Ruptured accelerator pump diaphragm, bad accelerator pump check valve, sticking/plugged accelerator pump check ball or missing weight or plugged orifice.

 

Have checked the new accelerator diaphragm looked good but replaced anyway. The orange check valve is in place and seem OK. The ball and weight are in place. Unclear as to where ' plugged orifice ' could be located. Thanks, Brian

 

"Plugged orifices" include the tube leading from the pump to the center well under the booster, the holes in the hollow bolt securing the booster or the discharge nozzles in the booster itself. The check valve could be preventing fuel from entering the pump chamber or allowing it to leak back into the float bowl.

 

 

Thanks for the explanation. The accelerator pump diaphragm area seems OK. Did an extensive cleanup of all tubes, chambers and vacuum passages but it's possible I missed something. Could the problem be on the ignition side? Brian

 

Anything is possible....

 

Thank you. Back to my original set of running conditions and stumble/hesitation. Probably not the accelerator pump or assembly. Nor any of woodchucks other suggestions. From the drivers seat it feels akin to a flooded carb. causing the almost stall. Again, only happens under a load. Preforms fine in the garage. Accelerate and stumble. Get off the gas pedal the motor recovers and resumes idling get on the gas and the car speeds up. The initial hard gas pedal causes the problem. Thoughts? Thanks, Brian

 

Had the same symptoms as you and it turned out to be my electronic ignition unit (a Pertronix II installed inside the distributor). Idled fine, could rev it up with no misfiring or anything, but as soon as it was under load it was sluggish and stumbled. I messed with my carburetor FOREVER until I realized it wasn't a fuel or timing issue.

Could be a plethora of things, but in my case, the Pertronix unit wasn't getting a full 12 volts because of the resistance properties built into the pink resistor wire going to the coil. I set up a relay to bypass it and all was resolved.

Pertronix II Relay Installation

Hey all, I'm in the process of installing Pertronix's relay kit (PN 2001) for my Pertronix II system because it's not getting the full 12V. I have a few problems. 1. The Rally Pac tachometer does not work anymore. 2. The engine does not shut off when I turn the key off. Only point number two...

www.vintage-mustang.com

Pertronix actually sells a kit for this with good instructions, part number 2001. If this ends up being your issue as well, use the kit with the diagram provided in the above thread (post number 2) and you should be good.

More relevant information scraped from this thread:

Pertronix Relay Question

My Relay should arrive tonight, Hoping to solve my 5. 1 V at coil in run position (engine off) , 10.1 V at idle at coil. I already have questions. 1. Where the small purple relay line is attached to the Ig. wire /Resistance wire ( red/green ) on a 70. The black dot in diagram. The red...

www.vintage-mustang.com

 

most carburetor problems are electrical.

A bad condenser or worn distributor bushing will do it as well.


ken

 

Thanks guy. Very helpful and going in the right direction. Already have swapped distributor with a known good one. No Change. Have gone from points to pertronix and back again. No change. Have also changed condensers but will try anther new one tomorrow. Through all these changes/swaps the car starts right up idles and revs just fine. Brian

 

Lots of possibilities. Since it is running poor under load, I would check the flow through the jets to the boosters. Also, as soon as it runs poor, are you able to quickly pull the air horn and check the float level to confirm your fuel pump isn't falling behind? I had a similar situation with a 65 convertible that had a failing fuel pump.

 

Coil?

ken

 

Thanks for the help and suggestions. The coil? Dwell is reading at just under 27. Coil is probably OK. As suggested, a couple of times, I pulled the top plate and took a look around. Primary bowl; the orange tip for the accelerator inlet check was a bit long so cut it back. Small bit of silicone obstructing one primary jet 'orifice'. Removed the silicone obstruction. Put everything back together and went for a short drive. Acceleration is back. Some driving will tell the tale? Brian

 

Check the dwell not only at an idle but run it up to 2500 and see if it stays there.
27 is a little low shoot for 30

ken