In this system, "dwell" is not the same as "dwell" on a truck with distributor points ignition system. In this instance, "dwell' refers to the % on time time between pulses to the fuel control solenoid valves (FCSV) not the degree of time the ignition points are open.
As Mengr noted, there are differences to internal springs and gaskets between different models of "cobra" LPG regulators and they are not all the same, even if the outside looks the same, except for markings on 1 cover.
First let me run through a few checks that I consider 'first-checks' for this system.
There are 2 vac hoses on this trucks throttle body. it is a known problem that the orifice inside the throttle body can become clogged, and they are very small and fine orifice of which the size and position is important to the design of the truck, so I take a fine welding tip cleaner and run it up into the orifice to be sure no carbon buildup is restricting vac. Next it is important to be sure that the correct hose in in the correct position, so be sure to confirm that the hose that goes to the distributor vacuum advance is the hose that is closer to the mixer and away from the manifold (top of the 2 orifices). NEXT, if this unit has the computer module that when you take the round (computer cable access port) cover off, you see only plastic potting and not resistors and transistors (original), then the vac hose that runs from the throttle body to the computer module should seem rather long, and it's length is important, as the vac drop from the length of hose was included in the calculations as to how this machine reads it's 'manifold air pressure', and 30 inches is not wrong, so if you or someone else cut the hose to 'look' like the right length, then you will want to replace that hose with the correct length hose. That hose will seem long, and will hang down almost to be the lowest spot under the truck. Units where you see the resistors and transistors instead of potting material inside the computer module do not require the vac hose, and the plug on the ecm is a closed up plug (second generation ECM for these units as the first supplier went out of business).
Next (and by far the most common reason for this error actually, in my experiences) the engine tune up, so RE-tune up the engine completely including engine timing (checking with timing light, with vac line off and plugged at the distributor, then reinstall vac line and confirm distributor vac advance is working, watching the light on the timing mark to be sure it advances with rpm increases) and spark plug gap (gap really does make a difference in these trucks).
My W.A.G. off the top of my head is that you have the vac hoses on the throttle body in the wrong spots. Although it is also very easy to mistake some of the vac connections on this truck as elbows or tees when they are really valves. the elbow mounted on the air-horn over the engine valve cover is not just an elbow, and there is a tee to the vac lines near the FCSV that is not just a tee, and an improper connection between any of those hoses can make these trucks run terrible or not at all.
When ever teaching anyone about s15g fuel control systems, everyone should be warned not to test the FCSV on these trucks with anything other than the software, as there is a real possibility that testing the wires will let the magic smoke out of the ECM, and it is a pulsed signal anyway.