HOwdy John:
And Welcome to the FORUM! It's all about sixes over here. We can usually help.
A couple of questions first. Is your Mustang a Calif. car originally? When you say, "spark arrestor" I'm guessing you mean the Spark Control Valve on the side of the carb? Is that correct?
First check all connection on the metal line/rubber tube that connects the carb vacuum to the distributor vacuum advance canister to make sure there are no leaks and that it is not plugged. IF that is correct, disconnect and plug the vacuum line and plug it, then hook up your timing light- be sure to mark the timing mark on the damper with a piece of chalk. Now start the engine, blip the throttle while watching the mark with the timing light. You should see the mark move. If it does, proceed with timing. If it doesn't you have a problem somewhere. Is this a manual trans car? or au
FYI- there is a sticky in the small six forum somewhere that accurately describes the function of the SCV and the Load-O-Matic distributor. It is worth reading.
The Holley 1940 carb was a factory replacement carb for the 1100, after Autolite went out of business. They were a generic carb adapted to many different applications. For our engines they seldom work perfectly.
Where are you located? Elevation can be a factor as well as the choke system on these carbs. Is the Autolite 1100 you got a new carb/rebuild? Rebuilt carbs are notoriously poorly put together.
Keep it coming and again, welcome.
Adios, David