I went to the track last weekend and 8 out of the 9 passes I made, it would fall on its face after about 10 feet after my launch. When I give it full throttle it feels like it is going to die for a second or so, then it runs great. I am running a Edelbrock 750 carb with Edelbrock RPM heads,EDL cam, full MSD ignition, air gap intake, 3000-3500 stall tc on a 357ci motor. I jetted it up last night and changed the meetering rods too. I went with the biggest jets(.116)on the primaries and 2 sizes up on secondaries and the leanest rods. It still does the samething, but not as bad. It also has a little hesitation when I give it full throttle at speeds above 40mph too. I am going to recheck my plugs today and see if they still look to lean. The way it is set up it should be running rich as hell. any help would be great, thanks

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87'IROC-Z 350 TPI
Ported Plenum,Hypertech Air Foil,Ram Air kit,SLP headers,AFPR,80series flowmaster and MAF screens removed.
14.61@94mph(90+ degree weather)

 

*for at least the 5th time I will say the Edlebrock camshaft is junk. Any camshaft that doesn't produce enough signal at the carb to meter enough full to launch with a 3000rpm stall is junk and doesn't belong on a street car.

*You do not clear up bogs & stumbles by jetting the carb up. You change jets to change your A/F ratio at full throttle. Stumbling and bogging is caused from the transition to full throttle metering.

*The very first thing you do is to check how you are driving. Do you slam the pedal quickly to the floor? if so then this is wrong. A carburetor does its job much better when you roll the throttle smoothly, not in a stabbing action.

*Next you work with the accelerator pump-shot circuit on the carb. If you already have the lever in the top hole (closest to the plunger), then the next step is to go to a bigger pump nozzle (number printed on top of nozzle). You remove the air-horn and replace the nozzle with a larger one. I run a #45 or #46 on my 750.

*The next step is to try stiffer springs under your metering rods.

*The next step would be to increase the accelerator pump-shot stroke length (a custom mod). You can drill another hole in the bottom lever and straighten the rod slightly to increase the ratio of the pump. Then you must clearance the top of the air-horn around the plunger so the lever does not hit the carb.

*The next step would be to go richer (smaller) on the power-step of your metering rods.

You shouldn't have to go past these steps. These steps already assume that you have a good initial timing (15 to 20 degrees) and an aggressive timing curve. It also assumes that you have no vacuum leaks or internal engine problems like an over-tight rocker or cracked valve.
As a last resort I might try putting a 1" 4-hole spacer under the carb to increase low-rpm signal.
Still the best way to begin solving the problem is to put in a better camshaft.

good luck,
ODB

 

who rolls into the throttle when your drag racing. i don't. sounds to me like the air valve in the rear is opening to fast. like the air doors in a quadrajet. your right, he can jet all he wants and it won't help. there's nothing wrong with that edelbrock cam. if he's leaving the line at 3000rpm the carb has a pretty strong signal on it at that point. the cam has'nt got that much overlap.

 

I roll into the throttle always when I race. I leave the line at 1200rpm and pull 1.5 60-ft times @40mph.

 

so what does that mean? you can't make a smooth transition and go even faster by flooring it out of the hole? i suppose if your suspension could'nt hold it then that would make sense. otherwise it does'nt.

 

Sounds like a lean stumble. Increase the pump shot or increase the shooter size. I have no idea how to do it on an Edelbrock/AFB type carb but it's easy on a Holley.

