Hi John thank you so much for your reply, I bought a new ignition coil and the shop said that's the one don't need resistor so I cut both wires going to the resistor and wired together directly
I have measured the power to the coil is about 11.5v when battery posts show 12v
I also bought a inline spark test which lights up but spark is very dim ( I had to put spark plug in a cardboard box to see it)
Maybe distributor ? I took it off without moving the shaft not sure if that still moved the timing? Yes it's a flathead engine

If it backfires after you let go of switch
Try starting and let go of switch quickly
It sounds as if you have an issue with starting
Voltage. 1 -check your ignition switch as well as your distributor the distributor needs to be tight for ground 2- look at your wiring on coil make sure the wiring was put back in position
Is it a flathead engine ? If so check your timing
Since you had head off 3-make sure all of the ignition wires are tight especially on ignition switch 4- does it have an oil pressure or vacuum hooked to lp for safety switches
Lp units are supposed to have a vacuum switch or a oil pressure switch for safety in case unit gets a fire and engine dies it does not feed lp to engine 5 - if electric lp lock off put a meter or test light in it and see if it loses voltage if it is a vacuum lock off remove vacuum line and suck on it you should hear diaphragm close , if you get fuel it's bad
Anything I can do please let me know

If it backfires as you turn key off it may be a bad ignition switch , as you turn the key to start let go quickly and see if it starts
I've also seen starters that pull too much voltage and starves the ignition
My guess would be a bad switch or weak coil

Thanks for your help, the unit has been converted to propane only when i bought it, gas unit is no longer hooked up. here are somethings i did to it but im still having problems;
first i suspected there was a vacuum leak which not able to pull enough vacuum to open the regulator so i did smoke test, and smoke came out all around the old carburetor, and the throttle rod was slightly bent, and smoke came out from it.
Since its a BEAM brand and they were not longer making it, i bought a Impoco CA125M carburetor which throttle and bottom flange fits. i hooked up the vacuum line and propane line from regulator, when i try to start engine will crank but it wont start, right before i stop the key ignition the carburetor will have loud pop(backfire). I'm really lost and spent a lot time on it, any help will be appreciated and im willing to pay for help
Things i have done so far;
replaced new Propane Regulator
2. replaced head gasket and tested compression all 4 cylinders
145-150psi
3. installed new plugs.

You shouldn't hear LPG fuel at all. Disconnect propane tank and try it on the gasoline side of fuel system. If everything is ok on the gasoline side it should run ok once LPG is purged from lines. If it runs ok check your LPG regulator. Look for frost on regulator while running on LPG.

Start with your vaporizer
If you hear snd smell fuel it could be bypassing
And flooding unit

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