model a ford won't accelerate past 20

Did it always not accelerate past 20mph? If so, check the throttle linkage for lost motion due to a bend or wear.
 
If you have a RHD (not sure about LHD) car it is possible for some unpleasant person to connect your spark advance up to retard instead of advance.
 
FWIW:

This was a similar question (3) months ago by Maggie05 on 11-01-2015:

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"My Model A wont go over 20 MPH under power, this is what I checked so far, timing and points set properly, I have a good stream of fuel to the carb, float is set properly, no backfiring, the car revs up fine and idles great, when I take it out for a spin, it just won't go. I don't know what else to check. I'm lost."
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Then again three (3) months later on 03-01-2016:

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hello, My model a ford won't accelerate past 20 mph good flow of gas, timing set right, will accelerate while car is setting still, need some help with this one.

Thanks -

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I think "if" Maggie05 would give a "few" more specific details ....... everybody herein would offer to assist ........ even in stating the physical location of this Model A ...... some could possibly drop by to physically "see" the Model A problem and offer positive solutions.

Just one (1) opinion in trying to assist.
 
Make sure all four are firing had bad cap that killed spark to 1 cylinder ran fine but no top end my top speed was 45 but everything else was perfect so if there were a few other issues 3 might just get you 20. With it running one by one flick brass clip off distributer carefull with the voltage then replace if you pull one and there is no change in sound its not firing. Replace each one as you go so only one off at a time.
 
It's better to short the bronze straps, rather than remove them. With no place to go, the high voltage tries to jump internally inside the coil, or short across the coil tower.

Original straps were bronze, which is springier than brass.
When you buy new straps, look for the more reddish bronze rather than yellow brass.
 
Limited fuel flow can also cause your problem. Try opening the fuel line at the carburetor and see if if shoots almost a foot with an almost full tank. If it just dribbles, shoot in compressed air, even from a tire pump with a point nozzle for a temporary fix and see if an adequate fuel does not eliminate the problem.
 
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