I just had a carb problem that sounds like yours.  Get someone to press the gas pedal all the way to the floor (with the car off) then as the pedal is being held to the floor look down your carb and make sure the back secondaries are fully opening. 
When I did this mine were only opening up about 25%.  Adjust your linkage so, the back barrels are fully open.

Not a carbie guru butt.......I did sleep at a Holiday Inn once!
Anywho......ya'll do realize a proper adjusted vacuum accuated secondary 4 barrel carb it will be VERY card to tell when the back two barrels "come in". They slowly start opening when there is enough vacuum to pull them open. You really can't feel a 'pop' in the acceleration as they "snap open"....  because they don't!!
You cannnnnnn......find what holds them shut and reduce the weight(if they are held closed with a weight) or if there is a spring you can stretch the spring or get a softer one.
How do I know this...you may axe!?!
The Edlebrock carbs have a weight that holds the  plate shut on the secondaries. I drilled a hole in the WEIGHT to make it lighter and therefore  the back two barrels come open sooner and quicker. Too  big of a hole and its bog city!  Little bit is better in this case.
Scope it out and see what holds them shut then......hot rod it!!
6sal6

That's a big carb for a 302 especially for something near stock. You want something like 500 cfm. As 6saly6 said a properly operating vacuum secondary will only open as needed by the engine and you should not feel it open. It should be smooth and seamless. If you do feel them open with a sudden rush of power that means the power actually fell off before the secondaries opened. I know on the Edelbrock, the throttle plates are mechanically opened but on top there is the weighted air valve that opens to the air flow requirements. I'm honestly not totally familiar with the Carter. I know that part is slightly different in operation.

The reason these carbs get a bad rep is because people don't take the time to understand how a carb works, any carb.

woodie wrote:

Huskinhano wrote:

That's a big carb for a 302 especially for something near stock. You want something like 500 cfm. As 6saly6 said a properly operating vacuum secondary will only open as needed by the engine and you should not feel it open. It should be smooth and seamless. If you do feel them open with a sudden rush of power that means the power actually fell off before the secondaries opened. I know on the Edelbrock, the throttle plates are mechanically opened but on top there is the weighted air valve that opens to the air flow requirements. I'm honestly not totally familiar with the Carter. I know that part is slightly different in operation.

The reason these carbs get a bad rep is because people don't take the time to understand how a carb works, any carb.

 
That's how the car came to me, I am also thinking that the carb is too big for the engine, I think the primary bores are the same for the 500 and the 625, but the venturi boosters are different sizes, if I could get hold of a used 500 carb, could I swap the boosters over to the 625?

If you got a used 500 in good shape, I'd use that rather then swap boosters. I'm not even sure you could. The Edelbrock is just an updated version of the Carter. The difference in a 600 Edelbrock  and the 625 Carter could just be the testing standards used. Personally I'd try to get the Carter to work, you already have it. Here's a link to the Edelbrock owner's manual. it's an excellent manual that not only tells you how it works but has tuning charts. I would imagine that it relates very closely and should be a big help.

I would also check out the whole engine. With only 13 hg of vacuum, that's way too low for a stock engine. Either you have a vacuum leak or something is way out of wack with timing. You really need to give the whole engine a good look over. I think your timing is retarded or something. Try setting the initial timing with the vacuum line off the distributor to 16-18 BTDC. That's what these engines like, total timing should be about 36 to 38 degrees. I would also have a shop that works on distributors give it a good check up. Have them set the mechanical advance to the 271 hp version of the 289.

Tuning manual.  http://www.edelbrock.com/automotive/misc/tech-center/dl/carb-owners-manual.pdf
 

woodie wrote:

Huskinhano wrote:

woodie wrote:

 
That's how the car came to me, I am also thinking that the carb is too big for the engine, I think the primary bores are the same for the 500 and the 625, but the venturi boosters are different sizes, if I could get hold of a used 500 carb, could I swap the boosters over to the 625?

