like to see how close they are. I'm going to get some slicks once I get the carb squared away and run it again that 7.2 was only the second run at the track after I got the motor in. I think that's pretty good! Any thoughts?
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Old 12-27-2012, 03:27 PM
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Recurve the distributor for 24 to 26deg inital at idle and 38deg at max advance.
More base inital, less mech adv travel.
Connect the vac advance to the ported vacuum source.

To see the effect very quickly, to see if it helps, simply
ock out the mehcanical advance system ,, set the locked out timng to 38deg at idle.
( vac advance goes to the ported source.)

with the timing now fixed, remove the carb and flip it over and look at the throttle position. Reset the pri and sec throttle idle position at idle so both idle transfer slots have correct exposure at idle. .020-.030"

It will now respond much better off idle and idle nicer in general.

Once the timing curve and base inital is corrected, the carb may need minor fine tuning of the idle /off idle system
( idle fuel feed restrictions and idle air bleeds) Correct the timing curve, first.

Be sure the carb PCV hose is connected to the base of the carb and the PCV is functional.
It is a critical part of the carbs idle circuit.

Be sure the OEM rear starter motor brace bracket is installed.

What cam is in this motor?
What is the manifold vacuum at idle when idling in gear?

Last edited by F-BIRD'88; 12-27-2012 at 03:35 PM.
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Old 12-28-2012, 06:48 PM
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I think that's pretty good! Any thoughts?
Yeah, that sounds about right. As you get it all fine-tuned and dialed-in, you could expect to be in the 10's in the 1/4.

I guessed I would be in the 11's with my truck. But you never know until you run it!
Trucks are not aerodynamic, so you start pushing against that after a certain MPH. ( Maybe after 70mph or so? )

It's all fun. ( until the money runs out. )

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