Throttle hesitation - Impala Tech
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post #1 of 11 (permalink) Old 05-18-2010, 11:32 AM Thread Starter
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Throttle hesitation

I am having a the issue of throttle hesitation on my newly rebuilt 1964 283, the carb is a Rochester 2gv, I thoroughly cleaned it and installed a rebuilt kit and to the best of my knowledge installed all components and made all suggested adjustments correctly. 2 pumps of the peddle fires right up and idles good, quick acceleration takes right off with no hesitation, the issue is on slow acceleration there is significant hesitation. Any guidance why would be helpful. Timing and dwell is right where it should be. And I have adjusted the idle screws to max vacuum which is around 17..

HELP..

Check out my blog..1964 restore...
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1953 Chevrolet 3100 Pickup
261ci Stovebolt.

1966 Chevrolet El Camino
327ci Mouse.

1964 Chevrolet Impala
283ci little Mouse.

Next on the list....1962 Impala convertible..
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post #2 of 11 (permalink) Old 05-18-2010, 04:04 PM
BA.
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Do you have any mods like headers or free-flowing pipes/mufflers added? Stock intake?

in a general sense, it sounds to me like either pump adjustment, or not getting a good enough fuel shot when at part throttle. I'm more familiar with the ol' Holly 4 barrel but i'll be digging into my own 2 barrel soon enough! Eek!

You could also try a 1/2 enrichening of the idle circuit screws,.....to see if it affects your symptom. (trying to see if the problem is related to the transition between low speed fuel circuit and mid/high speed fuel circuit)

HOW A NOVICE REBUILDS A 66 IMPALA CONVERTIBLE:
http://www.impalas.net/forums/blog.php?u=1432
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post #3 of 11 (permalink) Old 05-18-2010, 04:56 PM Thread Starter
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No mods, stock intake, stock dual exhaust...mild Cam with the following spec's:

204/214 duration at .050 with .420/.443 lift...

It appears that the accelerator pump does not start squirting until about half throttle..?

Check out my blog..1964 restore...
http://www.impalas.net/forums/blog.php?u=2304

1953 Chevrolet 3100 Pickup
261ci Stovebolt.

1966 Chevrolet El Camino
327ci Mouse.

1964 Chevrolet Impala
283ci little Mouse.

Next on the list....1962 Impala convertible..
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post #4 of 11 (permalink) Old 05-18-2010, 07:25 PM
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On the carbs I've played with, the pump/squirter seemed to be squirting sooner than what you're seeing. I'm not sure if the 2 barrel should be the same but I would think so, just shooting a few cc's less fuel due to carb size.

On the Holley's there is the tiniest bit of play, hundredths of an inch, before the linkage started actuating the pump. This was adjustable.

Does your's look adjustable to take up any slack in the mechanism?

Also curious, since my car is not here for me to check,...does your linkage have a plastic 'cam'/follower that is in turn actuating the pump lever to shoot the extra fuel?
It's hard to explain succinctly, and also since I can only reference the Holley.
just trying to help though.

HOW A NOVICE REBUILDS A 66 IMPALA CONVERTIBLE:
http://www.impalas.net/forums/blog.php?u=1432
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post #5 of 11 (permalink) Old 05-18-2010, 08:26 PM
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there should not be any play from the time the accelerator linkage moves until the pump plunger starts to be pushed down. In fact, the pump plunger should actually hold the top of the carb away from the body slightly before the screws are tightened. That can be adjusted by moving the roll pin that holds the pump lever to another hole in the lever.
The other thing you want to check is to make sure the distributor advance diaphragm is not broken. just suck on the hose to it and block it with your tongue, it should hold vacuum. That hose should connect to the port above the throttle plates.
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post #6 of 11 (permalink) Old 05-18-2010, 11:49 PM
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Don has it right. Allan, The accelerator pump cam is specific to Holley and the like. A very much over-looked adjustment on tuning carbs. (I have a drawer full of those colored cams) Rochester carbs have the arm with several holes like Don mentioned. Sort of like an Edelbrock only different.

Don't listen to me, what do I know? I've only been doing this for 30 years.
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post #7 of 11 (permalink) Old 05-19-2010, 03:00 PM Thread Starter
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Thanks guys, I don't recall any adjustment on the the little 2 barrel accelerator pump, however, I am going to check it out when I get home and will report back my findings..

Check out my blog..1964 restore...
http://www.impalas.net/forums/blog.php?u=2304

1953 Chevrolet 3100 Pickup
261ci Stovebolt.

1966 Chevrolet El Camino
327ci Mouse.

1964 Chevrolet Impala
283ci little Mouse.

Next on the list....1962 Impala convertible..
Beetle is offline  
post #8 of 11 (permalink) Old 05-19-2010, 05:24 PM
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I wonder if maybe your float is set too low and not allowing the accel pump well to fill completely? Been a while since I've worked on a 2 bbl.
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post #9 of 11 (permalink) Old 05-19-2010, 08:27 PM Thread Starter
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ok...accelerator pump only squirts with a rapid quick acceleration, slow and easy no squirts...hmmm, and no adjustment ability.

Check out my blog..1964 restore...
http://www.impalas.net/forums/blog.php?u=2304

1953 Chevrolet 3100 Pickup
261ci Stovebolt.

1966 Chevrolet El Camino
327ci Mouse.

1964 Chevrolet Impala
283ci little Mouse.

Next on the list....1962 Impala convertible..

Last edited by Beetle; 05-19-2010 at 08:47 PM.
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post #10 of 11 (permalink) Old 06-07-2010, 10:43 AM Thread Starter
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popped the top and the accelerator pump cup was mush....purchased and installed a synthetic cup from Vintage Speed shop....all is well so far.

Check out my blog..1964 restore...
http://www.impalas.net/forums/blog.php?u=2304

1953 Chevrolet 3100 Pickup
261ci Stovebolt.

1966 Chevrolet El Camino
327ci Mouse.

1964 Chevrolet Impala
283ci little Mouse.

Next on the list....1962 Impala convertible..
Beetle is offline  
post #11 of 11 (permalink) Old 06-07-2010, 10:53 AM
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Quote:
Originally Posted by Beetle View Post
popped the top and the accelerator pump cup was mush....purchased and installed a synthetic cup from Vintage Speed shop....all is well so far.
A warning to all owners of antiques. Today’s fuel contains a minimum of 10% alcohol which will dissolve rubber. You car will also leak gas through the natural rubber lined hose unless it is also replaced with newer neoprene plastic lined fuel hose. Paper filters also slowly dissolve because alcohol is absorbent and sucks water vapor out of the air like a desiccant. That water causes paper fuel filters to also dissolve sending paper fibers through the passages of the carb to clog it up. New cars use fiberglass fuel filters to get out particles down to 100 micron size (the sintered brass filter some Rochester's have is a 10 micron filter).

Big Dave
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