Hi, my 351M Bronco has a few issues.

-runs beautiful 1500 rpm or more regardless of timing
-starts good cold but will not idle without help til warm
-takes a bit to start warm with gas pedal depressed.
-will not idle below 800 rpm, it lopes, misses etc.
-will not run with brake pedal applied, disconnected and capped brake vac line had to bump up idle huge to run, runs same.
-has lots of power and is running on all cylinders.
-I think I have a vac problem but I may have a problem with the propane system as well-power valve is backed out almost all the way, idle screw on back of carb doesn't seem to do anything (IMPCO 425)- regular throttle body idle screw is all I can use to affect idle.
-Engine was rebuilt 6000 km ago-no smoke-no leaks-good power.
-truck works great if idle is 1200 rpm+
-has aftermarket 4bbl intake and impco adapter
-throttle response is quick with no bogging or pinging.
any thoughts?
thanks
007BRONCO

 

i know nothing about propane but i would sugust taking it to a licenced propane mechanic and getting him to look it over, an hours labor to look over it could save ya a hole day of screwin arround

 

I hear ya, and I will consider it, I'll probably learn all there is to know after an hour with a good propane guru.
cheers
007BRONCO.

 

You can use an 18" length of 1/2" or 3/4" copper tubing to locate vacuum leaks. hold one end you your ear and point the other end toward the engine, sweeping it around until you hear the leak. Careful not to get too close to the fan, belts or pullies.

Was the valvetrain rebuilt? Missing at low rpm could be sticking valves and would definitely cause a vacuum problem.