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Follow my racing progress on Stephen's racing page
and check out the race car

87 IROC-Z Pro ET Bracket Race Car
383 stroker (carbed) with double hump cast iron heads and pump gas
461 Big Block installed and ready for the 2001 racing season

Best results before the 383 blew up
Best ET on a time slip: 11.857 altitude corrected to 11.163
Best MPH on a time slip: 117.87 altitude corrected to 126.10
Altitude corrected rear wheel HP based on power to weight ratio: 476.5
Best 60 foot: 1.662

Racing at 3500 feet elevation but most race days it's over 5000 feet density altitude!
Member of the Calgary Drag Racing Association

87 IROC bracket car, 91 454SS daily driver, 95 Homebuilt Harley

 

Kevin,
carbs do not inject fuel. Openning the throttle too quickly causes a quick pressure rise in the intake manifold. When the pressure shoots up, all the atomized or vaporized fuel instantly condenses back to a liquid and sticks to the manifold walls leaving nothing but extremely lean air going into the cylinders. Without using fuel injection there is nothing you can do to prevent a lean condition in this case. Both the pressure drop and the air are both much quicker than the fuel from the accelerator pump could ever be, and it takes time for fuel metering to start from the boosters.
Racers with carbs sometimes get around this problem by using stutter-boxes and high stall converters so that the boosters are metering immediately and there is no pressure rise in the intake.
Aside from that, you have to drive the carbureted car as I described. You will not go faster by flooring it quickly. The carb does not work best that way with a low rpm launch.



[This message has been edited by The ODB (edited March 04, 2001).]

 

Stabbing the throttle on a carburated car with a 3000 RPM stall converter that is apropiatly geared for the HP/Weight/RPM Range is perfectly ok and instant response can be expected. In the event the camshaft was at fault, the car would not pick up after the initial lay down/would show other signs such as idle quality,rpm range,dificulty in tuning,not allow the converter to flash as high as expected to name a few things there. I run a highly modified holley 750 on my '67 Camaro with 660 (1 to 1 after inital 10% opening)throttle shafts with very good response on the street as well as at the track. My '79 with a smog legal mild 383 and a GM3 converter that will flash to 1900 to 2100 with a modified 800cfm Qjet will also respond instantly. The secondary air valves are adjusted to where the slightest touch and they fall open. The diaphram is open in less than 1 second. I am not experienced with Carter style (Edelbrock) carbs to give any advice as to the proper tune up to eleviate his problem, although I believe someone here could. I am not an expert here but what I am saying is that although not as advanced as a fuel injected motor is, A carburetor can respond right up there and should be expected. This is only my opinion, but one that I felt I should let out when I see the response to the first question for advise or help received. Galen

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67 Camaro 10.5 tire street car 11:1 383 t350 4800 stall 4:30 9" 10.5X30 Mt ET Street 10.50'2@128 on motor
79 Z28 Restored w/9.17:1 383 Ca. smog legal S/RTorquers,modified by me Qjet,stock exh manifolds,3:73's 13.20's@103

 

4800 stall..
4.30:1 gears..
hmmm I wonder why you get instant response?
yeah that would be a good street setup.



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*I do custom performance mods on Edlebrock Performer carburetors (dualplane intake mods in the works),
White 1986 Irocz, 305 with iron #416 heads,
383 with aluminum TFS heads,
Edlebrock Performer-RPM intake and Performer #1407 carburetor, +110hp shot of crack, 700R-4 tranny, Vigilante 2400 lockup converter, 3.25:1 Ford 9" rear, Mcreary Road-Stars, SLP-stainless 1.75" shortie headers & Y-pipe, single 3" Borla exhaust, Linginfelter-TPI camshaft part number 74216 pulls 17" vacuum solid. Cam specs 213/219 @.050 114-LSA, sometimes advertised at 216/219 @.050 112-LSA .462/.470 lift @1.5:1 ratio. Using Harland Sharp 1.65:1 roller rockers. MSD-6AL, billet distributor, multi-retard, blaster-3 coil, and RPM switch. SouthSide machine subframe connectors, SSM lift-bars, Moroso 4" underdrive crank pulley.

N/A runs 10.9 @124,
Crack-runs 10.3 @135
haven't run at track since Oct-99

 

haha ODB




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60 Ranchero - Project ( Money Hole )
85 Sport Coupe LG4 - Daily Driver

Just another Hot Rod kid, or thats what they all tell me.
Livin' the Stereotype