i have a 1985 ford with a 300 in line. almost every time i drive it and then shut it down it wants to keep running and about every 3rd or 4th time i shut it down it back fires..... does anyone know why it would be doing this a have thought about replacing my carb but im not sure...

 

Is it backfiring thru the exhaust or the carb?

 

exhaust.... and i just replaced the head not too long ago because it was cracked.... it did this before the head was replaced too

 

I might be wrong here, but, if its backfiring thru the exhaust, its carb problems. If its backfiring thru the carb its timing. It might be worth it to spend the 15-20 bucks and buy a rebuild kit. Just out of curosity, have you pulled the codes?

 

what codes

 

The codes from the computer, that is if it hasn't had the duraspark conversion done to it.

 

not sure how to do that

 

Not shutting off when the key switch is turned to off is called "run-on" or "dieseling". It's commonly caused by the idle speed being set too high.

Backfiring in the exhaust system is caused by raw fuel entering the hot exhaust system components. Usually the carb is the culprit dumping too much fuel into the engine. More than likely, you have the idle speed set way high because it will not run properly if you slow it down. More than likely because it's dumping too much fuel and flooding the engine.

The previous poster is also correct, your truck originally had a computer controlled carb system on it with sensors and the timing was computer controlled. People get in there and rip this stuff off the engine, and then they really open up a can of worms, making the engine almost impossible to get running correctly.

See if hoses and sensors have been unplugged/disconnected. Then look to see if your distributor has a vacuum line connection on it and if it's connected. From what you report back, we can tell you what you need to do, and you might want to get another carb rather than rebuilding that one, if it's the computer type carb.

 

Backfiring through carburetors can be more than just timing. It can be burnt up intake valves, it can be sticking intake valves, or it can be bent valve stems on the intake valve (also causes sticking). Generally, backfiring through the carburetor is caused by anything that has to do with a cylinder firing and an intake valve being open.

'85 Inline: While your at it, go ahead and check the timing as well as going over the motor and checking for any of the usual stuff (fouled plugs, vacuum leaks, make sure all connections are tight, etc.). While the truck is running, I'd also hold a dollar bill loosely up to the exhaust, and if the exhaust pipe tries to pull the dollar in, then that means it has burnt up exhaust valves.

Since you've mentioned dieseling, I want to point out to you that dieseling can be caused by carbon buildup, and if severe enough, an improper fuel mixture (usually a lean mixture, as that causes the cylinders to run hotter). If you've got the computer controlled carburetor, pull codes and see what she says. If you've done a Duraspark II ignition swap, check your idle mixture and adjust it with a vacuum gauge. Then, get a fuel tank additive like Seafoam or something similar to a Fuel Injector Cleaner and add the correct amount to your gas tank to help clean out some of that carbon that's probably in there.

The first thing to do though, is pull codes.

 

I think while the L6 was computer controlled, it used an EEC-IV system, right Franklin?

If so,

Here is a list of what each code means:

TCCoA Tech Articles

And here is how to pull codes:

https://www.ford-trucks.com/forums/1...an-eec-iv.html

 

it does not have the dura spark ii conversion and it is computer control carb no vacume advance on the distributor. it is also missing the smog pump. when i had the head off i checked for carbon build up and there wasnt any that is more than normal. i have never adjusted the idle speed on the carb. it seems to idle at normal speed too.

 

i was thinking about doing a 2 barrel conversion but i want to make sure that the problem i am having is in fact a carb problem before i go ahead and do that.

 

That would be the Duraspark II conversion. Do a search here, theres a lot of good info on it. Good luck.