I have an 87 F250 5.8 liter 4barrel carb
On cold start the truck runs fine for about a minute and a half. Shortly after I kick down the high idle it will start to backfire then stall out shortly after. Turn the truck off wait a few minutes and the problem goes away. If I choose to just drive away after starting cold the problem remains;A minute or two down the road and the backfiring starts, then stall. Wait a minute or 2 and all is fine.

I have been through this truck a hundred times and cannot find a vacuum leak anywhere. I do know the egr valve is functional by putting a vacuum line directly to it. I haven't actually seen it move while connected as it should be. I do have a ported vacuum switch in the control scheme. It has 4 ports but I'm not sure how to check it. I'm sure one is a supply, one to the egr, one to the carb, and one to another control mechanism of some type.

^
||---Black: to some control valvel near the cannisters
||---Red: Vacuum supply
||---Green: to EGR valve
||---White: to carburetor
|==|
|==| Ford # looks something like E5TE-9D473-B2A

This problem has persisted for a couple of years believe it or not. During this time I replaced lots of stuff. Recently I replaced the ignition module. The carb and the rest of the motor is a factory rebuilt. Exhaust is also new.

Any ideas ? Please!

 

Is the choke shutting off too soon? Check the setting. Also check that the hot air intake system is functioning properly. Very often the vac-motor in the intake snorkle goes bad and you don't get any warm air off the exhaust manifold when the engine is cold. Then the cold air commng into the carb causes the fuel to condense. This makes for a super lean mix going into the engine which causes mis-fires. Good luck.

 

The valve on the air cleaner snorkle works. In fact I just ran the motor for about 15-20 minutes and the valve didn't change position from taking air off the exhaust manifold. The line to it has full vacuum. Begs a stupid question; when does it open so that it takes air from the outside? In between the vacuum source and the valve is a little gizmo on the inside of the air cleaner. I'm not sure if it senses temperature or pressure but it seems to be the switch that operates the valve at the intake snorkle.

A visual check on the carburetor choke shows that it seems to be operating as it should.

I keep coming back to that ported vacuum switch that controls the egr. I get full vacuum to it but it doesn't seem to provide vacuum to the egr either hot or cold. But then their are 2 other line on the switch. One from the carb and one from a control near the charcoal cannisters. How does one test this switch?

Thanks,

 

The hot air valve is temp regulated and is designed to open when the inside aircleaner temp reaches around 60-70 degrees. They usually take awhile to open.
It acts just like an icing up carb though due to no heated air.

 

Another check on the shroud over the exhaust manifold showed that about hald of it was rotted away. The reuslt of this is that most of the air flow wasn't being wrmed up by the manifold.
I fixed it all up with some sheet metal from my shed. So far on the past three cold mornings the motor started fine and warmed up without a single backfire. The next real test will be on a cold rainy morning.
It may have been the carburetor icing up after all.

 

Hopefully, you got it fixed