Hesitation/Dip in Acceleration
Thread Starter
Instructor
Joined: Jan 2015
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Hesitation/Dip in Acceleration
Hey there.
1973 350 4 barrel manual trans
When I accelerate I experience a momentary hesitation or dip in acceleration. Almost like the engine wants to stall. Then smooth up to shift point.
Happens in all gears at the same RPM (around 2k)
No other symptoms. Idle is good. No problem at start-up.
Carb/fuel system problems?
1973 350 4 barrel manual trans
When I accelerate I experience a momentary hesitation or dip in acceleration. Almost like the engine wants to stall. Then smooth up to shift point.
Happens in all gears at the same RPM (around 2k)
No other symptoms. Idle is good. No problem at start-up.
Carb/fuel system problems?
Last edited by Brother Rob; Jun 5, 2015 at 11:31 AM.
Advanced
Joined: Mar 2013
Posts: 72
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From: New York
Does this happen during normal or hard acceleration? If it is during hard acceleration, check the adjustment of the secondary air doors on your carb assuming you have a Quadrajet. They may be opening to late or too soon. There are videos on youtube on how to do this
Race Director
Joined: Jan 2000
Posts: 12,399
Likes: 247
From: San Jose CA
Lean anywhere can cause this.
A very common cause as well is accelerator pump failure - due to age/wear or fuel incompatibility (ethanol doesn't play nicely with many seal types).
An accelerator pump failure will exhibit itself with sharp throttle transitions as that is what the accelerator pump is designed to handle - compensating for the sudden inbound air well the main circuits "catch up".
Quadrajet secondaries do not come into play until roughly 2/3 throttle travel and there's a slight delay due to the choke pull off - so if you can do this by just lightly stabbing the gas and have an instant drop off in response, particularly if you aren't exceeding that 2/3 throttle travel - it's more likely primary side (accel pump or running too lean).
Thread Starter
Instructor
Joined: Jan 2015
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Yeah. It only happens at lower RPM. So if I'm cruising at 2,000 RPM in second and give it some gas (soft or hard acceleration) I get the dip. But if I'm cruising at 2,500 RPM, no dip when I accelerate. Sounds more like accel pump or lean mix.
Race Director
Joined: Jan 2000
Posts: 12,399
Likes: 247
From: San Jose CA
Verify that you've got an accel pump squirt by looking down the venture and giving the gas pedal a flick - make sure the engine's off. 
If it's a Q-Jet, you'll have 2 squirts and will have to hold the air flap open. If it's a Holley, you could have 2 or 4 depending on whether or not it's a "double pumper".
Many stock Q-Jets run very lean because (A) they were calibrated stoich or slightly leaner on the primaries for fuel economy and performance and (B) ethanol has a higher air content than gasoline, so e-blends run leaner than gas.
The secondary circuits have all been calibrated quite rich stock, possibly too rich, which would explain why higher RPM you don't have issues.
If it's a Q-Jet, you'll have 2 squirts and will have to hold the air flap open. If it's a Holley, you could have 2 or 4 depending on whether or not it's a "double pumper".
Many stock Q-Jets run very lean because (A) they were calibrated stoich or slightly leaner on the primaries for fuel economy and performance and (B) ethanol has a higher air content than gasoline, so e-blends run leaner than gas.
The secondary circuits have all been calibrated quite rich stock, possibly too rich, which would explain why higher RPM you don't have issues.
Thread Starter
Instructor
Joined: Jan 2015
Posts: 106
Likes: 0
Well I think I figured out what the problem might have been.
Bad batch of gas.
The other day I filled up at a random station. Then today I let the tank get almost empty and filled up at my usual place. The acceleration problem seems to be gone. It could have just been a clog that just happened to clear, but seems too coincidental.
Bad batch of gas.
The other day I filled up at a random station. Then today I let the tank get almost empty and filled up at my usual place. The acceleration problem seems to be gone. It could have just been a clog that just happened to clear, but seems too coincidental.
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