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Discussion Starter · #1 ·
I have two autolite/motorcraft 2100 carbs.

Number 1: year 1967, 1.14 with 50F jets (my original carb)

Number 2: year 1973, 1.08 with 47F jets

Both have just been rebuild with the same kind of rebuild kit, but carb number 1 bogs down on acceleration and causes the engine to knock on hard acceleration.

Carb number 2 runs great, no problems

What could be the cause of the problems with carb nr 1?

a 50F jet will gives a more rich mixture than a 47F, right?
(yes, I'm new at carbs :winks )
 

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Well, the first carb is set richer, so my best guess would be clogged/inop power valve, or clogged accelerator pump
 

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Make sure the accelerating pump is working right. Also, the knocking is more likely a timing issue. If the accelerating pump is working right, I'd back off the timing a degree or two and see how it runs like that. Also, what engines are you running them on?
 

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Discussion Starter · #4 ·
Make sure the accelerating pump is working right. Also, the knocking is more likely a timing issue. If the accelerating pump is working right, I'd back off the timing a degree or two and see how it runs like that. Also, what engines are you running them on?
I can see a accelerator pump shot in each barrel, when activating the throttle. I'll disassemble carb nr 1 again tomorrow and have a look at the accelerating pump and power valve.

I've tried the two carbs on the same stock 289 C engine without changing timing, which is set to 6 degrees initial, with a stock points distributor.
 

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50F is a richer jet but that shouldn't affect your bogging problem since the accelerator pump richens everything up so much that the jets are irrelevant at that time. Float level could affect the mixture almost as much as a difference of 47/50 on the jets. I also agree that your knock is a timing problem. A lean mixture could affect knock but not by much.

My guess would be that the 73 carb is overall setup to use less gas and have fewer emissions. They may well have restricted the accelerator pump outlet nozzles on that model which will give you less gas for each stroke of the pump or at least delivered more slowly. Also check the settings on how the accelerator pump is operated. Which of the 4 holes is the rod using on the throttle shaft end and which of the 2 holes is the rod in on its outer end?
 

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Discussion Starter · #6 ·
50F is a richer jet but that shouldn't affect your bogging problem since the accelerator pump richens everything up so much that the jets are irrelevant at that time. Float level could affect the mixture almost as much as a difference of 47/50 on the jets. I also agree that your knock is a timing problem. A lean mixture could affect knock but not by much.
My timing is set to 6 degrees initial timing and 33 degrees total timing with vacuum advance disabled. All timing is in at about 2700 RPM. Points set at 30 degrees.

With this timing my original 67 carb with 50F jets causes my std 289 C to knock on acceleration but the carb from 73 runs perfect without knocking with this timing.

So it must be an error in the 67' carb?

My guess would be that the 73 carb is overall setup to use less gas and have fewer emissions. They may well have restricted the accelerator pump outlet nozzles on that model which will give you less gas for each stroke of the pump or at least delivered more slowly. Also check the settings on how the accelerator pump is operated. Which of the 4 holes is the rod using on the throttle shaft end and which of the 2 holes is the rod in on its outer end?
Yes but my 73' carb runs much better than my 67' carb.

Number 2 from top on throttle shaft end and the outer hole on the other end.
 
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