Topic: Huge flat spot on acceleration

[Hipo giddyup]

Guessing this should be where this gets posted??

  I recently rebuilt an Autolite 4100 (C5ZF-D for a 289 automatic) to use on my 66' coupe project. I have set the timing on the car (10 degrees) and it starts and runs with no issues. Now, when you give it a quick hit of the throttle, it stumbles like it is not getting enough gas or too much? I took it out around the back field and noticed that if you accelerate while driving  it will do the same thing. It even shut off twice on me. I took the top off of the carb and the bowls have plenty of gas. I also checked the accelerator pump diaphragm by removing the primary booster and with moving the throttle, gas is indeed coming up to the booster assembly (also squirts fine when the booster is replaced). I then removed the carb and replaced the power valve just to be sure it hadn't ruptured. It hadn't but I replaced it anyway. Same issue.... 
  Now I'm questioning if something else it to blame such as the vacuum advance? It had gotten too late last night to continue on so I will likely pick up where I left off tonight. I have a known good working 4100 that I was going to put in place to fully rule out a carb issue. Any thoughts or ideas on this one? It's got me stumped....

[midlife]

Any chance the two booster assemblies were swapped?

[jwc66k]

I also checked the accelerator pump diaphragm by removing the primary booster and with moving the throttle, gas is indeed coming up to the booster assembly (also squirts fine when the booster is replaced).

Any chance the two booster assemblies were swapped?

Covered, but are the boosters correct? Is the accelerator pump arm in the correct hole?
Jim

[preaction]

What are the factory jet sizes ?

[Hipo giddyup]

 I will double check when I can get in front of my Mannel's book but yes, the boosters are correct for the carb and I would bet in the correct place. Since i was checking the booster yesterday I remember the primary was marked C H . I will say this DID NOT do this until recently. This car has sat for a few years and I am concerned there is something wrong with the distributor or vacuum advance. Maybe a bad advance plate? I did check the vacuum advance a few weeks ago (I have went over everything with a fine toothed comb) and it was not ruptured and I pulled vacuum on it. I have a new filter on the carb too so I was hoping nothing got sucked into the carb via the fuel line and clogged something? I'll let you know what I find as soon as I can. Thanks for the replies!

[Hipo giddyup]

What are the factory jet sizes ?

When I rebuilt this carb I double checked the boosters and jets. They were correct for this carb.

[midlife]

Just for kicks and giggles, exchange the two booster assemblies and take it out for a test drive.  Your symptoms are exactly what I experienced when I inadvertently swapped boosters on Midlife back in 2002 or so.

[67350#1242]

Primary booster will have nozzles and secondary won't.    Easy visual check without swapping.

[Hipo giddyup]

Thanks for the advice but the front booster is marked CH and the rear is marked A. I am very sure these are correct, and correct for this application. Let me double check my book.

[Dino1]

Also make sure you have the correct screw is holding the primary booster in place. It is hollow to allow gas to flow through it, the secondary one is solid.
Their is also a check ball and weight on the primary side inside the hollow screw when it is assembled.

[Hipo giddyup]

Wow!! That must be where I goofed. The primary has the solid screw ( I remember this when checking the acc. pump diaphragm)  so the secondary has to have the ball/weight and hollow screw. Darn it! Let me confirm and switch it if it's wrong. I was likely so excited during the rebuild I switched them by mistake. Surprising it ran at all. 

Thanks Dino1

[jwc66k]

Wow!! That must be where I goofed. The primary has the solid screw ( I remember this when checking the acc. pump diaphragm)  so the secondary has to have the ball/weight and hollow screw. Darn it! Let me confirm and switch it if it's wrong. I was likely so excited during the rebuild I switched them by mistake. Surprising it ran at all. 

No, the primary has a check ball, weight and hollow screw. The secondary has a solid screw.
Jim

[Hipo giddyup]

You guys are correct, I was just talking out loud that I had the solid screw with the primary booster. Sorry for the confusion.

[Hipo giddyup]

Bingo!!!! Dino1 hit the nail on the head. I had the primary and secondary booster screws reversed.    Lesson learned and not soon forgot.. The car is running 100 times better so thanks to all for the feedback and help in solving this.

[jwc66k]

Did you include the check ball and weight in the swap?
Jim