Ford 641 sputters at higher Rpm

I have a ford 641 work master.

It idles fine but when I turn the throttle higher, it starts to sputter and backfire. It also runs fine fine at a lower RPM, but when I move to half throttle...that’s when the sputtering starts.

It also backfires sometimes when I shut it off. Is it my carb? Not enough fuel being delivered at higher rpm? Clogged jets?
 
Could be the carb. What do you have the main adjuster set at?
Try backing it out 1/4 to a 1/2 turn from where it is.
 
No way to tell definitively from available information but sounds like ignition.

Never look to carburetion unless/until you are certain that ignition is spot on.

Dean
 
Could it be that the spark advance is stuck (centrifugal advance in the distributor)? This can be checked with a timing light.
 

I have no way of checking the centrifugal advance. I only have basic garage tools. I don’t have a timing light.

Is there another way?
 
I think its like a chevy distributor. When you take the distributor cap off you should be able to twist the shaft some with a little hand force, its spring loaded. do you have the little metal clip under the rotor? If not that will mess it up cause it will flop around, they are easy to lose.
 
I had a 960 do the same thing. I used a timing light to check if the timing advancing when the rpms went up. At
2400rpm, your timing should be 24-27 degrees advanced. Could be a spring in the distributor which was my
problem. I agree with not messing with the carburetor unless you are positive that is the problem.
 

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