holley 570 vs holley 650

[71coupe]

engine specs: engine rebuilt

306

comp cam (thumpr cam)

hypereutectic pistons

orig block

orig heads

long tube ceramic coated headers

flowmasters

pertronix dizzy

rearend - stock 3.0

the carb that the engine builder recommended was the street avenger - 570 vacuum secondaries, dual feed

The car has a slight hesitation when I stomp on it....just a half second maybe a second...then it blasts off. It runs pretty darn good; great top end. Of course with the 3.0 - doesn't help much. I plan on changing to 3.55. Had a ol timer adjust the carb the best he could, but still the hesitation exists.

Then I changed over to my "old" 4777-2; dual feed; true double pumper - carb was rebuilt so I just had to tune the rich/lean and float levels. took it for a spin and WOW...night and day! no hesitation....just spun the wheels like crazy! thought I'd never take off.

my question is: has anyone had issues with the street avenger - with vacuum secondaries.

how can an old carb perform much better than a newer one?

now don't get me wrong..the street avenger is nice if I'm cruising....great on gas as opposed to the double pumper...gas mileage went out the window!!! hahaha I'm loving the old carb..."I stomp and go and I control the secondaries"....car is just a weekend hotrod....

hahaha...here is pic...showing...good 'ol american muscle....stang!!!

 

[goodnigh]

You may need to tweak your timing.

My engine setup is similar to yours

although mine is a Cleveland with a 770

Avenger. The gas pedal is real twitchy.

Current timing is 9 BTDC.

mike

 

[TommyK]

engine specs: engine rebuilt

306

comp cam (thumpr cam)

hypereutectic pistons

orig block

orig heads

long tube ceramic coated headers

flowmasters

pertronix dizzy

rearend - stock 3.0

the carb that the engine builder recommended was the street avenger - 570 vacuum secondaries, dual feed

The car has a slight hesitation when I stomp on it....just a half second maybe a second...then it blasts off. It runs pretty darn good; great top end. Of course with the 3.0 - doesn't help much. I plan on changing to 3.55. Had a ol timer adjust the carb the best he could, but still the hesitation exists.

Then I changed over to my "old" 4777-2; dual feed; true double pumper - carb was rebuilt so I just had to tune the rich/lean and float levels. took it for a spin and WOW...night and day! no hesitation....just spun the wheels like crazy! thought I'd never take off.

my question is: has anyone had issues with the street avenger - with vacuum secondaries.

how can an old carb perform much better than a newer one?

now don't get me wrong..the street avenger is nice if I'm cruising....great on gas as opposed to the double pumper...gas mileage went out the window!!! hahaha I'm loving the old carb..."I stomp and go and I control the secondaries"....car is just a weekend hotrod....

hahaha...here is pic...showing...good 'ol american muscle....stang!!!

Disclaimer: This is the way I do it. It is not necessarily the right way or the best way. It is my limited experience playing with about 8 or ten carbs including a 670 SA over the last 9 years on several different engine combos.

The SA carbs are kind of notorious for tip in stumble issues and lean calibrations in general. There are also many reports of pump lever arm geometry issues. Specifically that the arm can end up riding on the throttle shaft spring rather than on the pump cam itself. This delays the pump shot which causes a stumble. First visually inspect how the lever contacts the pump cam. If it appears that the lever is not in conflict with the spring then the problem can be resolved with normal tuning of the accelerator pump circuit.

The first step in tuning the pump circuit is actually making sure your initial timing, transition slot exposure and idle mixture screw settings are correct as these have a large effect on tip in response. If you are sure these items are set correctly and the problem persists it is time to tune the pump circuit.

The first and most critical aspect of pump tuning is to make sure the lever is adjusted such that the nozzle squirts fuel at the slightest movement of the throttle while still maintaining about 010-.015 of clearance (free play) on the arm (piece attached to float bowl that adjusting bolt contacts) at WOT. This is necessary so the diaphragm in the bowl isn't damaged when you floor it. It is sometimes necessary to bend the lever (not the arm) to achieve the proper setting.

If you are sure that you have adjusted the lever correctly and the problem persists it is time to break out the pump cams and nozzles.

The blue cam is the most aggressive initially and is usually where I start. If changing to the blue cam immediately solves the issue then try a white cam as it is a bit less aggressive and may save some fuel if it works. The pump lever must be readjusted after every cam change.

If the blue cam does not resolve the issue increase the squirter size still using the blue cam. A larger squirter will provide a larger initial shot of fuel over a shorter duration. What you are trying to do is find the balance between the squirter and cam which provides the best tip in response using the least amount of fuel. The idea is to make a rich over correction then dial back the fuel to find the lean limit (the point the stumble returns) and then go back to last previous richer setting. This takes time and patience.

Also, it is a good idea to put a rag in the primary bores under the nozzle to catch the fuel which will be squirted while you are testing the pump circuit. You don't want raw fuel being dumped into the cylinders.

HTH