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350 starts, runs, but when floored chokes and dies

I love my Chevy Chevette!
Posts: 1
Joined: 05/10
Posted: 05/31/10 06:52 PM

Hi all,

I took a stock crate 350 with 30k that had a solid bottom end and rebuilt to top end. (Cracked head caused the inspiration. On the motor not me.)
I got it all back together with the following.

Rochester Quad (professionally rebuilt and tuned)
I reconnected the HEI vacuum hose to front and capped side vacuum that went to  stock air intake.

Summit HEI distributor w/ timing set 15 BTDC

Edelbrock 2116 intake

Vortec heads rebuilt with upgraded spring set stock exhaust manifolds

High flow water pump

The only smog on this stock was the PCV and that is still in place.

It runs great if I don't stomp on the gas. It will run smooth at 600 idle to 6000 rpm fine.
I stomp on the gas and it hesitates, backfires through carb and dies.

I had a GM ASE mechanic friend of mine check the timing and valve adjustment for me before I noticed this problem. So, I'm thinking the valves and ignition wires are all correct.

Any ideas? Smirk  

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Posts: 7841
Joined: 12/09
Posted: 05/31/10 09:42 PM

Sounds like your accelerator pump circuit is not working properly or your secondary air flap needs more spring tension.
The accelerator pump draws fuel from the float bowl past a ball check valve,,, so on the upstroke the accelerator pump bore is filled with fuel. When you hit the throttle, the pump plunger goes down and should deliver fuel to the two venturi nozzles.
IF,,, the ball is not seating properly or the screw that holds the ball in place is loose,,, then instead of fuel going to your venturis, it will simply return back to the float bowl.
The air flap spring is adjust able and if you hold the throttle open by hand (engine not running) it should open with light pressure and close with spring pressure without effort.
When The Flag Drops.,.


The Bull ***t Stops.,.

P. Engineer, Engine Builder

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I mow my lawn and find Chevys
Posts: 5119
Joined: 12/09
Posted: 06/01/10 04:30 AM

+1 tuff

It does sound like secondary air valve trouble (thats the big two flaps over the secondaries). The opening rate is controlled not only by that valve's spring but also by the "choke pull-off". If it has been removed or is defective, this could be your trouble. Check also to see if your secondary metering rods are on their hanger, and that it is not bent.  

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I mow my lawn and find Chevys
Posts: 2738
Joined: 08/09
Posted: 06/01/10 05:31 PM

Hi there U 2  

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