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..
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)
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.
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.
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.
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. Todays 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).