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Hello I'm new to this site just bought me my first vette 78, been finishing restoring it and i'm just about done working out all the kinks but one and just cant figure it out. This is the deal when i accelerate from a stop it goes fine but then while going at about 35-45 mph and stomp on it , it hesitates then picks up on acceleration but not as much as it probably should, dont know what it could be. This is what ive replaced so far,
1. cap, rotor, and coil pack on distributor
2.fuel lines
3.Brand new quadjet carburetor
4. spark plugs
5. spark plug wires
6.air filter
7. pcv valve
8. timed at 8
9. carb filter

Dont know what this problem could be? i thought maybe i need to re time it again but still not sure. Anyone have any ideas would really help thanks!
 

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Couple of things to look at. First, you pointed out the timing is at 8*. What is the total timing at 3000 RPM? you need to adjust it to get close to 36* at 3000.
Second, you can adjust the air valve on the Q-Jet- it could be opening early or late causing the hesitation..

:welcometoourcl
 

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I also thought it was somthing to do with the carb but I was told that when you put a new carb that adjustments were not necessary is that true?
Not true.

All I can say is that you are in good hands now.

The direct replacement carbs are pretty much setup to go right on the intended vehicle. In other words you don't have to mess with jets or metering rods. They also try to get the A/F mixture as close as possble. With anything, not two motors will react the same way, so a little tweaking will be nessessary.
 

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I noticed that you're running a Quadrajet. That has a vac. operated secondary system, and you don't get immediate throttle response. There is a lag before the secondaries open up and kick in. If you had a mechanical double pumper then I'd start looking for a problem, as with those, there should be immediate throttle response.
Duane
 

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Yep, the secondary air valve on a Q-Jet is adjustable too. Open too soon it'll bog, open too late and you get a hesitation. 2 different and distinct problems. Idea is to split the difference. When it's correct, it'll come on like a mechanical secondary carb.
 

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Also check to see that your throttle plates are open all the way when the pedal is to the floor. You mentioned you put on a new Q-jet. Check your linkage adjustment with the new carb. It could be a little different than the old one. With the engine off, have someone hold the pedal to the floor while you look down the carb to see where the throttle plates are positioned. Good luck!
 

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I'm leaning toward the fuel sock in the tank because you state accel from a stand still is OK but at speed it is lacking. My reasoning is that at a stand still, fuel bowl will be full due to fuel system can keep up with fuel flow and at cruise the fuel bowl may not be full because the fuel system cannot replace the fuel being used, due to a higher flow rate. Basically what I am saying is as fuel flow rate increases the fuel sock becomes more and more of a restriction and cannot provide the correct fuel level in the carb, so the fuel level drops and carb becomes unable to correctly meter fuel into the engine. Problems like the one you describe are hard to diagnose over the net. But you got a lot of options to try. First do the easy one like Air valve Adj. about 3/4 of a turn after spring contact is a good starting point, then adj. in 1/16 of a turn increments either way until it feels like acceleraration has increased. Next check float level. Then check fuel flow rate. If the fuel flow rate is good then it is probably not the sock. Ssorry but I don't know what your fuel flow rate should be. good Luck!!! Let us know what you find. :z1-2:
 

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I noticed that you're running a Quadrajet. That has a vac. operated secondary system, and you don't get immediate throttle response. There is a lag before the secondaries open up and kick in. If you had a mechanical double pumper then I'd start looking for a problem, as with those, there should be immediate throttle response.
Duane
Not true. A Qjet can and does have instant response when working correctly. The secondaries ARE NOT your typical vaccuum secondaries. The vacuum determines how far they are allowed to open but are not merely opened by vacuum, a common misconception, it is a unique style of carb that works very well... I have ran a Qjet at the strip, it can have as much instant response any carb out there. The secondary air valve spring needs to be adjusted just right or it will bog or heisitate, I step on mine and she jumps.
 
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