1979 F100 4x4
460 eng c6 trans
Demon carb
Just purchased have done all the basic tune up,timeing, oil change ect.
Symptoms 1. will not idle untill warm.
2. pressure in gas tank.
3. stalls and stumbles after excelleration when takeing off.
4. replaced fuel filter with a clear in line filter and noticed that it sometimes runs empty (the filter) or appears to not be getting gas from pump.
5. when filling with gas from a can it will at times boil back out.
6. when it stalls it blows air back through the carb.
This truck has had good and hack work done on it before I got it. The tank vents only through the a 12 inch hose at the tank! Most of the fuel line from tank to pump was replaced/cobbled. It has fresh gas, it sat for some time. old gas drained first (prior owner). When warm and throttled at the carb it sounds great lots of power no pings or stumbles.
Thank you.

 

Symptoms
1. will not idle until warm.
Check choke fast idle setting
Check idle mixture screws
Check for leaking butterfly rod on either side of carb for leaks ...with spray can of carb cleaner.

2. pressure in gas tank.
[b] Hot weather (temperature) causes fuel to expand.. is fuel cap correct for application ?

3. stalls and stumbles after Acelleration when taking off.

Carb probably needs rebuilding..
check indexing of accelerator rod on lever position
Check distributor for being worn out/....shaft does oblong turn and makes spark enabling difficult


4. replaced fuel filter with a clear in line filter and noticed that it sometimes runs empty (the filter) or appears to not be getting gas from pump.

Check for fatiqued rubber fuel lines ...for leaking air INTO fuel line

5.when filling with gas from a can it will at times boil back out.
Boil Back ?.. Nope... not a chance
Slow the pouring fuel in filler tube process to allow fuel to flow.
Check Tank filler hose for being pinched from body sagging

6. when it stalls it blows air back through the carb.
Severe timing problem
Check all Mechanical Timing adjustments and timing chain to gears slop

This truck has had good and hack work done on it before I got it. The tank vents only through the a 12 inch hose at the tank! Most of the fuel line from tank to pump was replaced/cobbled. It has fresh gas, it sat for some time. old gas drained first (prior owner). When warm and throttled at the carb it sounds great lots of power no pings or stumbles.

 

IF you are properly vented you should not have pressure in the tank. It originally vented through the charcoal canister. No vent will cause problems. Are you sure it's not a vacuum in the tank? That causes carb starvation.

You might want to replace the fuel lines or at least clean them out and check fuel pump pressure.

 

Thanks for the info! I will be checking into all this week end! cant rule out vacuum at tank although it did drench me in gas once but at other times it does sound like vacuum when opening. Did a check on timeing 8 btdc also checked spark advance and found that it worked when I applied vacuum however the vacuum port on the carb was not working, attached to a unused port and spark advance now works. hope this was the reason for the air blowing back through the carb. still has other issues mentioned above. The gas cap appears to be the correct one.
Thanks John

 

The original vaccum line from the dist vacuum advance ...ran to a thermal vacuum switch on the T-stat housing.
If all the emissions are removed it should run directly to the PORTED vacuum port so it only gets OFF IDLE vacuum ...(with the engine revved up)

 

Thank you, I can tell I am going to learn alt here! It was probably on the ported one and I assumed it had no vacuum, checked at idle. Yes all the emissions are removed.
John