31 coupe will not go over 40 mph

Not too much info.
How does it run otherwise?
Where is the base timing set?
Where is the spark lever set?
Have you adjusted the GAV [ carburetor]?
Have you checked for fuel flow?
Have you checked the gas cap?
Is it also overheating?
 
I would first check points gap and timing . Ford specs for points gap was eighteen to twenty two thousants . Most set at a happy medium of 20 thousants .
 
Has this motor always been like this or is the problem more recent. Are you sure the cam is timed correctly?
 
Check the throttle linkage, make sure the butterfly opens completely. Check timing, like it was said earlier too little info. All youre going to get are a bunch of basically worthless guesses, including mine.
 
Not much other info given about how car ran before, but many years ago we had a like problem. It was the carburetor. Popped and back fired over 35 mph.
 
I think you have not owned a mod. A for very long? Correct? How well does it start? Does it miss at any point in the RPM range? Does it do anything else? Did this just start? Did you just buy the car? Does it sound like it's over reving when you hit 40 MPH.
Some possible causes; Distributor malfunction, either something wrong inside or not timed correctly. Poor gas flow. Pedal not opening the butterfly all the way. Dragging brakes (The emergency brake is not fully released). The tires are not inflated enough. An outside possibility is that your cam timing gear has lost a tooth and it jumped timing. This is possible but check this last. Mod A s were designed to run at 60MPH and we hear of stock A s that will run at 70. So it's not that your car has reached it's design limits.
Terry
 
When you mention "backfire", does it sputter and backfire and loose power while driving?
If so I would pull over and open that gas cap. If you hear a hiss from the gas tank your cap vent is not venting properly.
I went through this recently and found that to be the culprit.

If not that I would do as suggested above and especially check the spark advance operation. Just my two cents worth, which could almost be considered worthless I guess. Please keep us posted, thanks. Chap
 
I believe the backfiring is when you turn off the engine. I think the timing may be off a bit. Backfire means you have unburied fuels hen you shut down. You could be running s bit rich and need to lean out the carb settings.
 
Running too rich and unburned gas collecting in the muffler . When the engine is shut down , the temperature in the muffler increases and if there is the slightest leak in the exhaust ,t the air from the exhaust leak sets of the explosion-backfire in the muffler
 
you may well have a problem but please be aware that more lies are told about the size of fish and especially ones that got away. And more lies told of the yields of crops. And more lies yet again told about the speed of Model A's.
 
Check the condition of your spark plugs as well. If any are wet or showing signs of poor performance, they could also be a cause of unburnt fuel going out to the exhaust and running on less than 4 cylinders.This can also restrict your top speed.
 
Here’s an easy thing to check that could cause your prob. Check the ferrule on your fuel line that goes into the carburetor. If more than about 1/16 in. of line is past the ferrule, it can hit the screen and restrict flow. Happens a lot.
 
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