Problem: Propane conversion not working

[quarterhorseman]

I recently bought an 88 f350 460efi, it has a propane conversion installed already(impco model e), but it does not seem to be working, i disconnected the fuel line going to the conversion and propane is being supplied to it, I also opened the bigger compartment which i think is the mixer/carburater, everything seems to be in working order inside, there dosent seem to be anything electronic about this, besides 2 little valves which i assume are the vaccums, the one going into the mixer seems to work because it clicks when i connect it to a power source, the one which appears to be an outlet for fumes or something dosent seem to make any noise when i do the same, could this be my problem.... there is a switch on the dash also which says "gasoline/alt fuel" which appears to me that all it does is turn off the fuel pump. anyone know anything about this, I couldnt find a manual or installation guide on this product online. thanks.. ps. when i turn my propane tank off and disconnect the line it seems that propane constantly flows, how long would it take to bleed a 3/8" 12 foot line... dosent seem to me like it should take long....

[Elderstarr]

Well, lets start off with the mixer (carbureator) first. Assuming all vacuum lines are properly connected, the only other problem I could see would be the port where the propane phisically is mixed with the incoming air, is plugged.

Did the assembly you bought come clean? Or recently cleaned? Something may have
been forced in there.

The vaporizer (before the carb) has a screen in place to capture any foregn material. Just pop the screws off and see if its clogged. Otherwise that should be in good order.

The Lockoff (main propane valve, between the propane line and vaporizer) should be connected only to a power source while the ingnition switch is in the ON position. If it clicks, it should be working. If your lockoff does not have a solonoid with wires attached, then it is a vacuum variety and should be connected to a vacuum line.

Do you have any clear pictures of your assembly?
Be sure to get your system tuned for the best performance.

[quarterhorseman]

ok, i just tried something else, the vacuum that is conntected to the mixer? that the propane goes into, is dead, I routed the vacuum line straight to it, by-passing the vacuum switch, vehicle turned over, ran fine... what is the importance of this vaccum switch.. and do i need it

[gasman6674]

The vac switch is a saftey to shut off fuel flow if the engine dies or you have an accident. They fail all the time. I would check to see what circut the 12v is coming from if its not from after the inertia switch I would move it there that way in a crash the inertia switch will shut down the lp system also. You can also put in a simple oil press switch to shut down the system if the engine dies which is more reliable than a vac switch but not as easy to hook up.

12 ft of hose may hold a couple of gallons of product and propane liquid expands 270 times when released to thr air so it may take a couple of minutes to bleed down if you only crack a line

regards