1987 f250 300 6cyl parked was running fine. now turns over fine wont start.

Here are Bunch of Well Known Pain in @#$@!%^^$# GM issues to check on ++ any or 2 or more will cause Surging and Stalling ....I am thinking O ring on Fuel injector gets Hot and Loss of Pressure occurs via Cracks in rubber seals ..This occurs in Turn motion when fuel in Rail is sloshed around . Even though pressure allows good starting the Heat 2 Plastic engine parts causes Expansion so a Crack that does not Visible Leak cold will If Searched for Hot ..Recommend a Strong Light even in Daytime to see if Leaks around Intake / Fuel injection areas .. Upper intake Plenum Air intake Gasket NOT SEALING right is common Hot issue on GM s ...Its a V6 the misfire tell me this right away ..Did it misfire before Tune Up ?? Even if it did the issue Now is Spark Plug wires have been Crossed or Wired out of Sequence / Order ...SOLUTION will cover many checks you must make . 1 st be sure Plug # 1 and each firing order Plug wire is matched up To proper ## on Magneto or Black plastic squarish box near valve cover ..OFTEN firing order was Clockwise on Ford / GM - that is old Distributor days ... Magneto driven is Left 2 Right.....Idle Surge is Present under Mis Wired conditions BUT you may check Magneto for Clean Ground surface in mounting bracket ...The Mags do false and wear out due 2 Heat so a Good Ground 2 Base mounting is needed .... Mechanics call them Coil Packs A Very Common since 1983 GM problem ....Buy aftermarket if Continuity Test proves they are Weak - Bosch or Standard make good Ignition parts . Since a Tune up was Done MAKE Sure plug wires have been Seated or Pushed on Tightly ..A CLEAN Air Filter can provide proper Air Flow at Start up - so Hold up to Sun Light then Flex it against Light 2 see if Lots of Light this will determine its Ability to Draw air ..NAPA sells a Spray Can = 10 bucks for Electronics cleaner that is used 2 Clean Mass air flow sensor visible when hose connections removed ..ALSO spray clean Intake air Temp sensor a small 2 wire attachment visible on outside of air intake hose .. The following is Called Fogging to Clean Fuel injection and is Best Done with Engine Running or Started right after all Below Items have been Spray cleaned +++ after 1 st spraying You must Rev engine 2 open Throttle plate and spray in direction of each Item .. The disconnected air box ++ intake Air Temp sensor with hose can be Off for Fogging Procedure but Wires plugged in .... Clean Throttle body intake at Plenum - or Intake at end of hose connection 2 main Fuel injectin Air intake .To do this remove Hose - Open Throttle via Someones help on Gas pedal or manually move it at Gas pedal cable connection - Then spray cleaner and be sure 2 get Idle control solenoid Tip visible when Throttle plate opens also on other side of this open plate is 2 nd Tip for EGR sensor be sure 2 spray it .. Final worst case issue is Fuel Pump in Tank can be Wearing out . Why you ask ?? You said young person Driving and If Poor Like me Then driving below 1 / 4 tank .. Yes electric fuel pumps when old Loose pressure when near bottom of Tank .. Stalling in / or after a Turn or sudden quick turn ++ Putting into Park after arriving in a sudden fashion will shift weight of car forward causing Gas in Tank To slosh / Move around .. All this Rapid movement moves Gas around and WILL Interfer with Inside of Tank electric Fuel pump ...Fuel supply interupt will cause misfire codes ...Driving habits by Old or Young often Help Pin Point a cause So see if any Trouble on Full Tank - If not Then Low fuel points Too electric fuel pump - Sadly it is inside of Tank = Major Job .. Try the Cheapest Ideals 1 st in my List ...Good Luck an hope it is simple Like a Loose plug wire ..............

Sounds like timing belt

If it sit for a long time the gas would not be any good no octane You can keep the critters out of the intake which i use lathing material cheep at hardware or scrap's around job sites free

Does this motor have a mass air flow sensor?

first thing to check is for fuel supply to the carb, it may be just a filter, worse off is you may have to rebuild the carb

clean out the intake and run a good fuel system cleaner through it. Giver some gas about 2000 rpm for 5 minutes.

Check the intake hose to make sure that it doesn't have a whole in it also check the air box that the filter goes in also. Mine was idling rough also and we found a whole in the filter box and the intake hose had a whole in it. Hope this helps.

Should be located behind the harmonic balancer on the bottom of the motor. You'll need a balancer puller to remove it.

Looks like gum and carbon deposits have clogged some critical parts in the engine, such as injectors.
You would need to get to the 'throttle-body'.
1. Trace the big intake-air hose to where it ends on top of the engine. This is the throttle -body.
2. Loosen the clamp that's holding the big-intake air hose. Pull off the intake air hose from the throttle-body.
3. Spray some "Throttle Body & Carburettor cleaner into the throttle-body. You could use the entire can.
4. Wipe off excess gas from the throttle-body with a piece of rag.
5. Reconnect the intake air hose, and check if the
problem has disappeared.

SOUNDS LIKE A VACUM LEAK,POSSIBLY INTAKE GASKET OR VACUM HOSES,GET SOME BERKIBLE 2+2 SPRAY DOWN INTAKES WHERE MEET HEAD AROND THROTTLE BODY AND ALSO EGR VALVE TO MAKE SURE IT ISN'T PARTAILY STUCK OPEN, VACUM LINES AIR INTAKE HOSE BETWEEN THROTTLE BODY AND AIR FLOW METER,ALSO MAKE SURE ITS NOT DRY ROTTED AND SUCKING AIR,THE ENGINE IDLE SHOULD GO UP OR DOWN DEPENDING HOW BIG THE LEAK