Author Topic: Huge flat spot on acceleration  (Read 2036 times)

Offline Hipo giddyup

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Huge flat spot on acceleration
« on: September 18, 2017, 09:50:01 AM »
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.... :-[
« Last Edit: September 18, 2017, 11:51:45 AM by carlite65 »
1967 Springtime Yellow Coupe, 289 2v 3spd, Metuchen built, Nov. 17th 66'
1966 Sahara Beige Coupe, 289 2v 3spd, Dearborn built, July 21st 66'
1964 1/2 Pagoda Green Coupe, 260 2v 3spd, Dearborn built, June 30th 64'
1966 GT350 Fastback clone, 289 HiPo, 725cfm Holley, 4spd, SanJose built, Nov 25th 65'

Offline midlife

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Re: Huge flat spot on acceleration
« Reply #1 on: September 18, 2017, 01:39:10 PM »
Any chance the two booster assemblies were swapped?
Midlife Harness Restorations - http://midlifeharness.com

Offline jwc66k

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Re: Huge flat spot on acceleration
« Reply #2 on: September 18, 2017, 03:20:52 PM »
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
« Last Edit: September 18, 2017, 03:23:27 PM by jwc66k »
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Offline preaction

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Re: Huge flat spot on acceleration
« Reply #3 on: September 18, 2017, 03:51:16 PM »
What are the factory jet sizes ?
8F02R218047-  July 18 1968   Dearborn

Offline Hipo giddyup

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Re: Huge flat spot on acceleration
« Reply #4 on: September 18, 2017, 03:51:59 PM »
 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!
1967 Springtime Yellow Coupe, 289 2v 3spd, Metuchen built, Nov. 17th 66'
1966 Sahara Beige Coupe, 289 2v 3spd, Dearborn built, July 21st 66'
1964 1/2 Pagoda Green Coupe, 260 2v 3spd, Dearborn built, June 30th 64'
1966 GT350 Fastback clone, 289 HiPo, 725cfm Holley, 4spd, SanJose built, Nov 25th 65'

Offline Hipo giddyup

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Re: Huge flat spot on acceleration
« Reply #5 on: September 18, 2017, 03:53:32 PM »
What are the factory jet sizes ?

When I rebuilt this carb I double checked the boosters and jets. They were correct for this carb.
1967 Springtime Yellow Coupe, 289 2v 3spd, Metuchen built, Nov. 17th 66'
1966 Sahara Beige Coupe, 289 2v 3spd, Dearborn built, July 21st 66'
1964 1/2 Pagoda Green Coupe, 260 2v 3spd, Dearborn built, June 30th 64'
1966 GT350 Fastback clone, 289 HiPo, 725cfm Holley, 4spd, SanJose built, Nov 25th 65'

Offline midlife

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Re: Huge flat spot on acceleration
« Reply #6 on: September 18, 2017, 06:55:11 PM »
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.
Midlife Harness Restorations - http://midlifeharness.com

Offline 67350#1242

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Re: Huge flat spot on acceleration
« Reply #7 on: September 18, 2017, 10:09:49 PM »
Primary booster will have nozzles and secondary won't.    Easy visual check without swapping.
67 Coupe SJ 11/16/66
67 GT350 SJ 2/01/67

Offline Hipo giddyup

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Re: Huge flat spot on acceleration
« Reply #8 on: September 18, 2017, 10:20:30 PM »
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.
1967 Springtime Yellow Coupe, 289 2v 3spd, Metuchen built, Nov. 17th 66'
1966 Sahara Beige Coupe, 289 2v 3spd, Dearborn built, July 21st 66'
1964 1/2 Pagoda Green Coupe, 260 2v 3spd, Dearborn built, June 30th 64'
1966 GT350 Fastback clone, 289 HiPo, 725cfm Holley, 4spd, SanJose built, Nov 25th 65'

Offline Dino1

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Re: Huge flat spot on acceleration
« Reply #9 on: September 19, 2017, 05:11:30 AM »
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.

Offline Hipo giddyup

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Re: Huge flat spot on acceleration
« Reply #10 on: September 19, 2017, 11:54:09 AM »
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.  :o

Thanks Dino1
1967 Springtime Yellow Coupe, 289 2v 3spd, Metuchen built, Nov. 17th 66'
1966 Sahara Beige Coupe, 289 2v 3spd, Dearborn built, July 21st 66'
1964 1/2 Pagoda Green Coupe, 260 2v 3spd, Dearborn built, June 30th 64'
1966 GT350 Fastback clone, 289 HiPo, 725cfm Holley, 4spd, SanJose built, Nov 25th 65'

Offline jwc66k

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Re: Huge flat spot on acceleration
« Reply #11 on: September 19, 2017, 12:16:06 PM »
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.  :o
No, the primary has a check ball, weight and hollow screw. The secondary has a solid screw.
Jim
I promise to be politically correct in all my posts to keep the BBBB from vociferating.

Offline Hipo giddyup

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Re: Huge flat spot on acceleration
« Reply #12 on: September 19, 2017, 01:36:44 PM »
You guys are correct, I was just talking out loud that I had the solid screw with the primary booster. Sorry for the confusion. ;)
1967 Springtime Yellow Coupe, 289 2v 3spd, Metuchen built, Nov. 17th 66'
1966 Sahara Beige Coupe, 289 2v 3spd, Dearborn built, July 21st 66'
1964 1/2 Pagoda Green Coupe, 260 2v 3spd, Dearborn built, June 30th 64'
1966 GT350 Fastback clone, 289 HiPo, 725cfm Holley, 4spd, SanJose built, Nov 25th 65'

Offline Hipo giddyup

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Re: Huge flat spot on acceleration
« Reply #13 on: September 25, 2017, 10:07:04 AM »
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.
1967 Springtime Yellow Coupe, 289 2v 3spd, Metuchen built, Nov. 17th 66'
1966 Sahara Beige Coupe, 289 2v 3spd, Dearborn built, July 21st 66'
1964 1/2 Pagoda Green Coupe, 260 2v 3spd, Dearborn built, June 30th 64'
1966 GT350 Fastback clone, 289 HiPo, 725cfm Holley, 4spd, SanJose built, Nov 25th 65'

Offline jwc66k

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Re: Huge flat spot on acceleration
« Reply #14 on: September 25, 2017, 04:31:56 PM »
Did you include the check ball and weight in the swap?
Jim
I promise to be politically correct in all my posts to keep the BBBB from vociferating.