If you got a used 500 in good shape, I'd use that rather then swap boosters. I'm not even sure you could. The Edelbrock is just an updated version of the Carter. The difference in a 600 Edelbrock  and the 625 Carter could just be the testing standards used. Personally I'd try to get the Carter to work, you already have it. Here's a link to the Edelbrock owner's manual. it's an excellent manual that not only tells you how it works but has tuning charts. I would imagine that it relates very closely and should be a big help.

I would also check out the whole engine. With only 13 hg of vacuum, that's way too low for a stock engine. Either you have a vacuum leak or something is way out of wack with timing. You really need to give the whole engine a good look over. I think your timing is retarded or something. Try setting the initial timing with the vacuum line off the distributor to 16-18 BTDC. That's what these engines like, total timing should be about 36 to 38 degrees. I would also have a shop that works on distributors give it a good check up. Have them set the mechanical advance to the 271 hp version of the 289.

Tuning manual.  http://www.edelbrock.com/automotive/misc/tech-center/dl/carb-owners-manual.pdf
 

 
Cheers, your last post sent me thinking I have had a good look over my engine as you said , and as this engine has no timing pointer on it, I decided to time it using the vacuum, it was was retarded, I now have 18 - 20 Hg of vacuum and the car pulls like a train, I appreciate your help.

Steve

What year is the engine? You can buy new timing pointers If the lower hose is on the RH side, the pulley on the harmonic balancer should have three bolts. If the lower hose is on the LH side, the pulley should have four bolts. Over time who knows what PO's do? If you have some other combination, it's wrong and you're never going to get a pointer to work.

Anytime you have a problem with something a quick look can fix a lot of them. I learned that a long time ago when I use to work on high speed packaging machinery with a customer looseing thousands of dollars and hour breathing down your neck.

woodie wrote:

The mustang is a 66 but the 302  engine is supposedly out of a 84 fox, the lower hose is on the passenger side, I know what you mean about a look at something can help, but some times a fresh set of eyes or some experience can really help,

Ok, you have a 50 oz balance motor. Your 66 would have a 28 oz balanced motor. I'm wondering if whom ever put the 84 motor in used or kept the 50 oz 84 balancer or bought the correct aftermarket balancer meant for these swaps? This could explain the lack of a timing mark. The timing cover on a 66 would have a cast on timing pointer. 70 and later went with a bolt on pointer, it's the same year Ford moved the lower hose to the fuel pump side and changed the bolt pattern on the balancer from three to four bolts.

If the person who installed the 84 motor kept the 84 balancer and you can't set timing, Proffessional Products makes a real nice balancer for about $100 US. The balancer has both sets of pulley bolt patterns, three different timing marks and the balancer weight bolts on so you can swap it to engines with different balance. Either zero, 28 or 50. Basically this will fit ANY SBF. I'm using one on the GT40P I swapped into my 66. Very nicely made.

ALWAYS determine true TDC.  I learned this the hard way when I found out my car was running about 50 degrees BTDC.  Ford used at least 3 different pointer locations and corresponding balancers over the years.  With mix and match parts its easy to have the wrong pointer for the wrong balancer and then you can be WAY off timing, even though the car still runs and drives seemingly okay.  Low compression engines don't really detonate even with the timing really far out of whack, so the whole "time it by ear" idea is worthless.  Its pretty easy to find true TDC with a piston stop and then use a timing tape or just a true TDC mark and a dial back timing light to properly time the engine.  Its worth checking to see which plate you have for the mechanical advance.  If you pull the breaker plate out of the distributor the advance plate is underneath and will have two slots marked with two numbers.  Whichever slot the tab is in is the slot you're working on, so double the stamped number is the total mechanical advance.  If it reads 13 for example total advance is 26 degrees.  Knowing that number you can then come up with a pretty reasonable idea as to where to set your initial timing.  If you had that 26 degrees foe example you wouldn't want to run more than 6-12 degrees depending on compression (less the higher the compression, more the lower).

While you're in there check the springs and put a nice light set in that will get total advance in by 3,000 RPM.  Low speed torque an seat of the pants grunt will be much improved.