My truck 1976 F-150, 390 FE, C6 trans.

[My issue] After sitting overnight, my truck only starts after I let go of the key after cranking. (Run position)

About 3 months ago I had bought a brand new Holley 4160 600 cfm carb for my truck.
The truck ran great for a while and then started to have a REALLY hard time starting after sitting overnight.


One morning I went to start my truck and it would just keep cranking and cranking, but as soon as i let go of the key,
I heard the loudest backfire in my entire life... After losing my hearing for a about 5 mins, I tried starting the truck again and it fired right up but it was SUPER loud.

My muffler had blown up (See pic below)

I assumed that my carb was dripping gas after the engine was shut off, so after that I went through the carb and found a piece of rubber hose stuck in the needle and seat.

After cleaning that out, I replaced the fuel line from the fuel pump to the engine and put 2 inline filters right before the carb.

I'm 99 percent sure that the carburetor has stopped dumping gas into the engine but now only starts when I let go of the key and it still backfires pretty bad about 90 percent of the time when starting in the morning. When it backfires, 70 percent of the time its through the intake and 30 percent of the time its through the exhaust.

[NOTE] This weird starting issue only happens after the truck sits overnight. Starting the truck during the day after sitting for 8 hours is no issue.

I have replaced the ignition switch and that made NO difference.
I have 5 volts at the ignition coil (When not cranking the engine)

(I don't know if this is related but, at the same time that all of this started, my headlights only stay on for a few mins and then they shut off and to get them back on, I have to turn them off and then back on a few times. When I turn my brights on, they stay on for like 10 sec and then they shut off. Marker lights, brake lights, and turn signals were not affected.

 

Wow.

A few questions...

Your intake is 4v correct?
Are you using a non-metallic carb spacer?
Is your metal fuel line from the pump to the carb laying against the block?
Where is the fuel in the site plug/How high is the float?
Have you adjusted the carb at all?
when was the last time you filled the truck up at the gas station and is it a pretty busy gas station?

 

Also, buy a fire extinguisher.

 

1. My intake is for a 4v carb
2. carb spacer appears to be aluminum 1-1.5 inch thick
3. metal fuel line is like 3-5 inches from the block. (half of it is metal, other half rubber)
4. in the sight plug, fuel is as the bottom of the threads
5. only thing ive adjusted is the idle mixture and the float level.
6. last time I got gas was at the Walmart gas station. (Which is always very busy)

 

Also I do have a fire extinguisher just in case hahaha

 

Check for condensation inside the distributor cap ... I have had similar issues and found moisture inside

 

You might also want to check and maybe replace your plugs if they've become fouled from all this.

 

Also run the truck at night, in the dark and look for spark plug wires arcing ... again morning moisture can mess with starting and backfire

 

what is the voltage at the + terminal of the coil during cranking?
does a test light connected to the - terminal of the coil flash during cranking?
if you don't have a test light or voltmeter handy (you should)...do you get the same results if you remove the brown wire from the solenoid and try to crank it??

 

Voltage while cranking stays at a steady 7.0V
Test light connected to negative on the coil flashes consistently while cranking

 

The distributor cap doesn't have any moisture but it is a little burnt for some reason

 

They do that, after several thousand miles it can buildup to quite a crust.

 

Is there good spark at the coil wire while cranking ?

 

the coil + terminal should be getting near battery voltage during cranking...supplied by the brown wire coming off the solenoid...

 

Any chance the distributor got loose? Retarded timing can lead to backfire, as well as very hot exhaust.