Engine Starts Then Dies

[Insayn]

The 76 Bronco has been sitting for awhile. It has a full tank of gas and I had it running a couple of months ago with no issues. I can crank it, get is started then it dies after a few seconds. It is getting fuel and the Duraspark was replaced recently after my last issue. I used a timing light to see if a plug wire was getting power while cranking it. Any ideas?

Motorcraft Carb w/electric choke
302

 

[Insayn]

https://classicbroncos.com/forums/media/img_4059-png.33002/

 

[DirtDonk]

So does it start, then die every time you try it? Or did it do it once, then no more starting at all?
Are you saying that you can still see gas shooting out of the squirters when you roll the throttle lever back?
And you’re seeing a very consistent spark event on multiple wires when you check with the timing light?

If all of the answers were yes, then disconnect the 3-wire distributor plug and check resistance between the Orange and Purple wires.
Should be between 400 and 700 ohms.

And check for voltage on the coil positive anyway.
Just in case.

And since you have a timing light handy, check the ignition timing while the starter is cranking.

 

[gnpenning]

Does it quit when you release the key from crank to run?

 

[Insayn]

Does it quit when you release the key from crank to run?

It quits shortly after that like it uses up all the fuel in the bowl.

 

[Oldtimer]

Mechanical or electric fuel pump?

 

[Chiclassics]

See if you can have someone keep spraying starter spray once you start , if it stays longer then it will confirm fuel delivery issue either to carb or idle circuit.

But if you are having fuel in the bowls it rules out your fuel pump issue, may be something is blocking idle circuit after you start it ...spray some carb cleaner and retry.

This assumes your electrical circuit is good

 

[DirtDonk]

And in case you're wondering why we're all on about the fuel Insayn, it's because mechanical fuel pumps of the modern era HATE sitting for any extended period of time.
Even as little as a couple of months is enough to kill some modern pumps. Seen it happen on almost every vehicle I know of.
The electric fuel pumps fare much better in this regard, but they're not completely immune either.

Paul

 

[Insayn]

I tried spraying starter fluid while it was running and it ran, but not for long.

A buddy told me to check the the accelerator pump. Since I was going to be in the carb I rebuilt it today. Turns out the diaphragm was torn a bit. Also found a dry rotted vacuum cap, some shellacking and a fuel filter full of what appears to be rust. Not too sure from where, the tank is new….maybe from PO running it on the old before replacing it.
https://classicbroncos.com/forums/media/img_4065-jpeg.33003/full

https://classicbroncos.com/forums/media/img_4068-jpeg.33006/


https://classicbroncos.com/forums/media/img_4068-jpeg.33005/

 

[Insayn]

https://classicbroncos.com/forums/media/img_4068-jpeg.33007/

https://classicbroncos.com/forums/media/img_4068-jpeg.33008/

 

[Insayn]

Now my issue is the vacuum leak at the EGR plate to the intake manifold. The paper gaskets don’t appear to do it. PO has a metallic gasket that I’m searching for.

 

[Chiclassics]

I tried spraying starter fluid while it was running and it ran, but not for long.

That confirms fuel delivery issue via idle circuit.

Typically when a car sits more than an year your old fuel chips away any rust in fuel lines etc which might be your case . Glad you found filter issue and also carb issue.

However a small crack in accelerator pump diaphragm is not your reason for idling issue. When you give throttle and you see squirt of gas coming out from nozzles is actually your accelerator pump working ok although it may give out in future based on what you described. But still is not the reason for car to die down. Idle circuit is not generally visible to naked eyes, it has path ways inside the carb body that uses engine vaccum to pull gas from float bowls to keep engine running. Even if a tiniest part of rust or debris blocks it , it will create issues idling.

Do this, take out the Idle mixture screw completely out ( I am guessing only one with your Motorcraft carb) and then spray compressed air or carb cleaner with a spray straw it will clear out any debris

Now coming to your vacuum leak given that you are unable to keep it up, how do you know there is a vacuum leak at the base of the carb? post some pics of it. Its quite easy to find gaskets. Is your intake stock given Motorcraft 2barrel?

 

[Jetfixr320]

The Ethanol alcohol in most gas sold today will dry out and clog jets if allowed to sit for extended periods of time.
It also has a lower boiling point and can cause vapor lock issues.

 

[Insayn]

That confirms fuel delivery issue via idle circuit.

Typically when a car sits more than an year your old fuel chips away any rust in fuel lines etc which might be your case . Glad you found filter issue and also carb issue.

However a small crack in accelerator pump diaphragm is not your reason for idling issue. When you give throttle and you see squirt of gas coming out from nozzles is actually your accelerator pump working ok although it may give out in future based on what you described. But still is not the reason for car to die down. Idle circuit is not generally visible to naked eyes, it has path ways inside the carb body that uses engine vaccum to pull gas from float bowls to keep engine running. Even if a tiniest part of rust or debris blocks it , it will create issues idling.

Do this, take out the Idle mixture screw completely out ( I am guessing only one with your Motorcraft carb) and then spray compressed air or carb cleaner with a spray straw it will clear out any debris

Now coming to your vacuum leak given that you are unable to keep it up, how do you know there is a vacuum leak at the base of the carb? post some pics of it. Its quite easy to find gaskets. Is your intake stock given Motorcraft 2barrel?

The leak at the base of the carb was the EGR blowing past the paper gasket I used out of the rebuild kit. I could literally feel it. I upgraded the gasket to a metal Edelbrock gasket and it solved that. That’s what was previously on there. I’ll try blowing the lines and the Jet next. Perhaps the fuel rotted the diaphragm in the mechanical pump too?

 

[Chiclassics]

if you install a fuel pressure gauze in line , you will know if your mechanical pump is within spec (3-7psi) ideally 5psi.