I have a 79 New Yorker with a 318 it has a hard time starting we just replaced air filter,fuel filter and lines plugs, Roter and cap it floods real easy

Have you rebuilt the carb? You may have a gasket gone bad, float not floating, or something similar.

Forgetting all you have done & why

What is or was the main issue & when did it start
& under what conditions ?

Does the thermostat open ?
Does it reach 195 or more coolant temp?
Does it go into Closed Loop? (Oxygen Sensors
in control of fuel)
Any OBD Codes in any system?
Fuel Pressure Correct as well as Volume
& under a load while driving

Sounds like you were going to say it only has a problem
AFTER it reaches operating coolant temp

SO- none of what you did appears to be related

First-- you find & resolve all problems. THEN do your
Preventative Maintenance-spark plugs,filters,etc.

Check or test the switch on the transaxle, that is for gear
selection,whatever you have there

May be the only problem you ever had or one of two problems

Only runs when cold-- in park only?
Won't run hot or cold-- in ANY gear?

Can you even drive it at all?

How about an Idle air control solenoid/valve
on or near the throttle body-got one of them?

i have a 1994 gmc jimmy 4.3l cpi. i could not pass the emission test.hard to start, spuddered, and smell of gas was strong. long story short, i replaced my spider injectors, good to go. $278 brand new from auto zone.

could be faulty crankshaft position sensor.

Have the fuel pressure checked if it is weak,it will cause this .

check for rusted leaking fuel lines in back near fuel tank , clogged fuel filter, low pressure in fuel pump check with gage , check air filter , loose plug wires , bad plugs and wires , timming, cap rotor, bad gas from the botton of the tank, see if you can get a code out of it that would help alot

time to buy a new van.. umm what about the alternator?

replace the fuel pump...if there was no pressure in the line going to the fuel filter, then there's only one thing that feeds that filter...its the fuel pump like i previously thought..the tune up? great idea...that's what made me think it wasn't spark related. good on you for doing the air filter...that is one of the most over looked parts for replacement on a car...it cost you as much as 10% more fuel if its bad...and long term hard start issues....replace you fuel pump AND the screen at the bottom at the same time...oh yeah, use a good quality pump....it has to last you a long time..

oh sorry, and when the fuel pump is out, check out the wiring harness, make sure no corrosion is present, no pins are bent, the connetor pigtail is not damaged etc...then replace the pump...thanks and let me know again.

could be the fuel pump a bad coil or pick up coil located under the cap and rotor fuel pressure should be at least 35psi.

what about the starter?