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Old 10-28-2003, 02:43 PM
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hesitation at full throttle

1978 Chevy El Camino, SB 350 V-8, double pump Holley 4 bbl carb, hei distributor. When I punch the pedal to the floor the engine hesitates, makes a loud pfffftttt, and then goes screaming down the road, leaving nothing but black exhaust behind me. If I ease on the throttle til about 5-10 mph and then punch it, it just screams (no pfffftttt and no black exhaust). I have read a lot about timing and am wondering if I need to adjust my curve, but nothing I read says anything about "initial" timing it all seems to refer to midrange timing. Is this possible a carb problem????
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Old 10-28-2003, 02:46 PM
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You may need to try a smaller squirter nozzle, or pump cam to take some of that fuel away initially.
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Old 10-28-2003, 02:50 PM
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somebody correct me if im wrong, but isnt that a case of not a big enough accelerator pump? that's the hesitation, and maybe the black smoke comes from being over jetted.
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Old 10-28-2003, 03:16 PM
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yep, sounds like not enough accelerator pump squirt along with too large of main jets. Now you did mean that the car drops on its face for a moment before recovering, and then the black smoke starts after it begins pulling?
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Old 10-28-2003, 03:56 PM
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If I give it wot while standing still, the engine will pause for a second and then kick in. If I give it wot after picking up some speed it wont pause but just go. Even if I'm going 40 mph down the road if a punch it it will sometimes pause before going. Some people say my carb is flooding, some say my timing is off????
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Old 10-28-2003, 04:19 PM
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Quote:
Originally posted by maddmaxx
Some people say my carb is flooding, some say my timing is off????
Would like to have additional info.

What is your timing? (both initial at idle and combined initial and mechanical @ 2500-3000 RPM)

What is your idle RPM?

What intake with the double pumper?

What squirter size?

Any engine mods?

Stock converter?

Rear gears and tire diameter?
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Old 10-28-2003, 04:33 PM
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If I understand your description, that is a classic description of your secondaries opening too soon and killing the gas feed venturi signal in the primaries. At slow speed, it takes the engine a moment or two to catch up and get the venturis flowing again. When you punch it at speed, the signal is strong enough to keep up with the sudden opening. This is a problem w/ double pumpers when trying to use them off-idle.

To test, disconnect and block the secondary butterflies closed. If that is the problem, your bottom end should come on strong. About all you can do is increase the pump size to supply gas with the squirter when the venturis won't. Reo pumps are common additions to double pumper carbs. Best bet on the street is to use a vacuum secondary which gives you the bottom end responsiveness you need but has all the top end you would ever want. Otherwise, increase the squirter size and plan on a little higher gas bill.
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Old 10-28-2003, 04:34 PM
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ok this is for bullheimer!
I guess I'm way out of my league on this one, but I'll answer what I can.
Timing at idle with vacuum unpluged is way above 14. My timing indicator only goes to 14. I already planned on buying a timing tape.
Initial and mechanical timing?????
Idle RPM???? No tach, any suggestions?
Intake. Edelbrock RPM Performer.
Squirter size?????
Engine mods. I don't think so, but I bought the car used.
Stock converter?????
Rear gears?????
Tires. Goodyear Wingfoot HP P225/70R14
I hate to sound ignorant, but I bought the car used and don't know what all was done to it. I do know that the engine was changed because the 78 El Camino only came with (up to) a 305 without hei distributor. mine is a 350 with hei distributor. I found this out after wiring the plugs wrong.
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Old 10-28-2003, 04:46 PM
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Since you are unsure of most of the questions I asked; I will tell you that you have a very good intake.(Edelbrock RPM) and the wrong carb.

As willys36 stated; get a vacuum secondary carb. I'd suggest around 600cfm.

Your initial timing sounds OK. When you do get the timing tape; try setting the Total (initial plus mechanical) timing around 34-36 degrees @ 2500-3000 RPM with the vacuum line disconnected and plugged. Plug it back up to 'FULL' vacuum source after you have adjusted the timing.

Set you idle RPM around 700-750 when in gear. (It will be slightly higher in Park.) Adjust the air/idle screws to get the highest vacuum at idle. You will probably have to re-adjust the curb idle speed after you adjust the air/idle screws.

ENJOY!!!

Last edited by Frisco; 10-28-2003 at 04:51 PM.
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