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Old 06-01-2011, 05:13 PM
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SBC hesitation.

I have a sbc 350 in a 85 chevy c10. When ever I start off the line, and want to floor it, it will fall on it's face to the point that it would stall, I have to ease on the gas, or power brake it for this not to happen, also if I'm going down the road about 40-45 and dump the accelerater it will also fall on it's face. It is also spark nocking alot off the line and just going down the road like up a hill or something. Any ideas about a solution to my problems?
I've listed the motor specs below.

Motor specs.
I've got the timing set at 10 degrees advanced
SBC 350 bored 40 over
Flat top pistons
Cam: .477 Int/ .480 Exh & 224 Int/ 230 Exh duration @ .050
Edelbrock alluminum heads 2.02 in/ 1.6 exh valves
Edelbrock alluminum intake air gap
Holley 670 cfm carb, vacuum secondary's

I also running a TH350 auto trans with a 2000 stall converter.

Thanks for the help.
Ben

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Old 06-01-2011, 05:30 PM
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The accelerator pump is likely worn out. Rebuild the carb.
As far as the detonation, "flat top pistons" means nothing to us. Post a part number or give full detailed description.
What's the static compression ratio?
What's the piston deck height?
What's the thickness of the head gasket?
EGR?
Cylinder head chamber cc's?
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Old 06-01-2011, 05:34 PM
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I would first start by upping the timing to around 18*ish

My engine did that a long time ago I found out that it was leaning out so much when I stepped on it. But I was able to fix that because I have adjustable secondaries on my eddy avs

Idk much about holleys but see what you can do to richen it up a bit when you go WOT. Bigger jets in the secondaries maybe. Again Im not a holley guy.

The pinging/knocking could be many things.
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Old 06-01-2011, 05:43 PM
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Couple more questions-

The initial timing is only part of the equation. What is the total timing? Do you use a vacuum advance and if so what type of vacuum is it using, ported or manifold? What RPM is the advance all in by? You using premium fuel?

Are you using the detent cable or manually downshifting when you hammer the throttle?

Does the accelerator pump squirters discharge fuel the instant the throttle shaft moves? There has to be no slack in the accelerator pump linkage at idle and some small amount of slack (0.015" or so) at WOT.
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Old 06-01-2011, 05:43 PM
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I just built the motor about a year ago. It's got about 7000 miles on it. So everything I used was brand new. The truck prevously had a v6. I didnt assemble the bottom half of the motor so I'm not sure what kind of pistons it has, all I know is their completly flat with four vavle releifs. The heads have a 72cc combustion chamber. So I'm not sure about the compression. With the heads I have on it they said I should have around 9.5 to 1 compression if not higher.
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Old 06-01-2011, 07:46 PM
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I'm not sure what the total timming is, I'll have to check. It is vacuum advance running off a port off the carb. I do run premium in it all the time. I mainly manually downshift because I could never get the detant cable to work. It works fine when I downshift but when I don't and tap the throttle it hesitates then goes. The accelerator pump I am assuming is set right as soon as I pull back the throttle it moves the pump. I wasn't sure of the best way to set that up.
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Old 06-01-2011, 08:05 PM
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Quote:
Originally Posted by 85c10SS
I'm not sure what the total timming is, I'll have to check. It is vacuum advance running off a port off the carb. I do run premium in it all the time. I mainly manually downshift because I could never get the detant cable to work. It works fine when I downshift but when I don't and tap the throttle it hesitates then goes. The accelerator pump I am assuming is set right as soon as I pull back the throttle it moves the pump. I wasn't sure of the best way to set that up.
Any idea of what the vacuum is at idle, and/or what the cam specs are?

Do the idle mixture screws do anything when you adjust them in or out- as in, do you have control of the idle mixture using the screws? If you turn the screws in does the engine want to stall (it should, BTW)? If the throttle blades are open too far because the cam is big and the vacuum is low at idle, this can cause off idle driveability problems.

That said, that you also have a problem even above idle when you open the throttle, tells us that the likeliest culprit is the accelerator pump circuit- either the shooter size or delivery from the pump cam or the adjustment or a combination of the three.

The more specifics you can provide about the engine will help. Like does it have a "radical" idle, does it 'smell' at idle from unburned fuel, does it ever pop through the carb when you open the throttle quickly, etc.

Also how is the carb set up for jets, what size is the shooter, what color pump cam is installed.
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Old 06-01-2011, 08:33 PM
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The vacuum at idle is 17 in Hg. The cam specs are .477 Int/ .480 Exh & 224 Int/ 230 Exh duration @ .050. the part number for the cam is CCA-12-242-2
The idle mixture screws do affect the idle.
The components in the carb is whatever came with it I bought it new from summit racing part number HLY-0-83670.
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Old 06-01-2011, 08:52 PM
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Quote:
Originally Posted by 85c10SS
The vacuum at idle is 17 in Hg. The cam specs are .477 Int/ .480 Exh & 224 Int/ 230 Exh duration @ .050. the part number for the cam is CCA-12-242-2
The idle mixture screws do affect the idle.
The components in the carb is whatever came with it I bought it new from summit racing part number HLY-0-83670.
That's good vacuum and it's a good sign that the carb mix screws do affect the idle.

So I'm thinking the accelerator pump is likely at the root of the problem like TI said.

Be sure the pump arm actuates the pump the instant the throttle shaft moves, and you might want to read up on adjusting the accelerator pump circuit. Holley's instructions are on page 6 and 7 HERE. I would put a shooter that is 3 sizes larger than what's in it now- if you have a 28, go to a 31, etc.
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Old 06-01-2011, 08:56 PM
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Alright thanks for all of the help.
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Old 06-02-2011, 11:46 AM
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A couple of other things to check is to see what spring you have in your secondary's and to check to make sure its not opening to quickly and also on the holley street avenger 670 they are jetted very lean and stock is 65 primary and 68 secondary on those carbs and I had to bump mine automatically to 67 front and 72 rear.

Not saying your have to change your primaries but you might want to jet up the secondary's some as from what I have seen the secondary's are normally 6 to 8 sizes bigger then the primary's because of no power valve in the secondary metering block. But as always check and only do one change at a time as if you do a bunch at once you can get into all kinds of problems like I have before. I have learned my lessons from the pros on here.
Eric
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