Most fuel related problems lead to the vaporizer (some people call it a regulator). The carb on most propane lifts is nothing more than a throttle butterfly, all fuel metering is controlled by the vaporizer. Vaporizers work off manifold vacuum to tell how much fuel to send.
That's entirely incorrect.
Vaporizers do exactly as the name suggests: They evaporate liquid propane, into gaseous. Vaporizers are also called 'coverters' or 'evaporators'.
Manifold vacuum is NOT used to determine 'how much' fuel to send, as it is actually OPPOSITE of demand. Think about this: When engine is running fast, and you close the throttle, manifold vacuum skyrockets, and fuel demand actually goes to nothing. When manifold vacuum is low, either the throttle is open wide, and the engine is heavily loaded, or the engine's not running. Manifold vacuum on a gaseous system is only used for one purpose: Fuel lockoff... meaning... shutting OFF the fuel. A vacuum-fuel lockoff is a fuel shutoff valve that opens only when vacuum is present.
The device which modulates fuel delivery, is called by several names: "negative regulator", "Zero Governor", or "Fuel Controller", and the principle is simple:
A common liquid-fuel carbeurator consists of a passage where incoming airflow is constricted through a narrow venturi. As the airflow approches the narrow spot (the convergent side), airspeed MUST increase. on the other side of the venturi, is a divergent segment. As air coming through the constriction enters the divergent zone, airflow slows down, and in the process, there is a PRESSURE DIFFERENTIAL between exterior (atmosphere) and the venturi divergent zone. This happens to be the point where a little suction tube drops down into a float bowl, and because of the difference between venturi-divergent pressure (top of the tube) and atmospheric (in the bowl), there's a natural suction of liquid fuel up into the tube. Just downstream of the divergent zone is a throttle plate, and with all this in place, the suction in that tube... aka 'fuel demand'... changes with respect to two things: THROTTLE POSITION and MANIFOLD VACUUM.
Imagine for a moment, that instead of drawing up liquid fuel, you draw up gaseous fuel. To do that, you'd need a special fuel-pressure regulator...
Consider the pressure regulator on your air compressor, torch tank... whatever. Let's say you've got somewhere between 80psi and 2500psi in the tank, but you need to feed out at no more than 20psi... you grab the handle, twist it backwards, which relieves pressure on a spring, and the OUTPUT PRESSURE Gauge drops to 20psi... now your hose only blows to 20psi. IF you put your thumb over it, and squeeze strong, you'll hold back that 20psi, right?
Now dial it down MORE. Dial it all the way down to ZERO. It won't flow at all.
Now put your lips over the hose and SUCK ON IT. It'll flow, because you're applying LESS THAN ZERO to it's output... and the reg is set to ZERO.
A ZERO GOVERNOR is exactly that: It regulates fuel gas pressure to zero. Actually, to slightly negative pressure... so slightly, that it's not measured in PSI... it's measured in INCHES OF WATER COLUMN... "-W.C. For propane and natural gas fueling, the general standard is 11" of water column... that's 0.397psi.
The biggest catch of the zero-governor... is that when the engine is off, it HAS TO STOP FUEL FLOW. When the engine is cranking, there ain't much of a demand signal, but the zero governor's gotta be sensitive enough to respond and flow a little fuel. Many people make a mistake of taking a screwdriver to the zero-gov, and after messing with it, don't realize that it's LEAKING FUEL... hour later, the shop roof flies into the neighbor's back forty.
The gaseous carb 'mixer', can be something as simple as a venturi with a 'spud'... a tube to connect to the zero governor. The 'spud' is an equivalent to the demand point of a liquid fuel carb, and in fact, many propane conversion systems consist of a 'spud' adapter that one simply removes the bowl, float, etc., and the main jet, slips the 'spud' into where the jet WAS, and connects a few draw hose from spud to the zero-governor valve.
Back to the vaporizer... the evaporator. A liquid withdrawl propane system is common to forklifts and other machines that have high fuel demand, but SMALL tanks. Propane assumes liquid form at any temperature below -35F or so... and in liquid state, you can pour yourself a quart of it, and throw it on a fire, and it will SNUFF OUT the fire. Why? Because LIQUID PROPANE doesn't burn.
Even when warm, it will not assume gaseous state unless it's given enough volume to expand, and sufficient time and latent heat, to expand and evaporate. After withdrawing liquid from the tank, it must be turned into vapor, and that's done by passing it through an orfice, into a heated area where it can expand. The evaporator is either mounted to something hot (air cooled engines) or fed warm water from the cooling system (liquid cooled) or in some cases, hydraulic fluid (from hydraulic machines with air-cooled engines) to provide heat.
Of all the propane-fueled engine problems I've resolved, I've found that most propane systems are a result of insufficient understanding of propane systems by a person. Everything from simply not having the bottle facing the right way (liquid withdrawl tube facing up, means no liquid, just vapor), or someone tinkering with the regulator, mixer, or evaporator when they should have been reading the book first.
It's very important to remember that a very quick and easy way to kill everyone within nintey feet of you, is to mess around with a gaseous fuel system while lacking the most important tool: Knowledge.
Impco, Garretson, and others have quite a bit of good information posted online, and you'll also find documents like the Propane Serviceman's Handbook... all good reads. Other excellent sources of information: Any antique tractor manuals from the 50's to '70's, as they show plumbing for early gaseous fuel systems by Ensign, Zenith, Beam, Garretson, and many others.
A liquid propane fueling system consists of:
Liquid-withdrawl tank ->fuel lockoff->filter->evaporator->first stage regulator (10psi or 2psi) regulator (11" w.c.)->zero governor->Mixer
A gaseous withdrawl system consists of:
Tank -> first-stage regulator (10psi or 2psi) -> seconds stage regulator (11" w.c.) -> zero governor -> Mixer.
In liquid withdrawl systems, it's not unusual to see the fuel lockoff and filter combined as one component. The Impco VFF-30 as an example. It is also not unusual to see the first and second stage regulator combined into the evaporator body as a 'converter unit', like the Impco J or Cobra.
An excellent example of a zero governor, is the Garretson KN. Look these up, read about 'em, and look at the prices for replacement units, and then look at the prices for rebuild kits, then ask yourself wether it's worth the peace of mind to save a few bucks on the rebuild kit.