Here is what my 75 f250 with a 390 4spd is doin:
It won't warm up right. it takes me three or 4 tries to get it to start and keep firing. After that I have to be very careful andquick to get it into gear, if I leave it in neutral it'll die. it does that for a couple miles after I start it.
It backfires like CRAZY. Sounds like every fire is a backfire. Mostly when it's in high rpms going down a hill (like when I'm engine braking)
The headers are open so the backfiring is even more pronounced.
Don't really know how to go about looking for a vacuum leak... or really even what it is.. kinda sad to admit!
Any ideas? I want to be able to give fixin it a try before taking it to my friend who's a mechanic (I owe him about 3 grand in beer for work he won't let me pay him for)

 

I really hate to say this b/c im sure it sounds great rackin off but if you are drivin this everyday then it needs to have a full exhaust system put on it(you said you were runnin open headers).

Also the truck should be warmed up properly before takin off to drive. May also be partly a malfuntioning choke. Does it idle in neutral when warm?

 

Once its running with no effort on you; (pumping gas pedal etc) run you hand along the side of the heads /header area ( where the headers bolt to the block of the engine) feel for air , might even have a graviest look on the heads & headers. if there's a leak try tighting down the headers bolts, Don't force them to tight; if that doesn't stop the leak; prolly need new header gaskets. ( I also recommend complete exhuast) If new cost is too much; try to find a similair model in the salavage yard for now and remove it and buy new header collectors and place the salavage yard exhaust pipes atleast run them towards the rear axle and use mufflers. This is only a quick fix and new is better.
Vaccum can be a headache; look along your carb with the air breather remove for any open ports, or ones with a black cap like rubber plug on them. I recommend replacing all of the vaccum lines & rubber plugs it that seem dry rotten or crack. You can purchase fairly cheap vaccum hose and plugs from any auto parts store. I recommend not to pull any hoses completely off both ends; but one end to measure diameter & lenght; then purchase the different type hoses and lenghts and remove only one hose at a time to replace them all. I would also replace the PCV valve on the valve cover that should he attaqch to your carb or intake by a piece of vaccum hose as well
hope this helps.

 

Every couple of months I have to replace my vacuum caps on my vacuum tree. If not one or two will decay and make my truck run just as you describe. The vacuum tree is behind the carburator on top of the manifold. I carry an assortment box of vacuum plugs in my toolbox. About 5 bucks at the parts store.

 

Vacuum Leak: Get a can of carb cleaner (or just soapy water) and spray around the manifold while the truck is running at idle warm. When you find the leak, the rpm will change immediately.

Backfire: Backfire is warm exhaust gas with unburnt fuel in it. IE Truck is running rich. The AIR circuit on the original configuration has an anti-backfire valve in it just for this reason, It diverts the AIR Pump air off as you release the pedal. Almost all backfires happen as throttle returns home. You are running with the fuel/air ratio turning rich temporarily as you shut off the air when the throttle butterfly closes. When the unburned fuel hits the open air, it will pop as it burns. Engine braking is perfect to cause a backfire.

You likely have two problems not one.
Vacuum Leak usually means you are running higher rpm and actually LEANS the mixture a bit. This can lead to a too lean hard start problem, etc.

Backfiring is a usually a rich mixture problem. It can however come from a an ignition source too but that is usually kind of obvious. The back firing will occur with no pedal movement as the fire is jumping around the dist. cap and often times, you can see/hear the fire jumping. You can also visually inspect the cap for irridescent coloring, and wires for white areas where fire is jumping.

Engine Warming: as the engine warms, the vacuum leak can actually go away as the parts warm up and expand. So looking for a vacuum leak can mean doing it warm AND cold at idle.

Without knowing how much of the emissions system is disabled, or knowing how the choke is working you may have several complicating factors.

 

I do have an exhaust system actually, but it's y piped and I have the caps off right now. I have dual flowmaster, which I re capped yesterday (it's only 3 wing nuts on each side)
I plan on trying all these ideas out later tonight when I get the time
Thanks! I'll let ya know if I find anything!

 

PCV valve hose was completely off for starters.. hahah so major vacuum leak! I guess I must have been a lil overworked when I gave the engine a once over last week. Any ideas how that could happen? it wont reconnect.. it runs way better when I just place it back on the valve and run the engine but it doesn't seal all the way.. not really sure where to go from here. New valve? the hose is pliable and new feeling, so the problem isn't there. Thanks for the help!

 

you can check for vacume leaks by conservitively spraying starting fluid around the vacume hoses (avoing spraying ignition parts) and you can find exhaust leaks by getting a peice metal tubeing and and listening in one end and running the other along your exhaust system. if you know for a fact that you do have a leak thats the easyest way to pinpoint its location

 

Mmmk well I feel stupid for not realizing this but the problem was that the tubing was not PCV hose, so the oil ate it up and it just got so old and pliable that it wouldnt seal. It was sucking flat too because the hose wasnt strong enough. Gonna head into town tomorrow and grab a new piece of hose and I'll be set! Easy fix! I love learning things from my awesome redneck mechanic neighbors!!

 

redneck is where its at !!