How is timing changed on propane vs gas? I have my plow truck that starts great when its cold but wont turn over when the engine is hot on propane....could that be a timing issue?

1941 Taylorcraft
1968 Charger
1994 Wrangler
1998 Wrangler
2008 Kia Rio
2017 Jetta

I didn't do 4 years and 9 months of Graduate School to be called Mister!