Callum pretty much has it.
If the problem is having to close the manifold valve too much to lower the flow, the reduction as the air passes through the valve will create turbulence. In a butterfly valve, the air is directed at an angle, creating a vector, increasing turbulence. In a gate valve, the air passes straight through a smaller space, so it isn't stopping the laminar flow as much (streamline airflow).
By adding a choke, you can lower the air flow into the blower, thus not closing the manifold valve as much. This makes a larger opening for the air to pass through, and decreases turbulence. At tuned full flame, the perfect setup is the manifold valve wide open and the gas needle valve wide open. The regulator valve and the choke are used to fine tune the flame to a smooth and even burn. Neutrality of the flame at full burn is the desired result when everything is tuned ( neither oxidizing nor reducing).
You should be able to tell if there is not enough air from your present blower because you will be running the blower with the manifold valve wide open and still can't get a good stable flame. When running at full tilt and properly tuned the flame should be stable and sound like a small jet engine at idle. The sound should be smooth and even.
As said, if the above things don't stop the problem (which it likely will), then a larger blower is probably needed. It is always better to have a slightly oversize blower. It will increase static pressure and can be choked back to lower the air volume if needed. If the blower is far too large, over choking can create starvation turbulence.
Adding VS to the blower is another good thing. A DC blower is easily speed adjusted, and 3Ph blowers c an run on a VFD. You still want the right size blower, though.
I will add some more info on blowers:
There are all sorts of things that will deliver air. They range from old fashioned bellows to digitally controlled toroidal blowers. THe most common is a centrifugal blower. The ones most folks use are the common squirrel cage type. They have even rows of closely spaced and slightly angled vanes. These work OK, but have lower static pressure, especially at low speed. The centrifugal blowers with angled larger vanes (often curved - toroidal) will deliver more air and at a higher static pressure. The static pressure does not drop as much when turning slowly. This is needed for larger burners and almost a requirement for a coal burning forge. These are very expensive compared to the squirrel cage types. In most cases, a Dayton squirrel cage blower that delivers 80-150 CFM is suitable for a blown forge.
THe cheap low cost blowers are fine for cooling computers and motors, but not so good for burners. A good rule of thumb is that if the output end isn't at least a 2.5X2.5"" opening it is probably too small. Hair dryers and leaf blowers are not the things to use. The better grade blowers have 3X3" or larger exit ports. 120VAC blowers can be speed controlled with a light dimmer. A dedicated fan speed control is a bit better, as the light dimmers aren't rated for the lode of many blowers. A DC blower can be controlled by many types of speed controllers.
Somer basic prices listed in order of power and usefulness ( 1-2-3):
1) Toroidal forge blower ( electric or hand crank) - $500-$600.
2) Dayton 120VAC squirrel cage forge blower, 125-150 CFM - $100-$120
2) Dayton 12VDC squirrel cage forge blower, 100-150 CFM - $125-$150
2) Used squirrel cage blower from a boiler or large gas/oil heating unit - Free to $100 (Can range greatly in CFM, so get the specs first. Many are far too big.)
3) Cheap squirrel cage blower from eBay, HF, or yard sale - 50-100 CFM - $20-$75
